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Invited review article
A review of volcanic electrification of the atmosphere
and volcanic lightning
Corrado Cimarelli a,
⁎, Kimberly Genareau b
a
Department of Earth and Environmental Sciences, Ludwig-Maximilians-Universität München, Theresienstrasse 41, 80333 Munich, Germany
b
Department of Geological Sciences, The University of Alabama, Box 870338, Tuscaloosa, AL 35404, USA
a b s t r a c t
a r t i c l e i n f o
Article history:
Received 11 July 2021
Received in revised form 1 December 2021
Accepted 5 December 2021
Available online 13 December 2021
The electrification of volcanic ash plumes and the occurrence of volcanic lightning are now known to be common
phenomena during explosive volcanic eruptions. This knowledge stems from centuries of anecdotal observations,
and in recent decades, from improved instrumentation and media attention. Following a summary of previous
reviews, this contribution will detail the most recent findings concerning electrification mechanisms of eruption
columns/plumes (triboelectrification, fracto-electrification) and how hydrometeor charging contributes to this
electrification depending upon the eruption style and abundance of external H2O. Field measurements to
determine the charge structure of volcanic ash and gas plumes reveal wide variability both spatially and
temporally, indicating the influence of these different charging mechanisms. The charge structure and
resulting lightning characteristics have been provided by a suite of both ground-based and satellite-based light-
ning detection methods and the various characteristics of each are summarized. As these detection methods have
revealed, the electrical properties of ash plumes can provide insight into their physical dynamics throughout the
course of an eruption. Lightning may therefore provide a means to track changing eruption conditions and the
associated hazards, providing another tool for monitoring efforts. Volcanic lightning also leaves physical evidence
in associated ashfall deposits. These lightning-induced textures have been documented and are summarized
here, in addition to the different experiments that have reproduced such textures. Lightning simulation experi-
ments provide information on changes to ash grain size, size distribution, chemical, and magnetic properties of
ash. Lightning discharge and the lightning-induced changes to ash grains potentially impact not only the hazards
induced by ashfall, but also changes in atmospheric chemistry relevant to biologic activity, the fluid dynamics of
eruption columns/plumes, and ash dispersion. Additionally, shock-tube experiments provide insight on the mi-
crophysical dynamics and environmental variables that influence electrification of dusty gas mixtures. Finally,
this review summarizes the challenges to volcanic lightning research and the future efforts that can aid in
addressing the unanswered questions regarding this phenomenon.
© 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://
creativecommons.org/licenses/by-nc-nd/4.0/).
Keywords:
Volcanic lightning
Volcanic electrification
Tephra
Volcanic ash
Hydrometeors
Volcanogenic ice
Remote sensing
Lightning monitoring array
Volcano monitoring
Radio frequency
Explosive volcanism
Atmospheric electrification
Fulgurite
Spherules
Tribo-electrification
Fracto-electrification
Ice electrification
Experimental volcanology
Lightning mapper
Continual radio frequencies
Contents
1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Previous reviews . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2. Topics of this review. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Mechanisms of electrification in volcanic plumes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
2.1. Fracto-electrification. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.2. Tribo-electrification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.3. Effects of water on plume electrification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.4. Other charging mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3. Charge structure of volcanic plumes and types of volcanic lightning. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
3.1. Previous studies on the charge structure of volcanic plumes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2. Volcanic lightning types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Journal of Volcanology and Geothermal Research 422 (2022) 107449
⁎ Corresponding author.
E-mail address: cimarelli@min.uni-muenchen.de (C. Cimarelli).
https://doi.org/10.1016/j.jvolgeores.2021.107449
0377-0273/© 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Contents lists available at ScienceDirect
Journal of Volcanology and Geothermal Research
journal homepage: www.elsevier.com/locate/jvolgeores
4. Volcanic lightning detection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.1. Ground-based methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
4.2. Satellite-based methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
4.3. Other detection methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
5. Revealing plume dynamics through electrical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
6. Effects of lightning on tephra properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
7. Experimental investigations of volcanic lightning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
7.1. Shock-tube decompression experiments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
7.2. Current impulse experiments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
8. Effects of lightning on atmospheric chemistry and processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
8.1. NOx fixation/ozone production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
8.2. Changes to ash plume convection and dispersion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
9. Challenges and future perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Declaration of Competing Interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Appendix A. Supplementary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17
1. Introduction
Although volcanic lightning has been reported for centuries
(e.g., Pliny the Younger; Volta, 1782; Fouqué, 1879; Symons, 1888;
Mercalli and Silvestri, 1891; Friedlaender, 1898; Fischer, 1893;
Anderson, 1903; Table 1 in Supplementary Material), an increasing
number of volcanic lightning reports and instrumental detections
(e.g., Stromboli 2003, 2007 and 2019, Eyjafjallajökull 2010, Puyehue
2011, Kirishima 2011, Etna 2013, 2015 and 2021, Sinabung 2014,
Villarrica 2015, Calbuco 2015, Colima 2015, 2016 and 2017, Pavlov
2016, Sakurajima 2009–2021, Bogoslof 2016–17, Ambae 2018, Fuego
2018, Anak Krakatau 2018–19, Taal 2020, St. Vincent 2021, Cumbre
Vieja of La Palma 2021) demonstrate that electrification is observed
over a wide range of explosive styles and extends to the lower end of
the volcanic explosivity index scale (VEI ~0) so that it can be considered
an intrinsic property of volcanic ash plumes (Fig. 1). Interest in this phe-
nomenon has been fostered by the opportunity to use detection of vol-
canic lightning as a viable real-time monitoring method for hazardous
volcanic activity. Besides advances in remote sensing methods, insight
gained from laboratory experiments and microanalyses of ashfall de-
posits has revealed that volcanic plume electrification has the potential
to impact both the hazards induced by volcanic ash and the fluid dy-
namics of the eruption column/plume. Consequently, there is much
still to be discovered. This contribution will summarize the current
state of knowledge on volcanic lightning and gaps that remain to be
filled.
1.1. Previous reviews
Previous reviews of this phenomenon include those of Mather and
Harrison (2006), James et al. (2008), McNutt and Williams (2010),
McNutt and Thomas (2015), and Aplin et al. (2016). These studies
have discussed a range of various mechanisms responsible for volcanic
lightning and (at the time of their publication) summarized known ob-
servations and general characteristics.
Although McNutt and Davis (2000) focused specifically on volcanic
lightning during the 1992 eruption of Mt. Spurr in Alaska, they included
a table that summarized the recorded observations of volcanic lightning
at the time. Here, their table is revised and updated with several new oc-
currences over the last decade, consisting of both visual observations
and instrumental detections (Table 1 in Supplementary Material).
Mather and Harrison (2006) added to this summary and focused mainly
on the electrification mechanisms within volcanic plumes by comparing
these mechanisms with those occurring in the multiphase environment
of thunderstorms. They also detailed methods of measuring atmo-
spheric electric fields and particle charge relevant to understanding
the electrification of volcanic plumes. Based upon the work of Basiuk
and Navarro-González (1996), Mather and Harrison (2006) provided
Fig. 1. Examples of volcanic lightning at different volcanoes of the world. From upper left: Surtsey, Iceland, December 1963 (photo Sigurgeir Jónasson); Eyjafjallajökull, Iceland, March 2010
(photo Marco Fulle); Taal, Philippines, January 2020 (photo Domcar C. Lagto); Sakurajima, Japan, March 2015 (photo Martin Rietze); Cumbre Vieja at La Palma, Canary Islands, October
2021 (photo INVOLCAN), Etna, Italy, November 2013 (photo Simona Scollo).
C. Cimarelli and K. Genareau Journal of Volcanology and Geothermal Research 422 (2022) 107449
2
a comparison between atmospheric electrical properties and lightning
characteristics during fair weather, thunderstorms, and explosive volca-
nic eruptions. They also discussed volcanic lightning as a process in ex-
traterrestrial environments and its potential role in early Earth as a
mechanism for generating life. James et al. (2008) also focused on elec-
trification mechanisms and volcanic particle charging, including fracto-
electrification, which will be discussed in more detail later, and further
expanded upon the potential for volcanic lightning in extraterrestrial
environments. These previous reviews, which provided key similarities
and differences between volcanic plumes and thunderstorms, revealed
that both granular interactions and hydrometeor interactions must be
considered in the attempt to explain volcanic electrification.
McNutt and Williams (2010) analyzed reports of volcanic lightning
during 212 eruptions at 80 different volcanoes to constrain the influ-
ence of location (i.e., latitude) and eruption characteristics on occur-
rence. They noted a higher likelihood of lightning during larger
explosions relative to smaller ones but noted the observational bias in
reports of volcanic lightning. Lightning is more easily observed in larger
eruptive columns and for those eruptions occurring at night, but the
prevalence of volcanic lightning observations at higher latitudes sug-
gested to the authors that magmatic H2O, instead of atmospheric
water vapor, was playing a dominant role. If atmospheric water vapor
was the controlling factor, then volcanic lightning observations would
positively correlate with eruptions occurring at lower latitudes, similar
to thunderstorm lightning. This previous hypothesis concerning the
fundamental role of magmatic water has further developed through
observations of eruptions with variable amounts of external H2O.
Comparison of dry magmatic eruptions with those in water-rich envi-
ronments reveals that more ice nucleation in the latter increases the
number of lightning events. The role of water in volcanic electrification
will be explored more fully in a subsequent section. In addition to the
role of water, the analysis of McNutt and Williams (2010) highlighted
many other important parameters of both the eruption and the regional
atmosphere that required more detailed examination in regard to volca-
nic lightning occurrence. Some of these atmospheric and eruption char-
acteristics have been the topics of more recent studies, and those will be
described in this review.
1.2. Topics of this review
Several works have progressively refined our understanding of vol-
canic plume electrification and volcanic lightning, targeting various as-
pects of the charging and discharging processes. Main topics of this
review involve: 1) the processes of plume charging; 2) the distribution
of charge within the eruptive column; and 3) the discharge processes.
Besides these, the discovery of volcanic lightning evidence within
erupted products (Genareau et al., 2015) has also furthered analytical
and experimental studies concerned with the effect of lightning activity
on the modification of both airborne volcanic ash and tephra deposits
on the ground. In this review all the aforementioned points will be ad-
dressed, reporting on the most recent advances achieved, both in the
context of field measurements and laboratory experiments, thus
expanding on the previous review papers (Mather and Harrison,
2006; James et al., 2008; McNutt and Thomas, 2015; Aplin et al., 2016).
2. Mechanisms of electrification in volcanic plumes
Explosive volcanic eruptions generate tephra through the mecha-
nism of magma fragmentation: transition of a magma from crystals
and gas bubbles suspended in a continuum of liquid silicate melt to a
turbulent suspension of solid particles in a mixture of expanding mag-
matic gases and entrained air (i.e., the eruption column). At these con-
ditions, the electrification of volcanic plumes can be better described
by the processes regulating the electrification of granular material
flows, where the composition, size, and kinetics of the solid particles,
in conjunction with the ambient conditions, are all relevant parameters.
Volcanic ash, the portion of tephra of nominal diameter smaller than
2 mm, is considered to be the main charge-carrying agent in a volcanic
plume. Volcanic ash electrification mechanisms proposed include
(Fig. 2): fracto-electrification (James et al., 2000; Büttner and
Zimanowski, 2000), tribo-electrification (Cimarelli et al., 2014;
Méndez Harper and Dufek, 2016), interaction with water and hydrome-
teors (Williams and McNutt, 2005; Arason et al., 2011), and natural ra-
dioactivity (Harrison et al., 2010; Nicoll et al., 2019). However, their
relative contributions in the eruptive column are not well understood.
Among these mechanisms, fracto- and tribo-electrification are of partic-
ular importance because they are intimately linked to the dynamics of
explosive eruptions, in which magma fragmentation and high particle
collision rates can efficiently charge the newly formed tephra within
the volcanic conduit and subsequently, during formation and evolution
of the eruption column and plume.
Charged particles can attract and repel each other according to their
polarity and their trajectories can be influenced by external electric
fields. Electrostatic forces are extremely effective on particles of smaller
mass and can be stronger than gravity (e.g., Jungmann et al., 2021),
hence affecting cohesion and adhesion properties of the material and
its ability to be transported, sedimented, and re-suspended. In this re-
spect, experiments (Matsusaka et al., 2001; James et al., 2002, 2003;
Alois et al., 2017) and field observations (Sorem, 1982; Gilbert and
Lane, 1994; Miura et al., 2002; Mueller et al., 2017) have shown that ag-
gregation and disaggregation of particles can be achieved under the ef-
fect of electrostatic forces. Such effects can determine the cohesion of
smaller particles into larger aggregates and therefore determine their
premature sedimentation from the ash plume (Taddeucci et al., 2011;
Van Eaton et al., 2012; Folch et al., 2016; Pollastri et al., 2021).
Conversely, it has been observed that charging may increase the res-
idence time of lofted mineral dust (Toth III et al., 2019 and references
therein) as evidenced by the anomalous sizes of transported particles
far from their source. This could well be the case for volcanic ash parti-
cles, which can match the size of mineral dust and can be ejected to
stratospheric altitudes during highly explosive volcanic eruptions.
Long residence times of ash at high levels in the atmosphere could influ-
ence radiative forcing, climate, and the global electrical circuit, as sug-
gested by numerical simulations (Genge, 2018). As a matter of fact,
electrification of volcanic particles has been measured by means of air-
borne electrostatic sensors in the 2010 Eyjafjallajökull volcanic ash
cloud thousands of kilometers away from the volcano (Harrison et al.,
2010). The cause for the observed unipolar charge was attributed to ra-
dioactive charging of the particles, since the original electrification ac-
quired at the volcano would be dissipated at such long distances.
Fig. 2. Proposed volcanic plume electrification mechanisms and observed volcanic
lightning (VL) type (with suited detection techniques) in each region of the volcanic
plume. The monitoring information derived from the detected VL type is also reported.
In purple is the range investigated by laboratory shock-tube experiments.
C. Cimarelli and K. Genareau Journal of Volcanology and Geothermal Research 422 (2022) 107449
3
Experiments on the ability of particles to retain their charge in absence
of contact with other surfaces and in variable environmental conditions
are lacking and should be further investigated.
2.1. Fracto-electrification
One mechanism of charging is produced during the fracturing of a
material. This phenomenon has been observed during the fracturing of
crystals (Donaldson et al., 1988), rocks (Enomoto and Hashimoto,
1990), glass (Langford et al., 1991), and other materials such as metal
and ice (Avila and Caranti, 1994). Depending on the type of strained ma-
terial, fracturing has been observed to promote the release of electrons,
positive ions, neutral atoms, and electromagnetic radiation (from radio
waves to visible light), hence promoting charging of the resulting
fragmented particles. Some of these occurrences have been explained
through the piezoelectric nature of certain substances (e.g., quartz),
which enhances charge separation at the tip of a propagating fracture,
generating large electric fields. In such cases, fracturing can charge ho-
mogeneous materials (Takeuchi et al., 2004). Laboratory experiments
conducted on cm-sized pumice clasts forced to collide and fracture
under vacuum conditions show that a net negative charge is held on
solid silicate particles and a net positive charge is either released as
ions or is carried on a very small proportion of fine particles generated
upon collision (James et al., 2000). Fracto-electrification, also referred
to as fracto-emission, is proposed to be a primary contributor to the
charge on tephra particles at the point of magma fragmentation, in the
volcanic conduit, and at exit from the vent (Smith et al., 2018). How-
ever, as magma fragmentation and particle collisions both happen in
the volcanic conduit almost simultaneously, in nature it is very difficult
to isolate the effect of fracto-electrification from that of tribo-
electrification. This may also be valid for some experiments where the
interaction of particles generated during the fracture propagation may
not be neglected.
2.2. Tribo-electrification
Despite being one of the most well-known forms of electrification,
the charging by collision and friction between bodies, also known as
tribo-electrification or contact electrification, is still intensely investi-
gated in the attempt to fully understand its underlying mechanisms
(Pähtz et al., 2010; Shinbrot, 2014). Tribo-electrification of granular ma-
terials is well known in industrial processing where electrostatic effects
may force particles to aggregate, adhere to surfaces, and generate haz-
ardous electrical discharges (Eden, 1973; Cross and FarrerD., 1982). Sev-
eral models exist for the tribo-electrification process (Lacks and
Sankaran, 2011) in which the charge exchange between particles in
contact with each other is regulated by their surface properties and
their ability to acquire or donate electrons (i.e., their work function).
The polarity of charge acquired by the two materials following contact
will thus depend on the difference of work function of said materials.
The tribo-electric series empirically predicts the polarity of one
charged material when in contact with another, according to the differ-
ence in their work functions. It follows that for the collision of two par-
ticles of the same material, the difference in work functions would be
zero thus precluding any charge transfer. Evidence from natural, exper-
imental, and industrial granular flows show instead that electrification
can also occur between particles of the same material (e.g., Alois et al.,
2018; Cimarelli et al., 2014), therefore contradicting the prediction of
the tribo-electric series. Charge transfer among chemically homoge-
neous particles would derive by the exchange of high-energy electrons
on particle surfaces. Particles having the same surface charge density
will have the same capacity of acquiring or losing electrons before the
collision. In this respect, the polarity of charge would relate to the di-
mensions (i.e., surface area) of the particles, with smaller particles gen-
erally charging negatively and bigger particles charging positively
(Lacks and Levandovsky, 2007). This phenomenon, also known as
size-dependent bipolar charging, has been sporadically investigated at
active volcanoes (Hatakeyama and Uchikawa, 1951; Kikuchi and
Endoh, 1982; Miura et al., 2002) and has also been reproduced in the
laboratory on homogeneous volcanic particles (Forward et al., 2009a,
2009b; Méndez-Harper et al., 2021). Although a volcano might display
a relatively homogeneous bulk composition during its eruptive history,
the main components of the resulting tephra may consist of particles of
glass, minerals, and rock fragments which display a high level of hetero-
geneity in terms of chemical composition and physical characteristics
(e.g., grain size, density, and grain shape), creating a favorable environ-
ment for charging and redistribution of charge through the collision of
different particles.
Tribo- and fracto-charging are still fields of study with many unan-
swered questions and unexplained observations, especially for the elec-
trification of particles of the same insulating material (Yair, 2008; Lacks,
2010; Shinbrot, 2014). One problem to solve is the relative contribution
of each of these mechanisms to the electrification of volcanic plumes. A
first step in this direction comes from studying the textures of volcanic
ash generated by single explosions producing electrical discharges at
Sakurajima volcano (Smith et al., 2018). Although textural evidence
points to fracto-charging as the prevalent mechanism of electrification,
a substantial contribution of tribo-electrification by particle collisions
cannot be completely ruled out. Experiments focused on isolating the
effects of tribo- and fracto-charging have been performed to constrain
electrification of tephra and volcanic analogue materials (James et al.,
2000; Houghton et al., 2013; Méndez Harper and Dufek, 2016). How-
ever, none of these have been able to generate electrical discharges,
making the link between mechanisms of charging and efficiency/mo-
dality of discharge still elusive. Recent experiments have shown nomi-
nal size-dependent bipolar charging in shock tube experiments of gas
and ash particle jets (Méndez-Harper et al., 2021). Charge polarity is
measured on single particles segregating downwind from the jet after
they have produced visible electrical discharges upon jet expansion
out of the shock tube nozzle. These results highlight the role of tribo-
electrification in the gas-thrust region of a volcanic plume and note
how particles traveling through this region still retain a considerable
charge, creating regions with volumetric charge densities high enough
to produce discharges.
Finally, electrification of liquids (especially hydrocarbon liquids) is
also well known in industrial processing (Klinkerberg and Van der
Minne, 1958; Koszman and Gavis, 1962). Water electrification can be
achieved by mechanical disruption, boiling, and freezing (Nolan and
McClelland, 1914; Mason and Maybank, 1960; Sun, 2020). As H2O is
one of the major volatile components dissolved in silicate magmas,
the effect of water on volcanic plume electrification will be examined
in the following section.
2.3. Effects of water on plume electrification
The presence of water in its different phases (vapor, liquid, and
solid) and the interaction of magma with external water has been pro-
posed to produce electrification during volcanic eruptions. As a matter
of fact, water is the dominant magmatic volatile, present in variable
abundance depending on bulk magma composition. A suggestion, in
analogy to the electrification observed in weather clouds, was first pro-
posed by Alessandro Volta in 1782 (Volta, 1782), where he refers to the
electrification phenomena (the occurrence of numerous lightning bolts)
observed in the 1779 eruption of Vesuvius and for which the explana-
tion was the amount and rapidity at which the “smoke” produced in
the eruption was liberated. In his communication, Volta extensively de-
scribes how the condensation of water in weather clouds would be the
cause of their charging, then finally concludes that one shouldn't be sur-
prised to see lightning accompanying volcanic eruptions. Calculations
suggest that water vapor condensation into liquid and solid phases
must be common during explosive eruptions as the concentration of
water in magmas may be sufficient to saturate the expanding plume
C. Cimarelli and K. Genareau Journal of Volcanology and Geothermal Research 422 (2022) 107449
4
(Textor et al., 2003; Williams and McNutt, 2005). In this respect, the co-
existence of supercooled water droplets and ice crystals in the
expanding saturated plume would suggest that the mechanism typical
for thunderstorm electrification, hydrometeor interaction (Reynolds
et al., 1957; Stolzenburg et al., 1998) could be active in volcanic erup-
tions (McNutt and Williams, 2010; Thomas et al., 2007; Arason et al.,
2011; Nicora et al., 2013). Moreover, ash particles in the volcanic
plume may act as ice nuclei once sufficient altitudes are achieved
(Durant et al., 2008; Schill et al., 2016; Genareau et al., 2018; Maters
et al., 2019), forming volcanogenic ice (Prata and Lynch, 2019), and fur-
ther promoting ice-ash charging. The eruption column must reach a
height coinciding with the local −10 °C or − 20 °C isotherm in order
to achieve volcanogenic ice nucleation.
The 2016–2017 eruption of Bogoslof (Alaska, U.S.A.) produced nu-
merous explosions, with lightning occurring in many, but not all, events.
A detailed analysis of several explosions by Van Eaton et al. (2020) re-
vealed the role of ice nucleation in both lightning generation and global
volcanic lightning detection. Using a combination of remote sensing
methods, a total of >4500 lightning discharges were detected during
the course of 70 individual explosions that produced ash plumes up to
12 km in height. Van Eaton et al. (2020) noted the higher likelihood of
lightning detection for those discharges occurring near or above the
−20 °C isotherm. This agrees with observations of Bennett et al.
(2010) and Arason et al. (2011) for the 2010 eruption of Eyjafjallajökull
in Iceland. These studies showed an increased number of lightning dis-
charges in taller plumes and correlated those occurrences to eruptive
phases in which the ash plume was approaching or overcoming the
−20 °C isotherm. Consequently, when the Eyjafjallajökull eruption
plume characteristics resembled those of thunderstorms (i.e., a mixed
phase environment with powerful updrafts), the resulting lightning
was larger and more apparent to global network sensors. The various
benefits and limits of these remote detection methods are discussed fur-
ther in a later section. In both the Bogoslof and Eyjafjallajökull eruptions,
external water was incorporated into the eruption column, seawater in
the former and glacial meltwater in the latter.
Several documented examples exist of volcanic lightning produced
by contact of seawater with magma or lava, as in the case of lava
flows entering the ocean and water entering the volcanic conduit dur-
ing emergent shallow submarine eruptions or partial collapse of the ed-
ifice. Such was the case for the 2018–2019 phreatomagmatic eruption of
Anak Krakatau, where partial collapse of the island caused the contact of
sea water with magma in the volcanic conduit, triggering explosions.
The eruption was characterized by a vapor-rich and ash-depleted
plume that reached the high troposphere through the vigorous convec-
tion propelled by the transfer of magmatic heat to the ocean water
(Prata et al., 2020). During this eruption, the record production of elec-
trical discharges (>150,000 in six days; Fig. 3) has been mainly attrib-
uted to the conversion of water vapor into ice during powerful updraft
of the plume (Prata et al., 2020).
Measurements of positive charging produced by the vaporization of
water by lava flows entering the sea during the 1964 eruption of Surtsey
volcano have been reported by Björnsson et al. (1967) and confirmed by
laboratory experiments by dropping water (sea water, glacial water,
and distilled water) on the surface of molten lava (Blanchard, 1964).
The increase in charging positively correlated with higher salinity of
the water samples and was explained by the authors in terms of solid-
solid contact of the precipitated salt particles. Büttner et al. (1997)
further investigated electrification during volcanic explosions by repro-
ducing magma fragmentation through the injection of both air and
water into a synthetic silicate melt to simulate the contrasting behavior
of magmatic vs. phreatomagmatic eruptions. Thermo-hydraulic fractur-
ing is demonstrated to be extremely efficient in fragmenting melt com-
pared to air-induced fragmentation, producing increased particle
surface area. Although the results seem to clearly show that thermo-
hydraulic fragmentation produces stronger electrical signals than
air-driven fragmentation, it appears difficult to discriminate between
the effects of fracto-charging from thermo-hydraulic fracturing alone
from that produced by the generation of water vapor as investigated
by Björnsson et al. (1967).
Other observations of electrification during magma-ocean interac-
tion include the eruptions of Capelinhos 1957–1958 (Machado et al.,
1962), Surtsey 1963 (Anderson et al., 1965), Bogoslof 2016–2017
(Haney et al., 2018; Van Eaton et al., 2020), and Anak Krakatau
2018–2019 (Prata et al., 2020). In all these cases, it is difficult to
Fig. 3. Time series of the 2018–19 Anak Krakatau eruption plume. (a) FPLUME (Folch et al., 2016) modelled water vapor, liquid and ice mass mixing ratio profiles with neutral buoyancy
level (NBL, dashed black line). ERA5 atmospheric profile at 12:00 UTC on 22 Dec. 2018 was used in the model. Shaded light blue horizontal area indicates lapse rate tropopause ± standard
deviation (σ) according to ERA5 data and radio occultation soundings (also shown in b). b) Plume height time series. Purple line indicates heights estimated using total flash rate at 10-min
intervals. Orange line indicates heights derived from Himawari-8 satellite images. All heights correspond to left axis of (a). Bottom grey shaded histograms indicate flash rates (CG =
cloud-to-ground and IC = in-cloud; right black axis; black y-label). Modified from Prata et al. (2020).
C. Cimarelli and K. Genareau Journal of Volcanology and Geothermal Research 422 (2022) 107449
5
distinguish the contribution of the water vapor alone to the electrifica-
tion of the plume, as the contact with external water produced en-
hanced magma fragmentation and generated energetic ash- and
vapor-rich volcanic jets. Observations consistently show that eruptions
influenced by external water typically display much more volcanic
lightning than those resulting from purely magmatic H2O. This is
indicated through comparison of the lightning abundance between
the 2018 eruption of Anak Krakatau (>150,000 events; Prata et al.,
2020) and the 2018 eruption of Volcán Fuego (<100 events; Schultz
et al., 2020), with the latter driven completely by magmatic volatiles.
Further observations derive from laboratory experiments of tribo-
electrification in volcanic particle-laden jets (Stern et al., 2019) and sus-
tained granular flows (Méndez Harper et al., 2020) under controlled
conditions of relative humidity, water content, and temperature. In
both experimental approaches, the electrification of volcanic ash parti-
cles (measured using Faraday cages and cups) is mainly achieved
through collisional and frictional contact between particles and is
shown to decrease in efficiency with increasing humidity and tempera-
ture up to the point of complete annealing. Among the two parameters,
relative humidity shows a stronger and clearer effect on electrification
(Fig. 4). Temporal and spatial changes in experimental variables
such as temperature, humidity, and particle density are matched by
the variability of an expanding volcanic plume. In the near-vent, gas-
thrust region high temperatures, high overpressures, and high particle
densities exist. Within the rising column, all these parameters progres-
sively decrease. Such variations are also expected to occur through
space and time in response to the changing thermodynamic conditions
of the hydrometeor phase and the fluid dynamic flow regime. Thus, lab-
oratory experiments of tribo-electrification provide a spatial and tem-
poral snapshot of the controls for this electrification mechanism. A
specific effort should be put into designing experiments able to isolate
the effects of different parameters on electrification, such as tempera-
ture and pressure, as together these variables might alter the dynamics
of the gas-particle mixture (number of collisions among particles, in-
creased fragmentation, condensation of water). As conditions in the vol-
canic plume quickly change from the vent into the umbrella region,
continued experiments on the laboratory scale to test the effects of dif-
ferent thermodynamic conditions will provide a more complete picture
of the natural process.
2.4. Other charging mechanisms
Other charging mechanisms have been invoked for the electrifica-
tion of particles in volcanic environments. One source of charging is pro-
vided by the radioactive decay of natural radioisotopes (U, Th, K; Aplin
et al., 2014, 2016) contained in both the mineral and glass components
of tephra and the gas phase (Rn) of the plume (Nicoll et al., 2019). How-
ever, this process is expected to diminish in intensity as the particle size
decreases, as demonstrated by measurements on micrometer-sized vol-
canic ash particles.
In photoelectric charging, free electrons are released from the sur-
face of a solid particle excited by incident energetic radiation
(e.g., ultraviolet radiation) (Lenard, 1902). However, this phenomenon
is more effective under vacuum conditions where the intensity of the in-
cident radiation is not diminished by atmospheric absorption
(Sickafoose et al., 2000). Consequently, this electrification mechanism
may be more relevant for extraterrestrial lightning, where conditions
in some cases may more closely approach those in a vacuum compared
to terrestrial volcanic systems.
Overall, the chemically complex and multiphase nature of volcanic
plumes suggests that several charging processes could operate during
explosive eruptions and that the dominant process may well change
with plume age and distance from the volcano (Mather and Harrison,
2006; James et al., 2008; McNutt and Williams, 2010; Aplin et al.,
2016; Van Eaton et al., 2016). Understanding these changes is key to in-
terpreting how the real-time variations in the plume's electrical signa-
ture reflects its evolving dynamics and how different types of volcanic
electrification are controlled by plume evolution, as described in the
section below.
3. Charge structure of volcanic plumes and types of volcanic
lightning
Established field techniques for measuring atmospheric electricity
have been applied to the study of volcanic plume electrification. Many
similarities have been noted between charging mechanisms in eruptive
columns/plumes and those in thunderstorms so that we may also ex-
pect similarities between the spatial distribution of both. This aspect is
particularly interesting as charge structure is responsible for the
Fig. 4. Effect of water on ash charging and discharging. a) Mean charge densities for experiments conducted at 25 °C and RH ranging from 0 to 60%. Each data point represents ∼100
individual particle measurements (Méndez Harper et al., 2020). b) Cumulative electrical discharges measured in expanding gas-ash jets in shock-tube experiments at variable wt% of
water in the expanding mixture (Stern et al., 2019).
C. Cimarelli and K. Genareau Journal of Volcanology and Geothermal Research 422 (2022) 107449
6
electrical potential within the plume and ultimately determines the fa-
vorable conditions for lightning generation. Both changes in the electri-
cal structure of the plume and consequent changes in volcanic lightning
may therefore track changes in space and time of the evolving physical
structure of the plume.
3.1. Previous studies on the charge structure of volcanic plumes
Early measurements of volcanic plume electrification mainly in-
volved collecting electric field gradient time series of the nearby or
overhead volcanic plumes and associated ashfall at single or multiple
stations. Such measurements showed either prevalent negative or pos-
itive perturbation of the electric field gradient caused by the presence of
the particle laden plumes (Hatakeyama, 1943, 1947, 1949; Nagata et al.,
1946; Hatakeyama and Uchikawa, 1951; Kikuchi and Endoh, 1982).
During the 1950 eruption of Aso, Hatakeyama and Uchikawa (1951) ob-
served a prevalent positive gradient associated with the ejection of
water vapor and ash from the active crater. This was perturbed by a neg-
ative transient when the plume front was directly above the measuring
stations and followed by positive gradients during the consequent ash-
fall. Such inversions of polarity were explained by the electric stratifica-
tion of the ash plume due to settling of particles of different sizes
carrying charge of opposite polarity. The electric stratification of the
plume derived from the early segregation of larger, positively charged
particles from the suspended smaller, negatively charged particles.
The dependence of charge polarity on particle size was confirmed in
the same study by laboratory measurements (Hatakeyama and
Uchikawa, 1951).
Using the same approach, observations employing improved instru-
mentation have provided further measurements of the electrification of
volcanic plumes (Gilbert et al., 1991; Miura et al., 2002; Lane and
Gilbert, 1992; James et al., 1998) and of ash clouds generated by pyro-
clastic density currents (Miura et al., 1996). The results of these studies
agree in assigning very strong charge anomalies to the plume, but con-
trast between each other in the determination of the overall electrical
structure of it, suggesting either a prevalently positive or negative polar-
ity, or the coexistence of both. Miura et al. (2002) proposed a vertically
stratified model dominated by gravitational settling which is character-
ized by a “PNP” (Positive-Negative-Positive) structure, where the lower,
positively charged part contains coarser ash particles; the middle, neg-
atively charged region contains finer ash; and the uppermost, positively
charged portion is dominantly composed of gases and aerosols (Fig. 5).
Harrison et al. (2010) conducted electrical measurements within the
distal plume resulting from the Eyjafjallajokull 2010 eruption. They
found that electrical effects persisted far from the eruption source.
Since then, Nicoll et al. (2019) performed in situ measurements of the
gaseous volcanic plume at Stromboli volcano (Italy) using disposable
sensors carried by weather balloons. The vertical profiles provided ther-
modynamic, electrical, and microphysical properties inside ash de-
pleted volcanic columns close to their source (~300 m above vent)
and provided new information about the magnitude, polarity, and ver-
tical distribution of charge. The space charge profiles showed notable
charge of up to ±8000 pC/m3
with well-defined layers of positive
charges at the cloud base and negative charges at the top. The highest
charge values were coincident with the highest concentration of SO2
droplets (Fig. 6) and measured charge values were 80 times higher
than those measured in conventional stratiform clouds (up to ±100
pC/m3
). Although these measurements relate to prevalently gaseous
plumes for which the total absence of fine ash particles cannot be
confirmed, they show we should expect charging close to the volcanic
vent even in conditions of very low explosivity.
Laboratory experiments reproducing electrical discharges in
particle-laden jets give further insight on the charge structure of
expanding granular flows and their electrification (Cimarelli et al.,
2014; Gaudin and Cimarelli, 2019). High-speed videos of these experi-
ments suggest that segregation of particles with different sizes
Fig. 5. Schematic illustration of the electrical structure of volcanic plumes and particle-laden jets in laboratory experiments. The electric charge distribution following the “PNP” model
proposed by Miura et al. (2002) where the upper part, dominated by volcanic gas and aerosols, has a prevalent positive charge, the middle part of mainly negatively charged fine ash
particles, and a lower part dominated by gravitational settling of coarser and positively charged ash particles. (a) Distribution of particles in jets with bimodal (c) and monomodal
(d) grain size distribution and their schematic charge structure (d and e, respectively). In the case with bimodal size distribution, larger and positively charged particles are confined in
the center of the flow while smaller and negatively charged particles follow the turbulence in the shear layer with the surrounding atmosphere. In the case with monodispersed
particles, clustering of particles with either prevalent positive or negative charge generate transient electrical dipoles.
C. Cimarelli and K. Genareau Journal of Volcanology and Geothermal Research 422 (2022) 107449
7
efficiently operates within the flow forming two regions where the par-
ticle motion is dominated by: 1) the inertia of the particles (larger par-
ticles at the center); and 2) the turbulence of the expanding gas (smaller
particles at the margins) (Fig. 5b and d). The segregation of particles
with different sizes would also account for the separation of charges of
opposite polarity (larger particles charged positively and smaller ones
charged negatively) that would be necessary for discharges to occur.
In the case of small particles with monodispersed size, the flow orga-
nizes into transient regions of variable particle volume density (clus-
ters) of prevalent charge polarity between which the discharges occur
(transient electric dipoles) (Fig. 5c and e). The effect of clustering in
generating electric fields has been addressed by numerical modelling
of turbulent flows laden with charged particles of bi-dispersed grain
size (Di Renzo and Urzay, 2018). The electric field generated by this pro-
cess would be of larger scale than the mean inter-particle distance and
the size of the smallest turbulent eddy. During the 2009 eruption of Re-
doubt (Alaska, U.S.A.) Behnke et al. (2013) utilized a Lightning Mapping
Array (LMA) to infer the charge structure of the eruption plume based
on the sizes and types of discharges occurring within it. Without the al-
titudes of the radio frequency (RF) sources the authors were unable to
give a detailed description of the charge structure in the plume, none-
theless they suggested that in the near-vent region charge was chaoti-
cally dispersed in clusters but became horizontally stratified within
the plume at higher altitudes, thus evolving into a charge structure sim-
ilar to that of a thunderstorm.
Considering all the field and laboratory observations, it appears that
no general model for the electrical structure of a volcanic plume can be
universally applied and that this structure will likely vary with time and
space depending on the phase of the explosive eruption and the pro-
gressive organization and stratification (Behnke et al., 2013;
Woodhouse and Behnke, 2014) of the plume. Thus, it appears that the
clustering of particles (and therefore of charges) observed in the exper-
iments (Fig. 5 b-e; Cimarelli et al., 2014, 2016) characterizes the initial
stage of a volcanic explosion and that stratification would progressively
ensue due to particle segregation within and from the plume (Fig. 5a).
This later aspect of the charge structure of volcanic plumes has so far
not been investigated in the laboratory and would require complex ex-
periments to incorporate convection and particle sedimentation with
jet expansion. The natural variability in these volcanic electrification
processes and relationship to the dynamics of individual eruptions
poses a unique challenge to the discipline but expands the potential
breadth of research opportunities in the future. Efforts to scale labora-
tory simulations with real-world phenomena have challenged volcanol-
ogists in previous studies of eruption dynamics, and volcanic
electrification is another example of such.
3.2. Volcanic lightning types
Volcanic lightning is produced within three regions of the eruption
column and plume, each of which is governed by very distinct micro-
physical dynamics (Fig. 2). Stemming from the former classification of
volcanic electrical activity into vent discharges, near-vent lightning, and
plume lightning based on the location, length, and timescale of occur-
rence (McNutt and Williams, 2010), continued analysis has provided in-
sight into the eruption characteristics that control this spectrum of
electrical activity at erupting volcanoes.
The onset of sudden volcanic explosions, as in the case of Vulcanian
style eruptions, is often characterized by a multitude of small electrical
discharges emerging from the vent as tephra is rapidly ejected
(Thomas et al., 2007, 2010; Behnke et al., 2013). This stage was origi-
nally referred to as vent discharges to contrast with actual lightning dis-
charges, mainly based on their characteristic very high (VHF;
30–300 MHz) signature. Behnke et al. (2018) further detailed that
vent discharges produce continual radio frequency (CRF) emissions
which consist of bursts of RF sources with duration of seconds to tens
of seconds, as opposed to the intermittent RF transients generated by
lightning events, which have durations of 10–100's of milliseconds.
Such RF bursts have been detected by LMA arrays deployed at several
tens of km from the volcanic crater (Thomas et al., 2007; Behnke et al.,
2011). CRF emissions have no parallel in thunderstorms, therefore
they could be discriminant for explosive volcanic eruptions. Vent dis-
charges have been related to the early dynamics of an evolving eruptive
column, where the electrification mechanism may be dominated by
fracturing and comminution of colliding particles (Smith et al., 2018)
and where the electrical structure of turbulent jets is not yet defined
by particle segregation and sedimentation (Smith et al., 2021) (Fig. 7b).
As shown by shock-tube laboratory experiments (Méndez-Harper
et al., 2018; von der Linden et al., 2021), the rapidly changing condition
of overpressure at the vent (e.g., shock waves; Fig. 7a) and the conse-
quent generation of an underpressurized region in the expanding jet be-
hind the shock front may cause a sudden decrease in the breakdown
voltage within the flow, thus enhancing the opportunity for inter-
particle spark discharges or corona discharge. The transient nature of
the CRF signal would hence be related to the stability of this region of
weakened dielectric strength close to the vent, so that CRF occurs as
long as conditions of overpressure persist.
Near-vent lightning was previously described as occurring in the
gas-thrust region immediately above the vent and within the
convection-driven expanding column. These discharges range from
hundreds of meters to several kms in length (Thomas et al., 2010).
Plume lightning was differentiated from these other types as occurring
Fig. 6. Vertical profiles through a gaseous volcanic plume at Stromboli at 11:40 UT on 30th Sept 2017 measured by a radiosonde and the VOLCLAB sensor package (Nicoll et al., 2019).
(a) Temperature (T, in grey) and relative humidity (RH, in black), (b) droplet concentration (black) and diameter (grey) measured by the optical backscatter sensor, (c) Space charge
density (grey) and SO2 concentration (black). Modified after Nicoll et al. (2019).
C. Cimarelli and K. Genareau Journal of Volcanology and Geothermal Research 422 (2022) 107449
8
both in the rising column several kilometers above the vent and in the
neutrally buoyant umbrella region (Fig. 2) and is most similar to thun-
derstorm lightning in terms of altitude and channel length (up to
>10 km). Besides their geographical position within the plume and
ash cloud, other characteristics that differentiate these previously de-
scribed lightning types stemmed primarily from the length of the
plasma arc and their timescale. With increasing altitude and expansion
of the eruption plume, lightning is larger and lasts for a longer time.
Both near-vent and plume lightning appear to be produced from
triboelectrification between tephra particles, between volcanogenic
ice particles, or between a combination of both depending on the atmo-
spheric conditions and amount of external water. As larger plume light-
ning tends to occur above the local level of ice nucleation, charging of
hydrometeors in the volcanic plume would explain the similarity with
thunderstorm lightning. It is worth noting that near-vent lightning oc-
currence can coincide and therefore be masked by the CRF signal in
LMA recordings, but it can still be discriminated with the use of high-
speed video imagery (Fig. 7a). Conditions for the simultaneous occur-
rence of CRF and all types of lightning may increase for eruptions
where the plume can be sustained for several hours to days producing
strong eruptive columns through vigorous convection. Moreover, CRF
have so far been detected at the inception of relatively violent explo-
sions generating jets likely exceeding the speed of sound and it is still
an open question if such signal can accompany also less energetic volca-
nic explosions. In this respect, to simplify our understanding of the dif-
ferent electrification mechanisms and the deriving charge structure, it
appears that impulsive short-lived eruptions may provide better insight
into the correlation between discharge types and changes in volcanic
activity over time.
4. Volcanic lightning detection
Lightning emits over a wide range of frequencies in the electromag-
netic spectrum. Besides the emission in the visible range (400–790
THz), significant electromagnetic energy is emitted by lightning
between 1 Hz and 300 MHz. An additional form of energy is generated
by lightning under the form of acoustic emissions associated with re-
sulting thunder (Haney et al., 2018; Haney et al., 2020). Using a micro-
phone array installed 60 km from Bogoslof, Haney et al. (2020) detected
volcanic lightning events that went unrecorded by the global lightning
monitoring networks during the 2016–2017 eruption. They captured
these events through electromagnetic pulses and also recorded thunder
produced by the volcanic lightning, in addition to those recordings first
described in Haney et al. (2018).
The occurrence of volcanic lightning during explosive eruptions has
been serendipitously observed as glitches (i.e., transient spiky interfer-
ence signals) on seismograms of the duration of 0.05 s (McNutt and
Davis, 2000) and on infrasound recordings (Anderson et al., 2018;
Haney et al., 2020). More commonly, lightning is detected by recording
the associated radio frequencies over a wide range from very low (VLF;
3–30 kHz) up to very high frequencies (VHF; 30–300 MHz). The detec-
tion of volcanic lightning has therefore made use of existing radio-
frequency techniques for the detection and study of thunderstorm light-
ning.
4.1. Ground-based methods
Two main types of ground-based thunderstorm detection antennas
have so far been used for locating and examining volcanic lightning:
1) regional networks of VLF antennas designed for long range (1000's
of km) thunderstorm detection (Lay et al., 2004; Bennett et al., 2010;
Nicora et al., 2013; Firstov et al., 2017); and 2) local arrays of VHF anten-
nas, suited for high spatial (1–10 m) and temporal (nanoseconds) reso-
lution studies of meteorological lightning (Thomas et al., 2004; Thomas
et al., 2010; Behnke et al., 2013, 2014).
VLF antennas suffer from having low sensitivity and variable detec-
tion efficiency (10–70%; Abarca et al., 2010). They are designed to detect
larger lightning and therefore prove inadequate to detect the lower in-
tensity volcanic discharges that occur in the near-vent region, as dem-
onstrated by long-term volcanic lighting recording at Sakurajima
Fig. 7. (a) High-speed (HS) video frame (10 kHz) of emerging ash jet with shock wave (note lofting of ash at the crater rim; white arrows) and vent lightning (~30 m; red arrows in a and
b). (b) multiparametric signals of explosion in (a), showing CRF coinciding with thermal anomaly and highest jet velocities. Grey lines are flashes detected by HS-video. (c) Discharge
(6 cm) in particle-laden jet in shock tube experiment (50 kHz; from Cimarelli et al., 2014, 2016). (d) Electrical and pressure signals recorded during a shock tube experiment. In
analogy to the CRF signals, the electrical discharges (blue spikes) occur during conditions of overpressure (hump in in the orange line) of the jet at the nozzle (after Gaudin and
Cimarelli, 2019).
C. Cimarelli and K. Genareau Journal of Volcanology and Geothermal Research 422 (2022) 107449
9
volcano (Aizawa et al., 2016; Vossen et al., 2021). As indicated by anal-
yses of volcanic lightning during the 2016–2017 Bogoslof eruption in
Alaska, the VLF antennas will have a higher likelihood of measuring
lightning discharges that occur at or above the level of ice nucleation,
where properties of the eruption plume more closely resemble those
of thunderstorms (Haney et al., 2020; Van Eaton et al., 2020). However,
thousands of VLF stations are already deployed all over the globe,
mainly for weather forecast and commercial applications. The sensor
networks mainly utilized for volcanic lightning studies include those
operated by Vaisala (Global Lightning Detection; GLD360), the Earth
Networks Total Lightning Network (ENTLN), the World Wide Lightning
Location Network (WWLLN) and the Arrival Time Difference Network
(ATDnet) (Fig. 8). They have been shown to detect lightning during
large explosive eruptions within the last decade (e.g., Calbuco 2015 (J.
Lapierre, pers. comm.); Bogoslof 2016 (Haney et al., 2018; Van Eaton
et al., 2020); Ambae 2018 (C. Vagasky, pers. comm.); Fuego 2018
(Schultz et al., 2020); Krakatau 2018 (Prata et al., 2020); Raikoke
2019; Ulawun 2019; Sinabung 2019; Taal 2020; La Soufriere at St.
Vincent 2021; see Table 1 in the Supplementary Material) although in
some cases with tens of minutes to hours of delay from the beginning
of the explosion. The persistent locations of these sensors allow for vol-
canic lightning detection without preliminary planning or field deploy-
ments, reducing effort and expense.
Alternatively, VHF antennas (or Lightning Mapping Arrays; LMA) are
custom-designed deployments built to unravel the physics of lightning
generation and propagation. They have been temporarily deployed at
erupting volcanoes for high resolution studies (Thomas et al., 2007,
2010; Behnke et al., 2014, 2018, 2021). Being sensitive to higher fre-
quency than VLF sensors, LMA are more efficient in detecting volcanic
lightning (Behnke et al., 2014, 2018), particularly near-vent lightning
that occurs at altitudes below the level of ice nucleation. The sensors
within the array allow the calculation of lightning time of occurrence
and location within the eruption column or plume when several sensors
detect the same discharge signal. As each lightning discharge generates
numerous electromagnetic signals, the LMA provides a three-
dimensional map of volcanic lightning activity over the course of an
eruption. This allows lightning properties to be linked with changes in
eruption dynamics.
More recently, the use of antennas measuring in the extremely low
frequency range (ELF; 1–45 Hz) have been successfully applied to the
continuous monitoring of persistently active volcanoes such as
Sakurajima in Japan (Fig. 9; Vossen et al., 2021) and Stromboli and
Etna in Italy. These antennas have a sampling rate of 100 Hz and there-
fore cannot rival the much higher temporal resolution of the LMA. How-
ever, their characteristics have some notable advantages: 1) the
antenna frequency range is not affected by common anthropogenic
radio emission; 2) the system is easily deployable and requires low
power consumption; 3) the lower sampling rate allows for the recogni-
tion of near-vent and plume lightning discharges from safe distance (up
to 40 km); 5) the unit price of the antennas is roughly half the price of
LMA antennas. A long-term deployment of these antennas at
Sakurajima volcano over 18 months of continuous observation allowed
the detection of electrical discharges in hundreds of relatively small vol-
canic plumes (up to about 5 km in height) with an accuracy of 73%, thus
surpassing the detection efficiency of regional VHF networks for the
same explosions (Vossen et al., 2021). In addition, the sensor can detect
the movement of spatial charge overhead or falling onto the antenna,
thus enabling the recording of electrically charged fallout. Similar
Fig. 8. Examples of volcanic lighting activity produced during explosive eruptions and sensed by ground-based lightning detection networks: a) 2010 Eyjafjallajökull eruption as detected
by ATDnet (modified after Bennet et al., 2010); b) 2015 Calbuco eruption detected by WWLLN; c) 2018 Ambae eruption detected by GLD360 (C. Vagasky, pers. comm.) and d) 2020 Anak
Krakatau eruption as detected by ENTLN (J. Lapierre, pers. comm.). See also Table 1 for further details on these eruptions.
C. Cimarelli and K. Genareau Journal of Volcanology and Geothermal Research 422 (2022) 107449
10
long-term studies of persistently active volcanoes will be necessary to
constrain both the evolution of electrical activity and the lightning dis-
charge properties for individual volcanoes displaying a range of explo-
sive styles and magnitudes. As eruptive activity is unique for every
active volcanic center, recent observations also indicate that electrical
activity is as well.
4.2. Satellite-based methods
In addition to the ground-based detection techniques previously de-
scribed, regional observation of lightning activity at active volcanoes can
be carried out by space-born sensors such as the Lightning Imaging Sen-
sor onboard the International Space Station (LIS-ISS) and the Geosta-
tionary Lightning Mapper (GLM) on the Geostationary Operational
Environmental Satellite (GOES). Satellite-based detection methods
have proven valuable for monitoring explosive eruptions from: 1)
volcanoes in remote locations; 2) volcanoes lacking instrumentation
to detect pre-eruptive activity (e.g., seismic, deformation); and 3) volca-
noes erupting after periods of extended repose over human historical
timescales (i.e., hundreds to thousands of years). Although the GOES
provides information on many volcanic centers in the western hemi-
sphere, the Himawari satellite (Japan) has been the primary space-
based tool for capturing volcanic plume data for persistently active
volcanoes in the Pacific ring of fire.
The GLM offers constant observation of active volcanoes within the
field of view of the GOES, which includes Central America, the Carib-
bean, the majority of South America, Hawaiian islands, and the conti-
nental United States. The first study to utilize GLM observations of
volcanic lightning is Schultz et al. (2020). Lightning that occurred during
the 2018 eruption of Volcán Fuego, Guatemala was analyzed from re-
cords obtained using both the GLM and the WWLLN. Comparison of
the satellite and ground-based sensors revealed that the GLM detected
less lightning than the WWLLN overall, but also detected lightning
events at different times from the WWLLN during the eruption. Lower
detection by the GLM may stem from absorption of the optical lightning
signal by the ash plume. Schultz et al. (2020) noted the occurrence of
lightning jumps during the 2018 Fuego eruption at the reported onset
of the initial explosion and also later in time, prior to collapse of the
eruption column into a pyroclastic density current. They suggested
that sudden increases in volcanic lightning may serve to warn of
impending hazards induced by changes in eruptive activity, and this is
a topic that should be explored further, particularly with continued
improvement of instrument capabilities. The ground-based detection
networks can discriminate between in-cloud and cloud-to-ground
lightning based upon peak current, while the GLM can capture the
two-dimensional spatial footprint of a lightning flash, which allows cal-
culation of channel length, and also measures the optical energy at a
wavelength of 777.4 nm (Rudlosky et al., 2019; Peterson, 2019). Schultz
et al. (2020) suggested that a combined approach utilizing both ground-
based and space-born sensors will provide a more robust dataset, as
capturing more lightning properties will enable a more complete anal-
ysis of the microphysical dynamics within an eruption column and
plume.
An additional benefit of satellite-based detection methods is the co-
incident examination of aerosol phase within the eruption plume. De-
pending on the instrument, detection of ash and/or volcanogenic ice
particles from satellite retrievals can provide information on the role
of these different phases in transferring charge and their microphysical
influence on lightning properties. Of course, accurate hydrometeor de-
tection and quantification may be limited in ash-rich eruption plumes
due to the optical opaqueness. However, despite any instrumental lim-
itations, the previous studies indicate that a combined dataset utilizing
both ground-based and satellite-based techniques can provide the
most complete story regarding the influence of plume properties on
lightning occurrence and discharge characteristics.
4.3. Other detection methods
An increasing number of volcanic lightning observations come from
multi-parametric measurements of explosive eruptions where lightning
detection is combined with high-speed videos and infrared thermal im-
aging, atmospheric vertical profiles, magnetotelluric measurements,
infrasound, and seismic measurements (Cimarelli et al., 2016; Aizawa
et al., 2016; Behnke et al., 2018).
Using a synchronized multiparametric array of visible high-speed
cameras, infrasound, and high sampling rate magnetotelluric stations,
Cimarelli et al. (2016) have monitored the electrical activity during
the persistent explosive activity at Sakurajima volcano (Suppl. Video
1). Their data show that intermittent Vulcanian explosions produce
plumes with average heights of <6 km above sea level and generate
mainly near-vent lightning in the gas thrust region (a few hundred me-
ters above the crater rim), where ice nucleation is negligible. Correlating
maximum height of lightning-bearing plumes with radiosonde atmo-
spheric profiles, this study highlights how the electrification is
Fig. 9. Extreme low frequency detection of volcanic lightning activity in Vulcanian explosions at the persistently active Sakurajima volcano (Japan). The long-term (18 months) time-series
compares the maximum height of lightning bearing volcanic plumes with the temperature of the surrounding atmosphere derived by weather balloon thermodynamic profiles. Only in
rare occasions the plumes reach freezing temperatures (−10 °C to −20 °C) ruling out the effect of ice nucleation in the electrification of the plumes. Over 724 explosions, 511 events have
been detected that produce lightning, of which only 2 were also detected by regional network antennas. Modified after Vossen et al. (2021).
C. Cimarelli and K. Genareau Journal of Volcanology and Geothermal Research 422 (2022) 107449
11
prevalently determined by the plume dynamics in absence of dominat-
ing external atmospheric factors such as high relative humidity and
freezing temperatures (Fig. 9). At the inception of an explosion, the tur-
bulence of the gas/particle jet generates a complex charge structure,
producing relatively small discharges (10–200 m channel length) with
no preferential directionality and with maximum length of flashes in-
creasing with time from the onset of the explosion (i.e., with the height
of the plume). Organization of charge is then achieved at later stages
with the transition from the jet to the convective phase and the gener-
ation of larger cloud-to-ground lightning. Also, the number of lightning
discharges detected is proportional to the pressure recorded by the
infrasound stations, hence showing a positive correlation of electrical
activity with the intensity of the explosions (Fig. 10).
5. Revealing plume dynamics through electrical properties
It is apparent that the successive occurrence of different lightning
“types,” characterized by location, size, and timescale, tracks the pro-
gressive development of the plume and that this can be used to monitor
plume evolution in time and space. Volcanic lightning detection and
mapping is therefore emerging as a powerful tool in the portfolio of vol-
cano monitoring techniques as it allows the detection of explosive activ-
ity from safe distances in unfavorable weather conditions (Behnke and
McNutt, 2014). More importantly, a key result emerging from multi-
parametric studies, in combination with laboratory experiments, is
that determining the characteristics of volcanic lightning (nature, inten-
sity and space-time distribution) may reveal the structure of the plume
and its source parameters (e.g., mass eruption rate), which are other-
wise very challenging to measure directly.
Studies have attempted to derive information on dynamic changes
during volcanic eruptions by correlating variations in recorded electrical
activity, as previous studies have proposed relationships between light-
ning flash rate and thunderstorm parameters such as cloud top height
and peak updraft speed (e.g., Price and Rind, 1992). Behnke and Bruning
(2015) calculated flash energy spectra from LMA data collected during
the 2009 eruption of Redoubt volcano in Alaska and compared them
to the evolving turbulence characteristics of the developing plume.
Their analysis confirms the proportionality between flash length scale
and energy with increasing volume of the plume. A combined analysis
of satellite images and lightning detection for the 2018–2019 eruption
of Anak Krakatau also reveals a good correlation between the height
of the plume and the lightning flash rate (Prata et al., 2020). The empir-
ical correlation found for the case of Anak Krakatau suggests that light-
ning flash rate could be used to derive information on the height of the
eruption column, thus providing important input for the modelling of
volcanic ash dispersal from the plume.
In general, the attempt to correlate variations in the volcanic light-
ning activity and the changing source parameters of the ash plume con-
stitutes one of the most challenging aspect of volcanic lightning
research as this requires systematic multiparametric studies where
the volcanic lightning activity can be measured against other indepen-
dent source parameters such as mass eruption rate, plume rise velocity,
plume height, etc., which are often very difficult to determine. In this re-
spect, combining field measurements with laboratory experiments and
numerical modelling may provide the necessary constraints to enable
such correlations.
6. Effects of lightning on tephra properties
Fulgurites are complex glassy structures generated by the melting of
coherent rocks or loose sediments by a cloud-to-ground lightning strike.
The production of fulgurites by lightning is well known and has been ex-
tensively reported in the literature. A vast collection of works dealing
with fulgurites can be found in the bibliography of McCollum and
Welsh (1999). Several descriptive studies have attempted to recon-
struct the physical conditions of the lightning strike by looking at the
textural modification and phase relationships of the exposed material.
The classification scheme of Pasek et al. (2012) defined various fulgurite
types (I through V) according to the affected material (e.g., sand, soil,
rock), but volcanic tephra is not listed among any of these starting ma-
terials. Tephra can be considered to fall between rocks and sediments,
potentially generating a different “type” of fulgurite.
As the physics of the volcanic lightning discharge process is compa-
rable to that of thunderstorm lightning (Aizawa et al., 2016), also the ef-
fects produced in materials are expected to be similar. Moreover, the
predicted temperatures achieved in the lightning channel (30,000 K;
Paxton et al., 1986; Rakov and Uman, 2003) exceed the glass transition
and melting temperature of natural silicate glasses and minerals. In this
Fig. 10. a) Correlation between number of discharges and height of the plume for 261 explosions recorded at Sakurajima volcano (Japan). Datapoints represent multiple plumes reaching
the same elevation above the crater rim while error bars show the variability in the number of discharges for plumes of the same height. b) Positive correlation between the number of
recorded flashes and maximum pressure peak produced by 32 explosions at Sakurajima volcano. Visible flashes were recorded with high-speed cameras while maximum average pressure
accompanying the explosions were measured with an infrasound array. Modified after Cimarelli et al. (2016) and Vossen et al. (2021).
C. Cimarelli and K. Genareau Journal of Volcanology and Geothermal Research 422 (2022) 107449
12
respect, it would be difficult to distinguish fulgurites on the ground pro-
duced from volcanic lightning from those produced by thunderstorm
lightning, since their appearance would chiefly depend on the charac-
teristics of the pristine starting material (Clocchiatti, 1990; Castro
et al., 2020) rather than the origin of the cloud-to-ground discharge.
Fewer studies have managed to produce fulgurites in the laboratory
(Fig. 12) by controlled electrical discharges on selected target materials
(Hachette, 1828; Butcher, 1908; Arai, 1969; Kumazaki et al., 1997;
Cimarelli et al., 2017; Elmi et al., 2018) to constrain how lightning prop-
erties (e.g., peak current, temperature, pressure, timescale) may be de-
termined from examination of fulgurite textural, chemical, and
mineralogical characteristics.
The high temperatures and high pressures produced in the ionized
lightning discharge channel can modify the structure and chemistry of
pristine tephra, producing a variety of shapes and textures (Fig. 11).
The effects of lightning on airborne tephra particles within the eruption
column and plume have been documented in ashfall deposits and
reproduced in the laboratory. First reported by Genareau et al. (2015),
lightning-induced volcanic spherules (LIVS) have been observed in ash-
fall deposits from several explosive eruptions. Initially identified in
products of the Redoubt 2009 and Eyjafjallajökull 2010 events, where
volcanic lightning was ubiquitous, similar textures were also generated
during high-voltage flashover experiments using electrical insulators
contaminated with volcanic ash (Wardman et al., 2014; Wardman
et al., 2012), suggesting that electrical discharge was a likely formation
mechanism. Theoretical calculations show that the timescale of ob-
served volcanic lightning discharge (≥ 0.1 ms) would allow for surface
tension-driven rounding of particles with sizes comparable to observed
LIVS (Wadsworth et al., 2017). Since the initial study of Genareau et al.
(2015), LIVS have been documented in products from other eruptions,
including the 2016 eruption of Pavlof (Genareau et al., 2020). Also
previously identified in ashfall deposits are spherule aggregates, com-
posed of several LIVS welded together, and pumiceous particles that dis-
play vesicles formed from secondary exsolution of remaining volatiles
bound in volcanic glass (Genareau et al., 2020).
LIVS provide physical evidence of lightning occurrence in the tephra
record. In order to quantify the total amount of erupted tephra affected
by volcanic lightning, several parameters must be known: 1) peak cur-
rent of the discharge, which will control the channel radius; 2) length of
the lightning channel; 3) ash concentration in erupted column or plume
where discharge occurs; 4) proportion of erupted volume that is fine
ash or smaller (<100 μm); and 5) total number of lightning discharges
during the eruption. Although the proportion of ash affected during
each discharge event may be small, eruptions with hundreds of thou-
sands of individual events, like that of Anak Krakatau in 2018–2019
(Prata et al., 2020) may have an impact on ash transport and resulting
hazards to the environment and infrastructure. An accurate quantifica-
tion of this impact will require combined efforts in constraining light-
ning parameters through remote sensing (e.g., channel length from
GLM observations) and modelling the resultant macro/micro-physical
effects on both atmospheric and ash plume properties. The various
lightning-induced textures previously identified in ashfall deposits are
comparable to those produced in current impulse experiments, and
each lightning effect has a direct impact on the associated ash-related
hazards, as described in the following section.
7. Experimental investigations of volcanic lightning
Laboratory experiments conducted to generate or simulate volcanic
lightning have been ongoing over the last several years in efforts to
reveal the microphysical dynamics contributing to electrical activity,
Fig. 11. Structural and textural modifications induced by lightning in volcanic tephra as produced by current impulse experiments. The secondary electron microscope images on the far
left (orange box) show textures identified in ashfall deposits and re-created during current impulse experiments. The high temperature in the lightning channel (dashed-line box inset),
exceeds the glass transition of igneous rocks and minerals, causing melting (orange fields on the lightning flash and in the inset), and/or vaporization and dissociation (yellow fields on the
lightning flash and the inset) of material compounds. The propagation of shock-waves (blue lines) may also stretch or compact melted air-born volcanic particles, forming the plate-like
and hair-like particles (images in the blue box) that have been produced in current impulse experiments, but not directly observed in ashfall deposits (*). Volcanic deposits (tephra and
lavas) on the ground may also transform into fulgurites (image in the dark grey box) by attachment of lightning to ground (dark grey field on the lightning flash).
C. Cimarelli and K. Genareau Journal of Volcanology and Geothermal Research 422 (2022) 107449
13
the role of environmental variables, and the resulting effects on ash
particles.
7.1. Shock-tube decompression experiments
Rapid decompression experiments of gas-particle mixtures (at sub-
and supersonic conditions) have been conducted to reproduce electrical
discharges at conditions similar to those found in volcanic plumes. Such
experiments have generated renewed interest in the discipline as they
provide the opportunity to visualize the electrical discharges with re-
spect to the evolving experimental jets and study processes of self-
electrification and discharge mechanisms in absence of imposed exter-
nal electric fields. Moreover, these experiments allow controlling key
parameters that would be difficult to constrain in nature, thus shedding
light on the microphysical dynamics at play within explosive eruption
columns.
In particular, shock-tube experiments have been performed to con-
strain the effects of particle size distribution (Suppl. Video 2), total
mass ejected, and initial overpressure of the gas-particle mixture on
the electrification of the expanding jets (Cimarelli et al., 2014; Gaudin
and Cimarelli, 2019). The results of these experiments show that the
total magnitude of electrical discharge generated is proportional to all
three parameters, although each play a different role in the process by
modulating the way discharges occur. The proportion of fine particles
in the original mixture appears to control the number of the discharges,
while their magnitude (i.e., the amount of charge neutralized by a single
discharge) is linearly correlated to the initial overpressure and total
mass of the system (Gaudin and Cimarelli, 2019). Stemming from
these observations, additional shock-tube experiments have also ex-
plored the roles of temperature and water content on ash charge trans-
fer and discharging processes. Stern et al. (2019) saturated their ash
samples with up to 27 wt% water and found that only 1.8 wt% signifi-
cantly decreased the total discharge generated during decompression.
Stern et al. (2019) also observed a higher number of smaller discharges
when temperature of the system was increased, which they attribute to
increased turbulence within the expanding sample mixture. Compara-
ble results were found in the tribo-electrification experiments of
Méndez-Harper et al. (2020). In this case, agitated particles produce
low-energy and low-frequency collisions under variable temperature
and relative humidity conditions over long time scales (60 s) to simulate
particle interaction in the convective portion of the plume. Results show
that the presence of external water inhibits charge transfer by coating
ash particle surfaces over sufficiently long timescales. Additionally, a
general decrease in charging with higher temperatures is also observed,
although the reason for this is less clear and requires further study.
There is evidence that the type of discharges produced in shock-tube
experiments resemble those generating the CRF signal at the onset of
explosive eruptions (Fig. 7) which would hence be determined by the
conditions of supersonic overpressure at the eruptive vent and the re-
duced dielectric strength in the gas-thrust region just above it
(Méndez-Harper et al., 2018; von der Linden et al., 2021). The CRF signal
could potentially be correlated with magnitude of overpressure at the
vent and CRF total duration would give a good estimate of the timescale
of tephra injection into the atmosphere, being this impulsive or sus-
tained, thus providing useful constraints on source parameters in
plume numerical models.
7.2. Current impulse experiments
To simulate the effects of lightning discharge on volcanic ash parti-
cles, current impulse experiments have been conducted using labora-
tory facilities originally designed for engineering applications and
materials testing. The impulse apparatus delivers a discharge with a
double exponential waveform and peak current magnitude similar to
cloud-to-ground lightning strikes. The size of the discharge channel
and temperature within it are both a function of peak current.
Experiments conducted on manufactured volcanic ash particles at
peak currents ranging from 7 kA to 100 kA cause a variety of structural
and chemical effects on particles (Genareau et al., 2017; Genareau et al.,
2019, 2020; Woods et al., 2021).
Regardless of ash composition, the grains are altered by the high
temperatures generated, which can reach almost 30,000 °C at the chan-
nel axis (Genareau et al., 2017). Individual grains are melted and/or
fused together, changing the overall size distribution of the sample
(Genareau et al., 2019). The instantaneous loss of smaller particles and
creation of larger particles will alter the fallout of tephra from the
transported plume. Resulting shapes of lightning-affected particles
range from individual solid or hollow spherules, spherule aggregates,
pumiceous particles, hair-like particles, and plate-like particles
(Fig. 13). All textures except the latter two have been documented in
ashfall samples from eruptions where lightning was documented. The
latter two textures (hair-like and plate-like particles) are hypothesized
to stem from the effects of particle melting combined with high pres-
sures (0.41–0.77 MPa) generated by expansion of the shock front.
These shock-induced textures may be overlooked in ashfall samples
due to their similarity to primary fragmentation textures (Genareau
et al., 2020).
Although studies involving the diffusion and volatilization of chem-
ical elements from ash particles exposed to electrical discharges are lim-
ited (Mueller et al., 2018; Woods et al., 2021), some preliminary
findings have emerged that warrant continued examination of these
phenomena. By injecting ash samples of different size and composition
into an artificially generated electric arc (welding arc), Mueller et al.
(2018) found that some elements (e.g., F, Cl, Na) were depleted from af-
fected grains compared to the starting material. They also documented
foaming and bubble expansion in the grains, as observed in comparable
experiments (Cimarelli et al., 2017). Extending from this study, Woods
et al. (2021) subjected andesite ash to high-current impulses (7–25
kA) and found that microlite minerals (plagioclase, magnetite, etc.)
within the grains were partially or completely melted. Melting of
these phases and heterogenous mixing with the melted glass compo-
nent created zones of chemical enrichment unique from the pure min-
eral and groundmass glass compositions in pre-experimental samples.
There was also clear compositional banding in post-experimental
lightning-induced particles (Woods et al., 2021). The chemical effects
of lightning on volcanic ash appears to be highly variable, as they are a
function of not only ash composition, but also ash grain size
(Wadsworth et al., 2017), peak current of the discharge, and proximity
of ash to the discharge channel axis (Genareau et al., 2017).
One apparent chemical impact on lightning-affected ash particles is
oxidation and/or reduction of compounds present in the glass and
microlite mineral phases. Previous analyses of rocks on the ground sur-
face struck by cloud-to-ground lightning have shown them to have a
unique magnetic signature (a.k.a. anomalous magnetization) relative
to surrounding areas (e.g., Cox, 1961; Sakai et al., 1998; Verrier and
Rochette, 2002). Additionally, examination of fulgurites has shown
them to be reduced in some cases (Jones et al., 2005; Pasek and Block,
2009; Hess et al., 2021). An analysis of volcanic ash samples subjected
to 7–100 kA current impulses, using vibrating sample magnetometry,
show that the particles have a higher saturation magnetization and
lower coercivity compared to pre-experimental samples (Genareau
et al., 2019b). Felsic ash appeared to display more significant magnetic
changes compared to mafic ash. These changes between pre- and
post-experimental magnetic properties suggest that certain compounds
within the ash grains are variably reduced. Further analyses of ash oxi-
dation states are ongoing to constrain which compounds are responsi-
ble for these magnetic changes, as results may pose important
implications for ash-related hazards to electrical infrastructure and effi-
cient cleaning of ashfall from metal structures. As experiments utilizing
both electrical arcs and current impulses have reproduced lightning-
induced textures within the laboratory, analyses on their chemical
properties continue.
C. Cimarelli and K. Genareau Journal of Volcanology and Geothermal Research 422 (2022) 107449
14
8. Effects of lightning on atmospheric chemistry and processes
Besides the consequence of tephra modification by lightning, as pro-
vided through current impulse experiments, the modification of the
suspended ash and gases may also pose important consequences for
chemical equilibria within volcanic plumes and the atmosphere. The
impacts of these chemical modifications on the ambient atmosphere
are a topic for future studies. Previous work, detailed below, indicate
that lightning is a significant contributor to changes in the atmosphere
that may affect biologic activity. Thermodynamic equilibrium models
of the effect of volcanic lightning on the composition of a plume have
so far neglected the chemical modification of volcanic ash (Martin and
Ilyinskaya, 2011). However, affected ash particles show evidence of sec-
ondary devolatilization (Fig. 12 and Fig. 13) and analyses indicate re-
duction of oxide compounds, indicating that volatiles are liberated
from lightning-affected ash particles. This is only one potential effect
of volcanic lightning on atmospheric chemistry, and others are detailed
below.
8.1. NOx fixation/ozone production
It is well known that the concentration of nitrogen oxides in the at-
mosphere can be strongly influenced by lightning as it is one of the
major abiotic contributors to nitrogen fixation and production of NO
and NO2 (i.e., NOx). In turn, through photochemical reactions, NOx
species are critical for the abundance of ozone (O3) a strong oxidant
which is a greenhouse gas and key absorber of ultraviolet radiation.
Lightning-induced NOx (LNOx) species contribute largely to the
abundance of NOx in the atmosphere, particularly in the upper
troposphere, thus having important implications for atmospheric
chemistry and climate (Navarro-González et al., 1998; Stark et al.,
1996; Höller et al., 1999; Gerlach, 2004; Schumann and Huntrieser,
2007). The production of reactive nitrogen species is also debated as
being a fundamental prerequisite for the emergence and early
evolution of life on Earth. The abundant volcanic activity during the
Archean and the associated generation of volcanic lightning could
have played an important role in the fixation of nitrogen in a mildly
reduced environment favored by volcanic activity (Navarro-González
et al., 2001). Lightning, and so volcanic lightning, is further considered
to be a viable mechanism for the production of reduced phosphorus
(Pasek and Block, 2009), a fundamental nutrient for marine and
terrestrial micro-organisms, which is otherwise almost exclusively
present in geological systems in its fully oxidized state. The content of
reduced phosphorus in natural fulgurites testifies to the ability of
cloud-to-ground lightning as a viable contributing mechanism to the bi-
ological cycle of phosphorus (Hess et al., 2021). Additionally, volcanic
lightning may have locally generated the reducing conditions for prebi-
otic synthesis of organic molecules (Johnson et al., 2008). Thus, addi-
tional simulation experiments able to reach the high temperatures of
natural lightning can be used to determine the impact on both volcanic
ash chemistry and ambient atmospheric chemistry, particularly in
regards to generating biologically important molecules and compounds.
8.2. Changes to ash plume convection and dispersion
The previously described current impulse experiments have re-
vealed that lightning discharge will change the grain size and size distri-
bution of tephra produced from primary magma fragmentation. These
textural changes will affect the dispersion of ash in the atmosphere, as
fusing of individual grains and aggregates into larger single particles
will increase the settling velocity of these lightning-induced particles
relative to that of the original constituent grains (Genareau et al.,
2019). Some of the smaller affected ash grains may even be destroyed
based upon their proximity to the discharge channel axis, removing
them completely from the total grain size distribution (Genareau
et al., 2017) and potentially impacting an accurate quantification of vol-
canic explosivity using distal ashfall deposits.
The shock wave produced by the lightning will not only contribute
to structural alteration of ash grains and aggregates but may potentially
alter fluid dynamic processes (e.g., turbulent eddies) occurring in the
convecting column and/or the laterally spreading plume. This effect is
evidenced by high-speed video footage of current impulse experiments
(Suppl. Video 3), which show expulsion of glowing ash particles tens of
centimeters from their original location following discharge of a plasma
Fig. 12. Experimental fulgurites. Micro- (a) and nano-tomographic (b, c) images of an experimental fulgurite showing melting features. The volcanic analogue starting material is
composed of micrometric glass fibers (short axis 14 μm). Voxel in (c) has 60 μm side (after Cimarelli et al., 2017). (d) Secondary electron image showing the globular shape of an
experimental fulgurite generated by an impulsive (500 ms) electrical discharge (227 A) on 63–90 μm volcanic ash from Laacher See volcano (Germany). (e) Backscattered electron
image revealing the internal glassy texture of the fulgurite in (d). The bimodal population of larger and smaller vesicles (black circular structures) suggests the entrapment of external
air and the degassing of volatile elements during the instantaneous melting of the ash.
C. Cimarelli and K. Genareau Journal of Volcanology and Geothermal Research 422 (2022) 107449
15
channel only a few millimeters in diameter. Although lightning dis-
charge deep within the convective column may not impact fluid dy-
namic processes, lightning at the column and plume margins may
arrest or enhance the incorporation of ambient atmosphere (Genareau
et al., 2020). Modelling the fluid dynamic effects of shock fronts gener-
ated by volcanic lightning would be an interesting topic for future study
as intense lightning activity may alter ash dispersion.
9. Challenges and future perspectives
The electrification of volcanic plumes and the generation of volcanic
lightning are common phenomena during explosive eruptions. After
centuries of anecdotal reports and observations we are now in posses-
sion of the appropriate technology to characterize and quantify these
phenomena. However, the use of volcanic lightning detection as a rou-
tine monitoring method is today still limited by three main factors:
1) a poor understanding of the electrification mechanisms at play in vol-
canic plumes, including the microphysical dynamics between volcanic
ash and hydrometeors; 2) the absence of systematic quantitative obser-
vations relating electrical activity to the plume dynamics; and 3)
the lack of sensors specifically tailored for the electrical monitoring of
volcanoes.
Microphysical observations derived from laboratory experiments
should address the different mechanisms of particle charging under
Fig. 13. Secondary electron images of ash particles following high-current (7–100 kA) impulse experiments. The various textures produced include (a), (b) spherules; (c), (d) hair-like
particles; (e), (f) plate-like particles. The starting materials were ash grains <32 μm in diameter, indicating that lightning can melt and fuse multiple particles together. Of these
experimentally produced textures, only spherules have been observed in natural ashfall samples in addition to spherule aggregates and pumiceous particles, which are not shown here
(Genareau et al., 2020).
C. Cimarelli and K. Genareau Journal of Volcanology and Geothermal Research 422 (2022) 107449
16
A review of volcanic electrification of the atmosphere and volcanic lightning
A review of volcanic electrification of the atmosphere and volcanic lightning
A review of volcanic electrification of the atmosphere and volcanic lightning
A review of volcanic electrification of the atmosphere and volcanic lightning

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A review of volcanic electrification of the atmosphere and volcanic lightning

  • 1. Invited review article A review of volcanic electrification of the atmosphere and volcanic lightning Corrado Cimarelli a, ⁎, Kimberly Genareau b a Department of Earth and Environmental Sciences, Ludwig-Maximilians-Universität München, Theresienstrasse 41, 80333 Munich, Germany b Department of Geological Sciences, The University of Alabama, Box 870338, Tuscaloosa, AL 35404, USA a b s t r a c t a r t i c l e i n f o Article history: Received 11 July 2021 Received in revised form 1 December 2021 Accepted 5 December 2021 Available online 13 December 2021 The electrification of volcanic ash plumes and the occurrence of volcanic lightning are now known to be common phenomena during explosive volcanic eruptions. This knowledge stems from centuries of anecdotal observations, and in recent decades, from improved instrumentation and media attention. Following a summary of previous reviews, this contribution will detail the most recent findings concerning electrification mechanisms of eruption columns/plumes (triboelectrification, fracto-electrification) and how hydrometeor charging contributes to this electrification depending upon the eruption style and abundance of external H2O. Field measurements to determine the charge structure of volcanic ash and gas plumes reveal wide variability both spatially and temporally, indicating the influence of these different charging mechanisms. The charge structure and resulting lightning characteristics have been provided by a suite of both ground-based and satellite-based light- ning detection methods and the various characteristics of each are summarized. As these detection methods have revealed, the electrical properties of ash plumes can provide insight into their physical dynamics throughout the course of an eruption. Lightning may therefore provide a means to track changing eruption conditions and the associated hazards, providing another tool for monitoring efforts. Volcanic lightning also leaves physical evidence in associated ashfall deposits. These lightning-induced textures have been documented and are summarized here, in addition to the different experiments that have reproduced such textures. Lightning simulation experi- ments provide information on changes to ash grain size, size distribution, chemical, and magnetic properties of ash. Lightning discharge and the lightning-induced changes to ash grains potentially impact not only the hazards induced by ashfall, but also changes in atmospheric chemistry relevant to biologic activity, the fluid dynamics of eruption columns/plumes, and ash dispersion. Additionally, shock-tube experiments provide insight on the mi- crophysical dynamics and environmental variables that influence electrification of dusty gas mixtures. Finally, this review summarizes the challenges to volcanic lightning research and the future efforts that can aid in addressing the unanswered questions regarding this phenomenon. © 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http:// creativecommons.org/licenses/by-nc-nd/4.0/). Keywords: Volcanic lightning Volcanic electrification Tephra Volcanic ash Hydrometeors Volcanogenic ice Remote sensing Lightning monitoring array Volcano monitoring Radio frequency Explosive volcanism Atmospheric electrification Fulgurite Spherules Tribo-electrification Fracto-electrification Ice electrification Experimental volcanology Lightning mapper Continual radio frequencies Contents 1. Introduction. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.1. Previous reviews . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.2. Topics of this review. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2. Mechanisms of electrification in volcanic plumes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 2.1. Fracto-electrification. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.2. Tribo-electrification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.3. Effects of water on plume electrification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 2.4. Other charging mechanisms . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3. Charge structure of volcanic plumes and types of volcanic lightning. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 3.1. Previous studies on the charge structure of volcanic plumes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 3.2. Volcanic lightning types . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 Journal of Volcanology and Geothermal Research 422 (2022) 107449 ⁎ Corresponding author. E-mail address: cimarelli@min.uni-muenchen.de (C. Cimarelli). https://doi.org/10.1016/j.jvolgeores.2021.107449 0377-0273/© 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Contents lists available at ScienceDirect Journal of Volcanology and Geothermal Research journal homepage: www.elsevier.com/locate/jvolgeores
  • 2. 4. Volcanic lightning detection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 4.1. Ground-based methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 4.2. Satellite-based methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 4.3. Other detection methods. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11 5. Revealing plume dynamics through electrical properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 6. Effects of lightning on tephra properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 7. Experimental investigations of volcanic lightning . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 7.1. Shock-tube decompression experiments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 7.2. Current impulse experiments. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14 8. Effects of lightning on atmospheric chemistry and processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 8.1. NOx fixation/ozone production . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 8.2. Changes to ash plume convection and dispersion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 9. Challenges and future perspectives . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Declaration of Competing Interest . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 Appendix A. Supplementary data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17 1. Introduction Although volcanic lightning has been reported for centuries (e.g., Pliny the Younger; Volta, 1782; Fouqué, 1879; Symons, 1888; Mercalli and Silvestri, 1891; Friedlaender, 1898; Fischer, 1893; Anderson, 1903; Table 1 in Supplementary Material), an increasing number of volcanic lightning reports and instrumental detections (e.g., Stromboli 2003, 2007 and 2019, Eyjafjallajökull 2010, Puyehue 2011, Kirishima 2011, Etna 2013, 2015 and 2021, Sinabung 2014, Villarrica 2015, Calbuco 2015, Colima 2015, 2016 and 2017, Pavlov 2016, Sakurajima 2009–2021, Bogoslof 2016–17, Ambae 2018, Fuego 2018, Anak Krakatau 2018–19, Taal 2020, St. Vincent 2021, Cumbre Vieja of La Palma 2021) demonstrate that electrification is observed over a wide range of explosive styles and extends to the lower end of the volcanic explosivity index scale (VEI ~0) so that it can be considered an intrinsic property of volcanic ash plumes (Fig. 1). Interest in this phe- nomenon has been fostered by the opportunity to use detection of vol- canic lightning as a viable real-time monitoring method for hazardous volcanic activity. Besides advances in remote sensing methods, insight gained from laboratory experiments and microanalyses of ashfall de- posits has revealed that volcanic plume electrification has the potential to impact both the hazards induced by volcanic ash and the fluid dy- namics of the eruption column/plume. Consequently, there is much still to be discovered. This contribution will summarize the current state of knowledge on volcanic lightning and gaps that remain to be filled. 1.1. Previous reviews Previous reviews of this phenomenon include those of Mather and Harrison (2006), James et al. (2008), McNutt and Williams (2010), McNutt and Thomas (2015), and Aplin et al. (2016). These studies have discussed a range of various mechanisms responsible for volcanic lightning and (at the time of their publication) summarized known ob- servations and general characteristics. Although McNutt and Davis (2000) focused specifically on volcanic lightning during the 1992 eruption of Mt. Spurr in Alaska, they included a table that summarized the recorded observations of volcanic lightning at the time. Here, their table is revised and updated with several new oc- currences over the last decade, consisting of both visual observations and instrumental detections (Table 1 in Supplementary Material). Mather and Harrison (2006) added to this summary and focused mainly on the electrification mechanisms within volcanic plumes by comparing these mechanisms with those occurring in the multiphase environment of thunderstorms. They also detailed methods of measuring atmo- spheric electric fields and particle charge relevant to understanding the electrification of volcanic plumes. Based upon the work of Basiuk and Navarro-González (1996), Mather and Harrison (2006) provided Fig. 1. Examples of volcanic lightning at different volcanoes of the world. From upper left: Surtsey, Iceland, December 1963 (photo Sigurgeir Jónasson); Eyjafjallajökull, Iceland, March 2010 (photo Marco Fulle); Taal, Philippines, January 2020 (photo Domcar C. Lagto); Sakurajima, Japan, March 2015 (photo Martin Rietze); Cumbre Vieja at La Palma, Canary Islands, October 2021 (photo INVOLCAN), Etna, Italy, November 2013 (photo Simona Scollo). C. Cimarelli and K. Genareau Journal of Volcanology and Geothermal Research 422 (2022) 107449 2
  • 3. a comparison between atmospheric electrical properties and lightning characteristics during fair weather, thunderstorms, and explosive volca- nic eruptions. They also discussed volcanic lightning as a process in ex- traterrestrial environments and its potential role in early Earth as a mechanism for generating life. James et al. (2008) also focused on elec- trification mechanisms and volcanic particle charging, including fracto- electrification, which will be discussed in more detail later, and further expanded upon the potential for volcanic lightning in extraterrestrial environments. These previous reviews, which provided key similarities and differences between volcanic plumes and thunderstorms, revealed that both granular interactions and hydrometeor interactions must be considered in the attempt to explain volcanic electrification. McNutt and Williams (2010) analyzed reports of volcanic lightning during 212 eruptions at 80 different volcanoes to constrain the influ- ence of location (i.e., latitude) and eruption characteristics on occur- rence. They noted a higher likelihood of lightning during larger explosions relative to smaller ones but noted the observational bias in reports of volcanic lightning. Lightning is more easily observed in larger eruptive columns and for those eruptions occurring at night, but the prevalence of volcanic lightning observations at higher latitudes sug- gested to the authors that magmatic H2O, instead of atmospheric water vapor, was playing a dominant role. If atmospheric water vapor was the controlling factor, then volcanic lightning observations would positively correlate with eruptions occurring at lower latitudes, similar to thunderstorm lightning. This previous hypothesis concerning the fundamental role of magmatic water has further developed through observations of eruptions with variable amounts of external H2O. Comparison of dry magmatic eruptions with those in water-rich envi- ronments reveals that more ice nucleation in the latter increases the number of lightning events. The role of water in volcanic electrification will be explored more fully in a subsequent section. In addition to the role of water, the analysis of McNutt and Williams (2010) highlighted many other important parameters of both the eruption and the regional atmosphere that required more detailed examination in regard to volca- nic lightning occurrence. Some of these atmospheric and eruption char- acteristics have been the topics of more recent studies, and those will be described in this review. 1.2. Topics of this review Several works have progressively refined our understanding of vol- canic plume electrification and volcanic lightning, targeting various as- pects of the charging and discharging processes. Main topics of this review involve: 1) the processes of plume charging; 2) the distribution of charge within the eruptive column; and 3) the discharge processes. Besides these, the discovery of volcanic lightning evidence within erupted products (Genareau et al., 2015) has also furthered analytical and experimental studies concerned with the effect of lightning activity on the modification of both airborne volcanic ash and tephra deposits on the ground. In this review all the aforementioned points will be ad- dressed, reporting on the most recent advances achieved, both in the context of field measurements and laboratory experiments, thus expanding on the previous review papers (Mather and Harrison, 2006; James et al., 2008; McNutt and Thomas, 2015; Aplin et al., 2016). 2. Mechanisms of electrification in volcanic plumes Explosive volcanic eruptions generate tephra through the mecha- nism of magma fragmentation: transition of a magma from crystals and gas bubbles suspended in a continuum of liquid silicate melt to a turbulent suspension of solid particles in a mixture of expanding mag- matic gases and entrained air (i.e., the eruption column). At these con- ditions, the electrification of volcanic plumes can be better described by the processes regulating the electrification of granular material flows, where the composition, size, and kinetics of the solid particles, in conjunction with the ambient conditions, are all relevant parameters. Volcanic ash, the portion of tephra of nominal diameter smaller than 2 mm, is considered to be the main charge-carrying agent in a volcanic plume. Volcanic ash electrification mechanisms proposed include (Fig. 2): fracto-electrification (James et al., 2000; Büttner and Zimanowski, 2000), tribo-electrification (Cimarelli et al., 2014; Méndez Harper and Dufek, 2016), interaction with water and hydrome- teors (Williams and McNutt, 2005; Arason et al., 2011), and natural ra- dioactivity (Harrison et al., 2010; Nicoll et al., 2019). However, their relative contributions in the eruptive column are not well understood. Among these mechanisms, fracto- and tribo-electrification are of partic- ular importance because they are intimately linked to the dynamics of explosive eruptions, in which magma fragmentation and high particle collision rates can efficiently charge the newly formed tephra within the volcanic conduit and subsequently, during formation and evolution of the eruption column and plume. Charged particles can attract and repel each other according to their polarity and their trajectories can be influenced by external electric fields. Electrostatic forces are extremely effective on particles of smaller mass and can be stronger than gravity (e.g., Jungmann et al., 2021), hence affecting cohesion and adhesion properties of the material and its ability to be transported, sedimented, and re-suspended. In this re- spect, experiments (Matsusaka et al., 2001; James et al., 2002, 2003; Alois et al., 2017) and field observations (Sorem, 1982; Gilbert and Lane, 1994; Miura et al., 2002; Mueller et al., 2017) have shown that ag- gregation and disaggregation of particles can be achieved under the ef- fect of electrostatic forces. Such effects can determine the cohesion of smaller particles into larger aggregates and therefore determine their premature sedimentation from the ash plume (Taddeucci et al., 2011; Van Eaton et al., 2012; Folch et al., 2016; Pollastri et al., 2021). Conversely, it has been observed that charging may increase the res- idence time of lofted mineral dust (Toth III et al., 2019 and references therein) as evidenced by the anomalous sizes of transported particles far from their source. This could well be the case for volcanic ash parti- cles, which can match the size of mineral dust and can be ejected to stratospheric altitudes during highly explosive volcanic eruptions. Long residence times of ash at high levels in the atmosphere could influ- ence radiative forcing, climate, and the global electrical circuit, as sug- gested by numerical simulations (Genge, 2018). As a matter of fact, electrification of volcanic particles has been measured by means of air- borne electrostatic sensors in the 2010 Eyjafjallajökull volcanic ash cloud thousands of kilometers away from the volcano (Harrison et al., 2010). The cause for the observed unipolar charge was attributed to ra- dioactive charging of the particles, since the original electrification ac- quired at the volcano would be dissipated at such long distances. Fig. 2. Proposed volcanic plume electrification mechanisms and observed volcanic lightning (VL) type (with suited detection techniques) in each region of the volcanic plume. The monitoring information derived from the detected VL type is also reported. In purple is the range investigated by laboratory shock-tube experiments. C. Cimarelli and K. Genareau Journal of Volcanology and Geothermal Research 422 (2022) 107449 3
  • 4. Experiments on the ability of particles to retain their charge in absence of contact with other surfaces and in variable environmental conditions are lacking and should be further investigated. 2.1. Fracto-electrification One mechanism of charging is produced during the fracturing of a material. This phenomenon has been observed during the fracturing of crystals (Donaldson et al., 1988), rocks (Enomoto and Hashimoto, 1990), glass (Langford et al., 1991), and other materials such as metal and ice (Avila and Caranti, 1994). Depending on the type of strained ma- terial, fracturing has been observed to promote the release of electrons, positive ions, neutral atoms, and electromagnetic radiation (from radio waves to visible light), hence promoting charging of the resulting fragmented particles. Some of these occurrences have been explained through the piezoelectric nature of certain substances (e.g., quartz), which enhances charge separation at the tip of a propagating fracture, generating large electric fields. In such cases, fracturing can charge ho- mogeneous materials (Takeuchi et al., 2004). Laboratory experiments conducted on cm-sized pumice clasts forced to collide and fracture under vacuum conditions show that a net negative charge is held on solid silicate particles and a net positive charge is either released as ions or is carried on a very small proportion of fine particles generated upon collision (James et al., 2000). Fracto-electrification, also referred to as fracto-emission, is proposed to be a primary contributor to the charge on tephra particles at the point of magma fragmentation, in the volcanic conduit, and at exit from the vent (Smith et al., 2018). How- ever, as magma fragmentation and particle collisions both happen in the volcanic conduit almost simultaneously, in nature it is very difficult to isolate the effect of fracto-electrification from that of tribo- electrification. This may also be valid for some experiments where the interaction of particles generated during the fracture propagation may not be neglected. 2.2. Tribo-electrification Despite being one of the most well-known forms of electrification, the charging by collision and friction between bodies, also known as tribo-electrification or contact electrification, is still intensely investi- gated in the attempt to fully understand its underlying mechanisms (Pähtz et al., 2010; Shinbrot, 2014). Tribo-electrification of granular ma- terials is well known in industrial processing where electrostatic effects may force particles to aggregate, adhere to surfaces, and generate haz- ardous electrical discharges (Eden, 1973; Cross and FarrerD., 1982). Sev- eral models exist for the tribo-electrification process (Lacks and Sankaran, 2011) in which the charge exchange between particles in contact with each other is regulated by their surface properties and their ability to acquire or donate electrons (i.e., their work function). The polarity of charge acquired by the two materials following contact will thus depend on the difference of work function of said materials. The tribo-electric series empirically predicts the polarity of one charged material when in contact with another, according to the differ- ence in their work functions. It follows that for the collision of two par- ticles of the same material, the difference in work functions would be zero thus precluding any charge transfer. Evidence from natural, exper- imental, and industrial granular flows show instead that electrification can also occur between particles of the same material (e.g., Alois et al., 2018; Cimarelli et al., 2014), therefore contradicting the prediction of the tribo-electric series. Charge transfer among chemically homoge- neous particles would derive by the exchange of high-energy electrons on particle surfaces. Particles having the same surface charge density will have the same capacity of acquiring or losing electrons before the collision. In this respect, the polarity of charge would relate to the di- mensions (i.e., surface area) of the particles, with smaller particles gen- erally charging negatively and bigger particles charging positively (Lacks and Levandovsky, 2007). This phenomenon, also known as size-dependent bipolar charging, has been sporadically investigated at active volcanoes (Hatakeyama and Uchikawa, 1951; Kikuchi and Endoh, 1982; Miura et al., 2002) and has also been reproduced in the laboratory on homogeneous volcanic particles (Forward et al., 2009a, 2009b; Méndez-Harper et al., 2021). Although a volcano might display a relatively homogeneous bulk composition during its eruptive history, the main components of the resulting tephra may consist of particles of glass, minerals, and rock fragments which display a high level of hetero- geneity in terms of chemical composition and physical characteristics (e.g., grain size, density, and grain shape), creating a favorable environ- ment for charging and redistribution of charge through the collision of different particles. Tribo- and fracto-charging are still fields of study with many unan- swered questions and unexplained observations, especially for the elec- trification of particles of the same insulating material (Yair, 2008; Lacks, 2010; Shinbrot, 2014). One problem to solve is the relative contribution of each of these mechanisms to the electrification of volcanic plumes. A first step in this direction comes from studying the textures of volcanic ash generated by single explosions producing electrical discharges at Sakurajima volcano (Smith et al., 2018). Although textural evidence points to fracto-charging as the prevalent mechanism of electrification, a substantial contribution of tribo-electrification by particle collisions cannot be completely ruled out. Experiments focused on isolating the effects of tribo- and fracto-charging have been performed to constrain electrification of tephra and volcanic analogue materials (James et al., 2000; Houghton et al., 2013; Méndez Harper and Dufek, 2016). How- ever, none of these have been able to generate electrical discharges, making the link between mechanisms of charging and efficiency/mo- dality of discharge still elusive. Recent experiments have shown nomi- nal size-dependent bipolar charging in shock tube experiments of gas and ash particle jets (Méndez-Harper et al., 2021). Charge polarity is measured on single particles segregating downwind from the jet after they have produced visible electrical discharges upon jet expansion out of the shock tube nozzle. These results highlight the role of tribo- electrification in the gas-thrust region of a volcanic plume and note how particles traveling through this region still retain a considerable charge, creating regions with volumetric charge densities high enough to produce discharges. Finally, electrification of liquids (especially hydrocarbon liquids) is also well known in industrial processing (Klinkerberg and Van der Minne, 1958; Koszman and Gavis, 1962). Water electrification can be achieved by mechanical disruption, boiling, and freezing (Nolan and McClelland, 1914; Mason and Maybank, 1960; Sun, 2020). As H2O is one of the major volatile components dissolved in silicate magmas, the effect of water on volcanic plume electrification will be examined in the following section. 2.3. Effects of water on plume electrification The presence of water in its different phases (vapor, liquid, and solid) and the interaction of magma with external water has been pro- posed to produce electrification during volcanic eruptions. As a matter of fact, water is the dominant magmatic volatile, present in variable abundance depending on bulk magma composition. A suggestion, in analogy to the electrification observed in weather clouds, was first pro- posed by Alessandro Volta in 1782 (Volta, 1782), where he refers to the electrification phenomena (the occurrence of numerous lightning bolts) observed in the 1779 eruption of Vesuvius and for which the explana- tion was the amount and rapidity at which the “smoke” produced in the eruption was liberated. In his communication, Volta extensively de- scribes how the condensation of water in weather clouds would be the cause of their charging, then finally concludes that one shouldn't be sur- prised to see lightning accompanying volcanic eruptions. Calculations suggest that water vapor condensation into liquid and solid phases must be common during explosive eruptions as the concentration of water in magmas may be sufficient to saturate the expanding plume C. Cimarelli and K. Genareau Journal of Volcanology and Geothermal Research 422 (2022) 107449 4
  • 5. (Textor et al., 2003; Williams and McNutt, 2005). In this respect, the co- existence of supercooled water droplets and ice crystals in the expanding saturated plume would suggest that the mechanism typical for thunderstorm electrification, hydrometeor interaction (Reynolds et al., 1957; Stolzenburg et al., 1998) could be active in volcanic erup- tions (McNutt and Williams, 2010; Thomas et al., 2007; Arason et al., 2011; Nicora et al., 2013). Moreover, ash particles in the volcanic plume may act as ice nuclei once sufficient altitudes are achieved (Durant et al., 2008; Schill et al., 2016; Genareau et al., 2018; Maters et al., 2019), forming volcanogenic ice (Prata and Lynch, 2019), and fur- ther promoting ice-ash charging. The eruption column must reach a height coinciding with the local −10 °C or − 20 °C isotherm in order to achieve volcanogenic ice nucleation. The 2016–2017 eruption of Bogoslof (Alaska, U.S.A.) produced nu- merous explosions, with lightning occurring in many, but not all, events. A detailed analysis of several explosions by Van Eaton et al. (2020) re- vealed the role of ice nucleation in both lightning generation and global volcanic lightning detection. Using a combination of remote sensing methods, a total of >4500 lightning discharges were detected during the course of 70 individual explosions that produced ash plumes up to 12 km in height. Van Eaton et al. (2020) noted the higher likelihood of lightning detection for those discharges occurring near or above the −20 °C isotherm. This agrees with observations of Bennett et al. (2010) and Arason et al. (2011) for the 2010 eruption of Eyjafjallajökull in Iceland. These studies showed an increased number of lightning dis- charges in taller plumes and correlated those occurrences to eruptive phases in which the ash plume was approaching or overcoming the −20 °C isotherm. Consequently, when the Eyjafjallajökull eruption plume characteristics resembled those of thunderstorms (i.e., a mixed phase environment with powerful updrafts), the resulting lightning was larger and more apparent to global network sensors. The various benefits and limits of these remote detection methods are discussed fur- ther in a later section. In both the Bogoslof and Eyjafjallajökull eruptions, external water was incorporated into the eruption column, seawater in the former and glacial meltwater in the latter. Several documented examples exist of volcanic lightning produced by contact of seawater with magma or lava, as in the case of lava flows entering the ocean and water entering the volcanic conduit dur- ing emergent shallow submarine eruptions or partial collapse of the ed- ifice. Such was the case for the 2018–2019 phreatomagmatic eruption of Anak Krakatau, where partial collapse of the island caused the contact of sea water with magma in the volcanic conduit, triggering explosions. The eruption was characterized by a vapor-rich and ash-depleted plume that reached the high troposphere through the vigorous convec- tion propelled by the transfer of magmatic heat to the ocean water (Prata et al., 2020). During this eruption, the record production of elec- trical discharges (>150,000 in six days; Fig. 3) has been mainly attrib- uted to the conversion of water vapor into ice during powerful updraft of the plume (Prata et al., 2020). Measurements of positive charging produced by the vaporization of water by lava flows entering the sea during the 1964 eruption of Surtsey volcano have been reported by Björnsson et al. (1967) and confirmed by laboratory experiments by dropping water (sea water, glacial water, and distilled water) on the surface of molten lava (Blanchard, 1964). The increase in charging positively correlated with higher salinity of the water samples and was explained by the authors in terms of solid- solid contact of the precipitated salt particles. Büttner et al. (1997) further investigated electrification during volcanic explosions by repro- ducing magma fragmentation through the injection of both air and water into a synthetic silicate melt to simulate the contrasting behavior of magmatic vs. phreatomagmatic eruptions. Thermo-hydraulic fractur- ing is demonstrated to be extremely efficient in fragmenting melt com- pared to air-induced fragmentation, producing increased particle surface area. Although the results seem to clearly show that thermo- hydraulic fragmentation produces stronger electrical signals than air-driven fragmentation, it appears difficult to discriminate between the effects of fracto-charging from thermo-hydraulic fracturing alone from that produced by the generation of water vapor as investigated by Björnsson et al. (1967). Other observations of electrification during magma-ocean interac- tion include the eruptions of Capelinhos 1957–1958 (Machado et al., 1962), Surtsey 1963 (Anderson et al., 1965), Bogoslof 2016–2017 (Haney et al., 2018; Van Eaton et al., 2020), and Anak Krakatau 2018–2019 (Prata et al., 2020). In all these cases, it is difficult to Fig. 3. Time series of the 2018–19 Anak Krakatau eruption plume. (a) FPLUME (Folch et al., 2016) modelled water vapor, liquid and ice mass mixing ratio profiles with neutral buoyancy level (NBL, dashed black line). ERA5 atmospheric profile at 12:00 UTC on 22 Dec. 2018 was used in the model. Shaded light blue horizontal area indicates lapse rate tropopause ± standard deviation (σ) according to ERA5 data and radio occultation soundings (also shown in b). b) Plume height time series. Purple line indicates heights estimated using total flash rate at 10-min intervals. Orange line indicates heights derived from Himawari-8 satellite images. All heights correspond to left axis of (a). Bottom grey shaded histograms indicate flash rates (CG = cloud-to-ground and IC = in-cloud; right black axis; black y-label). Modified from Prata et al. (2020). C. Cimarelli and K. Genareau Journal of Volcanology and Geothermal Research 422 (2022) 107449 5
  • 6. distinguish the contribution of the water vapor alone to the electrifica- tion of the plume, as the contact with external water produced en- hanced magma fragmentation and generated energetic ash- and vapor-rich volcanic jets. Observations consistently show that eruptions influenced by external water typically display much more volcanic lightning than those resulting from purely magmatic H2O. This is indicated through comparison of the lightning abundance between the 2018 eruption of Anak Krakatau (>150,000 events; Prata et al., 2020) and the 2018 eruption of Volcán Fuego (<100 events; Schultz et al., 2020), with the latter driven completely by magmatic volatiles. Further observations derive from laboratory experiments of tribo- electrification in volcanic particle-laden jets (Stern et al., 2019) and sus- tained granular flows (Méndez Harper et al., 2020) under controlled conditions of relative humidity, water content, and temperature. In both experimental approaches, the electrification of volcanic ash parti- cles (measured using Faraday cages and cups) is mainly achieved through collisional and frictional contact between particles and is shown to decrease in efficiency with increasing humidity and tempera- ture up to the point of complete annealing. Among the two parameters, relative humidity shows a stronger and clearer effect on electrification (Fig. 4). Temporal and spatial changes in experimental variables such as temperature, humidity, and particle density are matched by the variability of an expanding volcanic plume. In the near-vent, gas- thrust region high temperatures, high overpressures, and high particle densities exist. Within the rising column, all these parameters progres- sively decrease. Such variations are also expected to occur through space and time in response to the changing thermodynamic conditions of the hydrometeor phase and the fluid dynamic flow regime. Thus, lab- oratory experiments of tribo-electrification provide a spatial and tem- poral snapshot of the controls for this electrification mechanism. A specific effort should be put into designing experiments able to isolate the effects of different parameters on electrification, such as tempera- ture and pressure, as together these variables might alter the dynamics of the gas-particle mixture (number of collisions among particles, in- creased fragmentation, condensation of water). As conditions in the vol- canic plume quickly change from the vent into the umbrella region, continued experiments on the laboratory scale to test the effects of dif- ferent thermodynamic conditions will provide a more complete picture of the natural process. 2.4. Other charging mechanisms Other charging mechanisms have been invoked for the electrifica- tion of particles in volcanic environments. One source of charging is pro- vided by the radioactive decay of natural radioisotopes (U, Th, K; Aplin et al., 2014, 2016) contained in both the mineral and glass components of tephra and the gas phase (Rn) of the plume (Nicoll et al., 2019). How- ever, this process is expected to diminish in intensity as the particle size decreases, as demonstrated by measurements on micrometer-sized vol- canic ash particles. In photoelectric charging, free electrons are released from the sur- face of a solid particle excited by incident energetic radiation (e.g., ultraviolet radiation) (Lenard, 1902). However, this phenomenon is more effective under vacuum conditions where the intensity of the in- cident radiation is not diminished by atmospheric absorption (Sickafoose et al., 2000). Consequently, this electrification mechanism may be more relevant for extraterrestrial lightning, where conditions in some cases may more closely approach those in a vacuum compared to terrestrial volcanic systems. Overall, the chemically complex and multiphase nature of volcanic plumes suggests that several charging processes could operate during explosive eruptions and that the dominant process may well change with plume age and distance from the volcano (Mather and Harrison, 2006; James et al., 2008; McNutt and Williams, 2010; Aplin et al., 2016; Van Eaton et al., 2016). Understanding these changes is key to in- terpreting how the real-time variations in the plume's electrical signa- ture reflects its evolving dynamics and how different types of volcanic electrification are controlled by plume evolution, as described in the section below. 3. Charge structure of volcanic plumes and types of volcanic lightning Established field techniques for measuring atmospheric electricity have been applied to the study of volcanic plume electrification. Many similarities have been noted between charging mechanisms in eruptive columns/plumes and those in thunderstorms so that we may also ex- pect similarities between the spatial distribution of both. This aspect is particularly interesting as charge structure is responsible for the Fig. 4. Effect of water on ash charging and discharging. a) Mean charge densities for experiments conducted at 25 °C and RH ranging from 0 to 60%. Each data point represents ∼100 individual particle measurements (Méndez Harper et al., 2020). b) Cumulative electrical discharges measured in expanding gas-ash jets in shock-tube experiments at variable wt% of water in the expanding mixture (Stern et al., 2019). C. Cimarelli and K. Genareau Journal of Volcanology and Geothermal Research 422 (2022) 107449 6
  • 7. electrical potential within the plume and ultimately determines the fa- vorable conditions for lightning generation. Both changes in the electri- cal structure of the plume and consequent changes in volcanic lightning may therefore track changes in space and time of the evolving physical structure of the plume. 3.1. Previous studies on the charge structure of volcanic plumes Early measurements of volcanic plume electrification mainly in- volved collecting electric field gradient time series of the nearby or overhead volcanic plumes and associated ashfall at single or multiple stations. Such measurements showed either prevalent negative or pos- itive perturbation of the electric field gradient caused by the presence of the particle laden plumes (Hatakeyama, 1943, 1947, 1949; Nagata et al., 1946; Hatakeyama and Uchikawa, 1951; Kikuchi and Endoh, 1982). During the 1950 eruption of Aso, Hatakeyama and Uchikawa (1951) ob- served a prevalent positive gradient associated with the ejection of water vapor and ash from the active crater. This was perturbed by a neg- ative transient when the plume front was directly above the measuring stations and followed by positive gradients during the consequent ash- fall. Such inversions of polarity were explained by the electric stratifica- tion of the ash plume due to settling of particles of different sizes carrying charge of opposite polarity. The electric stratification of the plume derived from the early segregation of larger, positively charged particles from the suspended smaller, negatively charged particles. The dependence of charge polarity on particle size was confirmed in the same study by laboratory measurements (Hatakeyama and Uchikawa, 1951). Using the same approach, observations employing improved instru- mentation have provided further measurements of the electrification of volcanic plumes (Gilbert et al., 1991; Miura et al., 2002; Lane and Gilbert, 1992; James et al., 1998) and of ash clouds generated by pyro- clastic density currents (Miura et al., 1996). The results of these studies agree in assigning very strong charge anomalies to the plume, but con- trast between each other in the determination of the overall electrical structure of it, suggesting either a prevalently positive or negative polar- ity, or the coexistence of both. Miura et al. (2002) proposed a vertically stratified model dominated by gravitational settling which is character- ized by a “PNP” (Positive-Negative-Positive) structure, where the lower, positively charged part contains coarser ash particles; the middle, neg- atively charged region contains finer ash; and the uppermost, positively charged portion is dominantly composed of gases and aerosols (Fig. 5). Harrison et al. (2010) conducted electrical measurements within the distal plume resulting from the Eyjafjallajokull 2010 eruption. They found that electrical effects persisted far from the eruption source. Since then, Nicoll et al. (2019) performed in situ measurements of the gaseous volcanic plume at Stromboli volcano (Italy) using disposable sensors carried by weather balloons. The vertical profiles provided ther- modynamic, electrical, and microphysical properties inside ash de- pleted volcanic columns close to their source (~300 m above vent) and provided new information about the magnitude, polarity, and ver- tical distribution of charge. The space charge profiles showed notable charge of up to ±8000 pC/m3 with well-defined layers of positive charges at the cloud base and negative charges at the top. The highest charge values were coincident with the highest concentration of SO2 droplets (Fig. 6) and measured charge values were 80 times higher than those measured in conventional stratiform clouds (up to ±100 pC/m3 ). Although these measurements relate to prevalently gaseous plumes for which the total absence of fine ash particles cannot be confirmed, they show we should expect charging close to the volcanic vent even in conditions of very low explosivity. Laboratory experiments reproducing electrical discharges in particle-laden jets give further insight on the charge structure of expanding granular flows and their electrification (Cimarelli et al., 2014; Gaudin and Cimarelli, 2019). High-speed videos of these experi- ments suggest that segregation of particles with different sizes Fig. 5. Schematic illustration of the electrical structure of volcanic plumes and particle-laden jets in laboratory experiments. The electric charge distribution following the “PNP” model proposed by Miura et al. (2002) where the upper part, dominated by volcanic gas and aerosols, has a prevalent positive charge, the middle part of mainly negatively charged fine ash particles, and a lower part dominated by gravitational settling of coarser and positively charged ash particles. (a) Distribution of particles in jets with bimodal (c) and monomodal (d) grain size distribution and their schematic charge structure (d and e, respectively). In the case with bimodal size distribution, larger and positively charged particles are confined in the center of the flow while smaller and negatively charged particles follow the turbulence in the shear layer with the surrounding atmosphere. In the case with monodispersed particles, clustering of particles with either prevalent positive or negative charge generate transient electrical dipoles. C. Cimarelli and K. Genareau Journal of Volcanology and Geothermal Research 422 (2022) 107449 7
  • 8. efficiently operates within the flow forming two regions where the par- ticle motion is dominated by: 1) the inertia of the particles (larger par- ticles at the center); and 2) the turbulence of the expanding gas (smaller particles at the margins) (Fig. 5b and d). The segregation of particles with different sizes would also account for the separation of charges of opposite polarity (larger particles charged positively and smaller ones charged negatively) that would be necessary for discharges to occur. In the case of small particles with monodispersed size, the flow orga- nizes into transient regions of variable particle volume density (clus- ters) of prevalent charge polarity between which the discharges occur (transient electric dipoles) (Fig. 5c and e). The effect of clustering in generating electric fields has been addressed by numerical modelling of turbulent flows laden with charged particles of bi-dispersed grain size (Di Renzo and Urzay, 2018). The electric field generated by this pro- cess would be of larger scale than the mean inter-particle distance and the size of the smallest turbulent eddy. During the 2009 eruption of Re- doubt (Alaska, U.S.A.) Behnke et al. (2013) utilized a Lightning Mapping Array (LMA) to infer the charge structure of the eruption plume based on the sizes and types of discharges occurring within it. Without the al- titudes of the radio frequency (RF) sources the authors were unable to give a detailed description of the charge structure in the plume, none- theless they suggested that in the near-vent region charge was chaoti- cally dispersed in clusters but became horizontally stratified within the plume at higher altitudes, thus evolving into a charge structure sim- ilar to that of a thunderstorm. Considering all the field and laboratory observations, it appears that no general model for the electrical structure of a volcanic plume can be universally applied and that this structure will likely vary with time and space depending on the phase of the explosive eruption and the pro- gressive organization and stratification (Behnke et al., 2013; Woodhouse and Behnke, 2014) of the plume. Thus, it appears that the clustering of particles (and therefore of charges) observed in the exper- iments (Fig. 5 b-e; Cimarelli et al., 2014, 2016) characterizes the initial stage of a volcanic explosion and that stratification would progressively ensue due to particle segregation within and from the plume (Fig. 5a). This later aspect of the charge structure of volcanic plumes has so far not been investigated in the laboratory and would require complex ex- periments to incorporate convection and particle sedimentation with jet expansion. The natural variability in these volcanic electrification processes and relationship to the dynamics of individual eruptions poses a unique challenge to the discipline but expands the potential breadth of research opportunities in the future. Efforts to scale labora- tory simulations with real-world phenomena have challenged volcanol- ogists in previous studies of eruption dynamics, and volcanic electrification is another example of such. 3.2. Volcanic lightning types Volcanic lightning is produced within three regions of the eruption column and plume, each of which is governed by very distinct micro- physical dynamics (Fig. 2). Stemming from the former classification of volcanic electrical activity into vent discharges, near-vent lightning, and plume lightning based on the location, length, and timescale of occur- rence (McNutt and Williams, 2010), continued analysis has provided in- sight into the eruption characteristics that control this spectrum of electrical activity at erupting volcanoes. The onset of sudden volcanic explosions, as in the case of Vulcanian style eruptions, is often characterized by a multitude of small electrical discharges emerging from the vent as tephra is rapidly ejected (Thomas et al., 2007, 2010; Behnke et al., 2013). This stage was origi- nally referred to as vent discharges to contrast with actual lightning dis- charges, mainly based on their characteristic very high (VHF; 30–300 MHz) signature. Behnke et al. (2018) further detailed that vent discharges produce continual radio frequency (CRF) emissions which consist of bursts of RF sources with duration of seconds to tens of seconds, as opposed to the intermittent RF transients generated by lightning events, which have durations of 10–100's of milliseconds. Such RF bursts have been detected by LMA arrays deployed at several tens of km from the volcanic crater (Thomas et al., 2007; Behnke et al., 2011). CRF emissions have no parallel in thunderstorms, therefore they could be discriminant for explosive volcanic eruptions. Vent dis- charges have been related to the early dynamics of an evolving eruptive column, where the electrification mechanism may be dominated by fracturing and comminution of colliding particles (Smith et al., 2018) and where the electrical structure of turbulent jets is not yet defined by particle segregation and sedimentation (Smith et al., 2021) (Fig. 7b). As shown by shock-tube laboratory experiments (Méndez-Harper et al., 2018; von der Linden et al., 2021), the rapidly changing condition of overpressure at the vent (e.g., shock waves; Fig. 7a) and the conse- quent generation of an underpressurized region in the expanding jet be- hind the shock front may cause a sudden decrease in the breakdown voltage within the flow, thus enhancing the opportunity for inter- particle spark discharges or corona discharge. The transient nature of the CRF signal would hence be related to the stability of this region of weakened dielectric strength close to the vent, so that CRF occurs as long as conditions of overpressure persist. Near-vent lightning was previously described as occurring in the gas-thrust region immediately above the vent and within the convection-driven expanding column. These discharges range from hundreds of meters to several kms in length (Thomas et al., 2010). Plume lightning was differentiated from these other types as occurring Fig. 6. Vertical profiles through a gaseous volcanic plume at Stromboli at 11:40 UT on 30th Sept 2017 measured by a radiosonde and the VOLCLAB sensor package (Nicoll et al., 2019). (a) Temperature (T, in grey) and relative humidity (RH, in black), (b) droplet concentration (black) and diameter (grey) measured by the optical backscatter sensor, (c) Space charge density (grey) and SO2 concentration (black). Modified after Nicoll et al. (2019). C. Cimarelli and K. Genareau Journal of Volcanology and Geothermal Research 422 (2022) 107449 8
  • 9. both in the rising column several kilometers above the vent and in the neutrally buoyant umbrella region (Fig. 2) and is most similar to thun- derstorm lightning in terms of altitude and channel length (up to >10 km). Besides their geographical position within the plume and ash cloud, other characteristics that differentiate these previously de- scribed lightning types stemmed primarily from the length of the plasma arc and their timescale. With increasing altitude and expansion of the eruption plume, lightning is larger and lasts for a longer time. Both near-vent and plume lightning appear to be produced from triboelectrification between tephra particles, between volcanogenic ice particles, or between a combination of both depending on the atmo- spheric conditions and amount of external water. As larger plume light- ning tends to occur above the local level of ice nucleation, charging of hydrometeors in the volcanic plume would explain the similarity with thunderstorm lightning. It is worth noting that near-vent lightning oc- currence can coincide and therefore be masked by the CRF signal in LMA recordings, but it can still be discriminated with the use of high- speed video imagery (Fig. 7a). Conditions for the simultaneous occur- rence of CRF and all types of lightning may increase for eruptions where the plume can be sustained for several hours to days producing strong eruptive columns through vigorous convection. Moreover, CRF have so far been detected at the inception of relatively violent explo- sions generating jets likely exceeding the speed of sound and it is still an open question if such signal can accompany also less energetic volca- nic explosions. In this respect, to simplify our understanding of the dif- ferent electrification mechanisms and the deriving charge structure, it appears that impulsive short-lived eruptions may provide better insight into the correlation between discharge types and changes in volcanic activity over time. 4. Volcanic lightning detection Lightning emits over a wide range of frequencies in the electromag- netic spectrum. Besides the emission in the visible range (400–790 THz), significant electromagnetic energy is emitted by lightning between 1 Hz and 300 MHz. An additional form of energy is generated by lightning under the form of acoustic emissions associated with re- sulting thunder (Haney et al., 2018; Haney et al., 2020). Using a micro- phone array installed 60 km from Bogoslof, Haney et al. (2020) detected volcanic lightning events that went unrecorded by the global lightning monitoring networks during the 2016–2017 eruption. They captured these events through electromagnetic pulses and also recorded thunder produced by the volcanic lightning, in addition to those recordings first described in Haney et al. (2018). The occurrence of volcanic lightning during explosive eruptions has been serendipitously observed as glitches (i.e., transient spiky interfer- ence signals) on seismograms of the duration of 0.05 s (McNutt and Davis, 2000) and on infrasound recordings (Anderson et al., 2018; Haney et al., 2020). More commonly, lightning is detected by recording the associated radio frequencies over a wide range from very low (VLF; 3–30 kHz) up to very high frequencies (VHF; 30–300 MHz). The detec- tion of volcanic lightning has therefore made use of existing radio- frequency techniques for the detection and study of thunderstorm light- ning. 4.1. Ground-based methods Two main types of ground-based thunderstorm detection antennas have so far been used for locating and examining volcanic lightning: 1) regional networks of VLF antennas designed for long range (1000's of km) thunderstorm detection (Lay et al., 2004; Bennett et al., 2010; Nicora et al., 2013; Firstov et al., 2017); and 2) local arrays of VHF anten- nas, suited for high spatial (1–10 m) and temporal (nanoseconds) reso- lution studies of meteorological lightning (Thomas et al., 2004; Thomas et al., 2010; Behnke et al., 2013, 2014). VLF antennas suffer from having low sensitivity and variable detec- tion efficiency (10–70%; Abarca et al., 2010). They are designed to detect larger lightning and therefore prove inadequate to detect the lower in- tensity volcanic discharges that occur in the near-vent region, as dem- onstrated by long-term volcanic lighting recording at Sakurajima Fig. 7. (a) High-speed (HS) video frame (10 kHz) of emerging ash jet with shock wave (note lofting of ash at the crater rim; white arrows) and vent lightning (~30 m; red arrows in a and b). (b) multiparametric signals of explosion in (a), showing CRF coinciding with thermal anomaly and highest jet velocities. Grey lines are flashes detected by HS-video. (c) Discharge (6 cm) in particle-laden jet in shock tube experiment (50 kHz; from Cimarelli et al., 2014, 2016). (d) Electrical and pressure signals recorded during a shock tube experiment. In analogy to the CRF signals, the electrical discharges (blue spikes) occur during conditions of overpressure (hump in in the orange line) of the jet at the nozzle (after Gaudin and Cimarelli, 2019). C. Cimarelli and K. Genareau Journal of Volcanology and Geothermal Research 422 (2022) 107449 9
  • 10. volcano (Aizawa et al., 2016; Vossen et al., 2021). As indicated by anal- yses of volcanic lightning during the 2016–2017 Bogoslof eruption in Alaska, the VLF antennas will have a higher likelihood of measuring lightning discharges that occur at or above the level of ice nucleation, where properties of the eruption plume more closely resemble those of thunderstorms (Haney et al., 2020; Van Eaton et al., 2020). However, thousands of VLF stations are already deployed all over the globe, mainly for weather forecast and commercial applications. The sensor networks mainly utilized for volcanic lightning studies include those operated by Vaisala (Global Lightning Detection; GLD360), the Earth Networks Total Lightning Network (ENTLN), the World Wide Lightning Location Network (WWLLN) and the Arrival Time Difference Network (ATDnet) (Fig. 8). They have been shown to detect lightning during large explosive eruptions within the last decade (e.g., Calbuco 2015 (J. Lapierre, pers. comm.); Bogoslof 2016 (Haney et al., 2018; Van Eaton et al., 2020); Ambae 2018 (C. Vagasky, pers. comm.); Fuego 2018 (Schultz et al., 2020); Krakatau 2018 (Prata et al., 2020); Raikoke 2019; Ulawun 2019; Sinabung 2019; Taal 2020; La Soufriere at St. Vincent 2021; see Table 1 in the Supplementary Material) although in some cases with tens of minutes to hours of delay from the beginning of the explosion. The persistent locations of these sensors allow for vol- canic lightning detection without preliminary planning or field deploy- ments, reducing effort and expense. Alternatively, VHF antennas (or Lightning Mapping Arrays; LMA) are custom-designed deployments built to unravel the physics of lightning generation and propagation. They have been temporarily deployed at erupting volcanoes for high resolution studies (Thomas et al., 2007, 2010; Behnke et al., 2014, 2018, 2021). Being sensitive to higher fre- quency than VLF sensors, LMA are more efficient in detecting volcanic lightning (Behnke et al., 2014, 2018), particularly near-vent lightning that occurs at altitudes below the level of ice nucleation. The sensors within the array allow the calculation of lightning time of occurrence and location within the eruption column or plume when several sensors detect the same discharge signal. As each lightning discharge generates numerous electromagnetic signals, the LMA provides a three- dimensional map of volcanic lightning activity over the course of an eruption. This allows lightning properties to be linked with changes in eruption dynamics. More recently, the use of antennas measuring in the extremely low frequency range (ELF; 1–45 Hz) have been successfully applied to the continuous monitoring of persistently active volcanoes such as Sakurajima in Japan (Fig. 9; Vossen et al., 2021) and Stromboli and Etna in Italy. These antennas have a sampling rate of 100 Hz and there- fore cannot rival the much higher temporal resolution of the LMA. How- ever, their characteristics have some notable advantages: 1) the antenna frequency range is not affected by common anthropogenic radio emission; 2) the system is easily deployable and requires low power consumption; 3) the lower sampling rate allows for the recogni- tion of near-vent and plume lightning discharges from safe distance (up to 40 km); 5) the unit price of the antennas is roughly half the price of LMA antennas. A long-term deployment of these antennas at Sakurajima volcano over 18 months of continuous observation allowed the detection of electrical discharges in hundreds of relatively small vol- canic plumes (up to about 5 km in height) with an accuracy of 73%, thus surpassing the detection efficiency of regional VHF networks for the same explosions (Vossen et al., 2021). In addition, the sensor can detect the movement of spatial charge overhead or falling onto the antenna, thus enabling the recording of electrically charged fallout. Similar Fig. 8. Examples of volcanic lighting activity produced during explosive eruptions and sensed by ground-based lightning detection networks: a) 2010 Eyjafjallajökull eruption as detected by ATDnet (modified after Bennet et al., 2010); b) 2015 Calbuco eruption detected by WWLLN; c) 2018 Ambae eruption detected by GLD360 (C. Vagasky, pers. comm.) and d) 2020 Anak Krakatau eruption as detected by ENTLN (J. Lapierre, pers. comm.). See also Table 1 for further details on these eruptions. C. Cimarelli and K. Genareau Journal of Volcanology and Geothermal Research 422 (2022) 107449 10
  • 11. long-term studies of persistently active volcanoes will be necessary to constrain both the evolution of electrical activity and the lightning dis- charge properties for individual volcanoes displaying a range of explo- sive styles and magnitudes. As eruptive activity is unique for every active volcanic center, recent observations also indicate that electrical activity is as well. 4.2. Satellite-based methods In addition to the ground-based detection techniques previously de- scribed, regional observation of lightning activity at active volcanoes can be carried out by space-born sensors such as the Lightning Imaging Sen- sor onboard the International Space Station (LIS-ISS) and the Geosta- tionary Lightning Mapper (GLM) on the Geostationary Operational Environmental Satellite (GOES). Satellite-based detection methods have proven valuable for monitoring explosive eruptions from: 1) volcanoes in remote locations; 2) volcanoes lacking instrumentation to detect pre-eruptive activity (e.g., seismic, deformation); and 3) volca- noes erupting after periods of extended repose over human historical timescales (i.e., hundreds to thousands of years). Although the GOES provides information on many volcanic centers in the western hemi- sphere, the Himawari satellite (Japan) has been the primary space- based tool for capturing volcanic plume data for persistently active volcanoes in the Pacific ring of fire. The GLM offers constant observation of active volcanoes within the field of view of the GOES, which includes Central America, the Carib- bean, the majority of South America, Hawaiian islands, and the conti- nental United States. The first study to utilize GLM observations of volcanic lightning is Schultz et al. (2020). Lightning that occurred during the 2018 eruption of Volcán Fuego, Guatemala was analyzed from re- cords obtained using both the GLM and the WWLLN. Comparison of the satellite and ground-based sensors revealed that the GLM detected less lightning than the WWLLN overall, but also detected lightning events at different times from the WWLLN during the eruption. Lower detection by the GLM may stem from absorption of the optical lightning signal by the ash plume. Schultz et al. (2020) noted the occurrence of lightning jumps during the 2018 Fuego eruption at the reported onset of the initial explosion and also later in time, prior to collapse of the eruption column into a pyroclastic density current. They suggested that sudden increases in volcanic lightning may serve to warn of impending hazards induced by changes in eruptive activity, and this is a topic that should be explored further, particularly with continued improvement of instrument capabilities. The ground-based detection networks can discriminate between in-cloud and cloud-to-ground lightning based upon peak current, while the GLM can capture the two-dimensional spatial footprint of a lightning flash, which allows cal- culation of channel length, and also measures the optical energy at a wavelength of 777.4 nm (Rudlosky et al., 2019; Peterson, 2019). Schultz et al. (2020) suggested that a combined approach utilizing both ground- based and space-born sensors will provide a more robust dataset, as capturing more lightning properties will enable a more complete anal- ysis of the microphysical dynamics within an eruption column and plume. An additional benefit of satellite-based detection methods is the co- incident examination of aerosol phase within the eruption plume. De- pending on the instrument, detection of ash and/or volcanogenic ice particles from satellite retrievals can provide information on the role of these different phases in transferring charge and their microphysical influence on lightning properties. Of course, accurate hydrometeor de- tection and quantification may be limited in ash-rich eruption plumes due to the optical opaqueness. However, despite any instrumental lim- itations, the previous studies indicate that a combined dataset utilizing both ground-based and satellite-based techniques can provide the most complete story regarding the influence of plume properties on lightning occurrence and discharge characteristics. 4.3. Other detection methods An increasing number of volcanic lightning observations come from multi-parametric measurements of explosive eruptions where lightning detection is combined with high-speed videos and infrared thermal im- aging, atmospheric vertical profiles, magnetotelluric measurements, infrasound, and seismic measurements (Cimarelli et al., 2016; Aizawa et al., 2016; Behnke et al., 2018). Using a synchronized multiparametric array of visible high-speed cameras, infrasound, and high sampling rate magnetotelluric stations, Cimarelli et al. (2016) have monitored the electrical activity during the persistent explosive activity at Sakurajima volcano (Suppl. Video 1). Their data show that intermittent Vulcanian explosions produce plumes with average heights of <6 km above sea level and generate mainly near-vent lightning in the gas thrust region (a few hundred me- ters above the crater rim), where ice nucleation is negligible. Correlating maximum height of lightning-bearing plumes with radiosonde atmo- spheric profiles, this study highlights how the electrification is Fig. 9. Extreme low frequency detection of volcanic lightning activity in Vulcanian explosions at the persistently active Sakurajima volcano (Japan). The long-term (18 months) time-series compares the maximum height of lightning bearing volcanic plumes with the temperature of the surrounding atmosphere derived by weather balloon thermodynamic profiles. Only in rare occasions the plumes reach freezing temperatures (−10 °C to −20 °C) ruling out the effect of ice nucleation in the electrification of the plumes. Over 724 explosions, 511 events have been detected that produce lightning, of which only 2 were also detected by regional network antennas. Modified after Vossen et al. (2021). C. Cimarelli and K. Genareau Journal of Volcanology and Geothermal Research 422 (2022) 107449 11
  • 12. prevalently determined by the plume dynamics in absence of dominat- ing external atmospheric factors such as high relative humidity and freezing temperatures (Fig. 9). At the inception of an explosion, the tur- bulence of the gas/particle jet generates a complex charge structure, producing relatively small discharges (10–200 m channel length) with no preferential directionality and with maximum length of flashes in- creasing with time from the onset of the explosion (i.e., with the height of the plume). Organization of charge is then achieved at later stages with the transition from the jet to the convective phase and the gener- ation of larger cloud-to-ground lightning. Also, the number of lightning discharges detected is proportional to the pressure recorded by the infrasound stations, hence showing a positive correlation of electrical activity with the intensity of the explosions (Fig. 10). 5. Revealing plume dynamics through electrical properties It is apparent that the successive occurrence of different lightning “types,” characterized by location, size, and timescale, tracks the pro- gressive development of the plume and that this can be used to monitor plume evolution in time and space. Volcanic lightning detection and mapping is therefore emerging as a powerful tool in the portfolio of vol- cano monitoring techniques as it allows the detection of explosive activ- ity from safe distances in unfavorable weather conditions (Behnke and McNutt, 2014). More importantly, a key result emerging from multi- parametric studies, in combination with laboratory experiments, is that determining the characteristics of volcanic lightning (nature, inten- sity and space-time distribution) may reveal the structure of the plume and its source parameters (e.g., mass eruption rate), which are other- wise very challenging to measure directly. Studies have attempted to derive information on dynamic changes during volcanic eruptions by correlating variations in recorded electrical activity, as previous studies have proposed relationships between light- ning flash rate and thunderstorm parameters such as cloud top height and peak updraft speed (e.g., Price and Rind, 1992). Behnke and Bruning (2015) calculated flash energy spectra from LMA data collected during the 2009 eruption of Redoubt volcano in Alaska and compared them to the evolving turbulence characteristics of the developing plume. Their analysis confirms the proportionality between flash length scale and energy with increasing volume of the plume. A combined analysis of satellite images and lightning detection for the 2018–2019 eruption of Anak Krakatau also reveals a good correlation between the height of the plume and the lightning flash rate (Prata et al., 2020). The empir- ical correlation found for the case of Anak Krakatau suggests that light- ning flash rate could be used to derive information on the height of the eruption column, thus providing important input for the modelling of volcanic ash dispersal from the plume. In general, the attempt to correlate variations in the volcanic light- ning activity and the changing source parameters of the ash plume con- stitutes one of the most challenging aspect of volcanic lightning research as this requires systematic multiparametric studies where the volcanic lightning activity can be measured against other indepen- dent source parameters such as mass eruption rate, plume rise velocity, plume height, etc., which are often very difficult to determine. In this re- spect, combining field measurements with laboratory experiments and numerical modelling may provide the necessary constraints to enable such correlations. 6. Effects of lightning on tephra properties Fulgurites are complex glassy structures generated by the melting of coherent rocks or loose sediments by a cloud-to-ground lightning strike. The production of fulgurites by lightning is well known and has been ex- tensively reported in the literature. A vast collection of works dealing with fulgurites can be found in the bibliography of McCollum and Welsh (1999). Several descriptive studies have attempted to recon- struct the physical conditions of the lightning strike by looking at the textural modification and phase relationships of the exposed material. The classification scheme of Pasek et al. (2012) defined various fulgurite types (I through V) according to the affected material (e.g., sand, soil, rock), but volcanic tephra is not listed among any of these starting ma- terials. Tephra can be considered to fall between rocks and sediments, potentially generating a different “type” of fulgurite. As the physics of the volcanic lightning discharge process is compa- rable to that of thunderstorm lightning (Aizawa et al., 2016), also the ef- fects produced in materials are expected to be similar. Moreover, the predicted temperatures achieved in the lightning channel (30,000 K; Paxton et al., 1986; Rakov and Uman, 2003) exceed the glass transition and melting temperature of natural silicate glasses and minerals. In this Fig. 10. a) Correlation between number of discharges and height of the plume for 261 explosions recorded at Sakurajima volcano (Japan). Datapoints represent multiple plumes reaching the same elevation above the crater rim while error bars show the variability in the number of discharges for plumes of the same height. b) Positive correlation between the number of recorded flashes and maximum pressure peak produced by 32 explosions at Sakurajima volcano. Visible flashes were recorded with high-speed cameras while maximum average pressure accompanying the explosions were measured with an infrasound array. Modified after Cimarelli et al. (2016) and Vossen et al. (2021). C. Cimarelli and K. Genareau Journal of Volcanology and Geothermal Research 422 (2022) 107449 12
  • 13. respect, it would be difficult to distinguish fulgurites on the ground pro- duced from volcanic lightning from those produced by thunderstorm lightning, since their appearance would chiefly depend on the charac- teristics of the pristine starting material (Clocchiatti, 1990; Castro et al., 2020) rather than the origin of the cloud-to-ground discharge. Fewer studies have managed to produce fulgurites in the laboratory (Fig. 12) by controlled electrical discharges on selected target materials (Hachette, 1828; Butcher, 1908; Arai, 1969; Kumazaki et al., 1997; Cimarelli et al., 2017; Elmi et al., 2018) to constrain how lightning prop- erties (e.g., peak current, temperature, pressure, timescale) may be de- termined from examination of fulgurite textural, chemical, and mineralogical characteristics. The high temperatures and high pressures produced in the ionized lightning discharge channel can modify the structure and chemistry of pristine tephra, producing a variety of shapes and textures (Fig. 11). The effects of lightning on airborne tephra particles within the eruption column and plume have been documented in ashfall deposits and reproduced in the laboratory. First reported by Genareau et al. (2015), lightning-induced volcanic spherules (LIVS) have been observed in ash- fall deposits from several explosive eruptions. Initially identified in products of the Redoubt 2009 and Eyjafjallajökull 2010 events, where volcanic lightning was ubiquitous, similar textures were also generated during high-voltage flashover experiments using electrical insulators contaminated with volcanic ash (Wardman et al., 2014; Wardman et al., 2012), suggesting that electrical discharge was a likely formation mechanism. Theoretical calculations show that the timescale of ob- served volcanic lightning discharge (≥ 0.1 ms) would allow for surface tension-driven rounding of particles with sizes comparable to observed LIVS (Wadsworth et al., 2017). Since the initial study of Genareau et al. (2015), LIVS have been documented in products from other eruptions, including the 2016 eruption of Pavlof (Genareau et al., 2020). Also previously identified in ashfall deposits are spherule aggregates, com- posed of several LIVS welded together, and pumiceous particles that dis- play vesicles formed from secondary exsolution of remaining volatiles bound in volcanic glass (Genareau et al., 2020). LIVS provide physical evidence of lightning occurrence in the tephra record. In order to quantify the total amount of erupted tephra affected by volcanic lightning, several parameters must be known: 1) peak cur- rent of the discharge, which will control the channel radius; 2) length of the lightning channel; 3) ash concentration in erupted column or plume where discharge occurs; 4) proportion of erupted volume that is fine ash or smaller (<100 μm); and 5) total number of lightning discharges during the eruption. Although the proportion of ash affected during each discharge event may be small, eruptions with hundreds of thou- sands of individual events, like that of Anak Krakatau in 2018–2019 (Prata et al., 2020) may have an impact on ash transport and resulting hazards to the environment and infrastructure. An accurate quantifica- tion of this impact will require combined efforts in constraining light- ning parameters through remote sensing (e.g., channel length from GLM observations) and modelling the resultant macro/micro-physical effects on both atmospheric and ash plume properties. The various lightning-induced textures previously identified in ashfall deposits are comparable to those produced in current impulse experiments, and each lightning effect has a direct impact on the associated ash-related hazards, as described in the following section. 7. Experimental investigations of volcanic lightning Laboratory experiments conducted to generate or simulate volcanic lightning have been ongoing over the last several years in efforts to reveal the microphysical dynamics contributing to electrical activity, Fig. 11. Structural and textural modifications induced by lightning in volcanic tephra as produced by current impulse experiments. The secondary electron microscope images on the far left (orange box) show textures identified in ashfall deposits and re-created during current impulse experiments. The high temperature in the lightning channel (dashed-line box inset), exceeds the glass transition of igneous rocks and minerals, causing melting (orange fields on the lightning flash and in the inset), and/or vaporization and dissociation (yellow fields on the lightning flash and the inset) of material compounds. The propagation of shock-waves (blue lines) may also stretch or compact melted air-born volcanic particles, forming the plate-like and hair-like particles (images in the blue box) that have been produced in current impulse experiments, but not directly observed in ashfall deposits (*). Volcanic deposits (tephra and lavas) on the ground may also transform into fulgurites (image in the dark grey box) by attachment of lightning to ground (dark grey field on the lightning flash). C. Cimarelli and K. Genareau Journal of Volcanology and Geothermal Research 422 (2022) 107449 13
  • 14. the role of environmental variables, and the resulting effects on ash particles. 7.1. Shock-tube decompression experiments Rapid decompression experiments of gas-particle mixtures (at sub- and supersonic conditions) have been conducted to reproduce electrical discharges at conditions similar to those found in volcanic plumes. Such experiments have generated renewed interest in the discipline as they provide the opportunity to visualize the electrical discharges with re- spect to the evolving experimental jets and study processes of self- electrification and discharge mechanisms in absence of imposed exter- nal electric fields. Moreover, these experiments allow controlling key parameters that would be difficult to constrain in nature, thus shedding light on the microphysical dynamics at play within explosive eruption columns. In particular, shock-tube experiments have been performed to con- strain the effects of particle size distribution (Suppl. Video 2), total mass ejected, and initial overpressure of the gas-particle mixture on the electrification of the expanding jets (Cimarelli et al., 2014; Gaudin and Cimarelli, 2019). The results of these experiments show that the total magnitude of electrical discharge generated is proportional to all three parameters, although each play a different role in the process by modulating the way discharges occur. The proportion of fine particles in the original mixture appears to control the number of the discharges, while their magnitude (i.e., the amount of charge neutralized by a single discharge) is linearly correlated to the initial overpressure and total mass of the system (Gaudin and Cimarelli, 2019). Stemming from these observations, additional shock-tube experiments have also ex- plored the roles of temperature and water content on ash charge trans- fer and discharging processes. Stern et al. (2019) saturated their ash samples with up to 27 wt% water and found that only 1.8 wt% signifi- cantly decreased the total discharge generated during decompression. Stern et al. (2019) also observed a higher number of smaller discharges when temperature of the system was increased, which they attribute to increased turbulence within the expanding sample mixture. Compara- ble results were found in the tribo-electrification experiments of Méndez-Harper et al. (2020). In this case, agitated particles produce low-energy and low-frequency collisions under variable temperature and relative humidity conditions over long time scales (60 s) to simulate particle interaction in the convective portion of the plume. Results show that the presence of external water inhibits charge transfer by coating ash particle surfaces over sufficiently long timescales. Additionally, a general decrease in charging with higher temperatures is also observed, although the reason for this is less clear and requires further study. There is evidence that the type of discharges produced in shock-tube experiments resemble those generating the CRF signal at the onset of explosive eruptions (Fig. 7) which would hence be determined by the conditions of supersonic overpressure at the eruptive vent and the re- duced dielectric strength in the gas-thrust region just above it (Méndez-Harper et al., 2018; von der Linden et al., 2021). The CRF signal could potentially be correlated with magnitude of overpressure at the vent and CRF total duration would give a good estimate of the timescale of tephra injection into the atmosphere, being this impulsive or sus- tained, thus providing useful constraints on source parameters in plume numerical models. 7.2. Current impulse experiments To simulate the effects of lightning discharge on volcanic ash parti- cles, current impulse experiments have been conducted using labora- tory facilities originally designed for engineering applications and materials testing. The impulse apparatus delivers a discharge with a double exponential waveform and peak current magnitude similar to cloud-to-ground lightning strikes. The size of the discharge channel and temperature within it are both a function of peak current. Experiments conducted on manufactured volcanic ash particles at peak currents ranging from 7 kA to 100 kA cause a variety of structural and chemical effects on particles (Genareau et al., 2017; Genareau et al., 2019, 2020; Woods et al., 2021). Regardless of ash composition, the grains are altered by the high temperatures generated, which can reach almost 30,000 °C at the chan- nel axis (Genareau et al., 2017). Individual grains are melted and/or fused together, changing the overall size distribution of the sample (Genareau et al., 2019). The instantaneous loss of smaller particles and creation of larger particles will alter the fallout of tephra from the transported plume. Resulting shapes of lightning-affected particles range from individual solid or hollow spherules, spherule aggregates, pumiceous particles, hair-like particles, and plate-like particles (Fig. 13). All textures except the latter two have been documented in ashfall samples from eruptions where lightning was documented. The latter two textures (hair-like and plate-like particles) are hypothesized to stem from the effects of particle melting combined with high pres- sures (0.41–0.77 MPa) generated by expansion of the shock front. These shock-induced textures may be overlooked in ashfall samples due to their similarity to primary fragmentation textures (Genareau et al., 2020). Although studies involving the diffusion and volatilization of chem- ical elements from ash particles exposed to electrical discharges are lim- ited (Mueller et al., 2018; Woods et al., 2021), some preliminary findings have emerged that warrant continued examination of these phenomena. By injecting ash samples of different size and composition into an artificially generated electric arc (welding arc), Mueller et al. (2018) found that some elements (e.g., F, Cl, Na) were depleted from af- fected grains compared to the starting material. They also documented foaming and bubble expansion in the grains, as observed in comparable experiments (Cimarelli et al., 2017). Extending from this study, Woods et al. (2021) subjected andesite ash to high-current impulses (7–25 kA) and found that microlite minerals (plagioclase, magnetite, etc.) within the grains were partially or completely melted. Melting of these phases and heterogenous mixing with the melted glass compo- nent created zones of chemical enrichment unique from the pure min- eral and groundmass glass compositions in pre-experimental samples. There was also clear compositional banding in post-experimental lightning-induced particles (Woods et al., 2021). The chemical effects of lightning on volcanic ash appears to be highly variable, as they are a function of not only ash composition, but also ash grain size (Wadsworth et al., 2017), peak current of the discharge, and proximity of ash to the discharge channel axis (Genareau et al., 2017). One apparent chemical impact on lightning-affected ash particles is oxidation and/or reduction of compounds present in the glass and microlite mineral phases. Previous analyses of rocks on the ground sur- face struck by cloud-to-ground lightning have shown them to have a unique magnetic signature (a.k.a. anomalous magnetization) relative to surrounding areas (e.g., Cox, 1961; Sakai et al., 1998; Verrier and Rochette, 2002). Additionally, examination of fulgurites has shown them to be reduced in some cases (Jones et al., 2005; Pasek and Block, 2009; Hess et al., 2021). An analysis of volcanic ash samples subjected to 7–100 kA current impulses, using vibrating sample magnetometry, show that the particles have a higher saturation magnetization and lower coercivity compared to pre-experimental samples (Genareau et al., 2019b). Felsic ash appeared to display more significant magnetic changes compared to mafic ash. These changes between pre- and post-experimental magnetic properties suggest that certain compounds within the ash grains are variably reduced. Further analyses of ash oxi- dation states are ongoing to constrain which compounds are responsi- ble for these magnetic changes, as results may pose important implications for ash-related hazards to electrical infrastructure and effi- cient cleaning of ashfall from metal structures. As experiments utilizing both electrical arcs and current impulses have reproduced lightning- induced textures within the laboratory, analyses on their chemical properties continue. C. Cimarelli and K. Genareau Journal of Volcanology and Geothermal Research 422 (2022) 107449 14
  • 15. 8. Effects of lightning on atmospheric chemistry and processes Besides the consequence of tephra modification by lightning, as pro- vided through current impulse experiments, the modification of the suspended ash and gases may also pose important consequences for chemical equilibria within volcanic plumes and the atmosphere. The impacts of these chemical modifications on the ambient atmosphere are a topic for future studies. Previous work, detailed below, indicate that lightning is a significant contributor to changes in the atmosphere that may affect biologic activity. Thermodynamic equilibrium models of the effect of volcanic lightning on the composition of a plume have so far neglected the chemical modification of volcanic ash (Martin and Ilyinskaya, 2011). However, affected ash particles show evidence of sec- ondary devolatilization (Fig. 12 and Fig. 13) and analyses indicate re- duction of oxide compounds, indicating that volatiles are liberated from lightning-affected ash particles. This is only one potential effect of volcanic lightning on atmospheric chemistry, and others are detailed below. 8.1. NOx fixation/ozone production It is well known that the concentration of nitrogen oxides in the at- mosphere can be strongly influenced by lightning as it is one of the major abiotic contributors to nitrogen fixation and production of NO and NO2 (i.e., NOx). In turn, through photochemical reactions, NOx species are critical for the abundance of ozone (O3) a strong oxidant which is a greenhouse gas and key absorber of ultraviolet radiation. Lightning-induced NOx (LNOx) species contribute largely to the abundance of NOx in the atmosphere, particularly in the upper troposphere, thus having important implications for atmospheric chemistry and climate (Navarro-González et al., 1998; Stark et al., 1996; Höller et al., 1999; Gerlach, 2004; Schumann and Huntrieser, 2007). The production of reactive nitrogen species is also debated as being a fundamental prerequisite for the emergence and early evolution of life on Earth. The abundant volcanic activity during the Archean and the associated generation of volcanic lightning could have played an important role in the fixation of nitrogen in a mildly reduced environment favored by volcanic activity (Navarro-González et al., 2001). Lightning, and so volcanic lightning, is further considered to be a viable mechanism for the production of reduced phosphorus (Pasek and Block, 2009), a fundamental nutrient for marine and terrestrial micro-organisms, which is otherwise almost exclusively present in geological systems in its fully oxidized state. The content of reduced phosphorus in natural fulgurites testifies to the ability of cloud-to-ground lightning as a viable contributing mechanism to the bi- ological cycle of phosphorus (Hess et al., 2021). Additionally, volcanic lightning may have locally generated the reducing conditions for prebi- otic synthesis of organic molecules (Johnson et al., 2008). Thus, addi- tional simulation experiments able to reach the high temperatures of natural lightning can be used to determine the impact on both volcanic ash chemistry and ambient atmospheric chemistry, particularly in regards to generating biologically important molecules and compounds. 8.2. Changes to ash plume convection and dispersion The previously described current impulse experiments have re- vealed that lightning discharge will change the grain size and size distri- bution of tephra produced from primary magma fragmentation. These textural changes will affect the dispersion of ash in the atmosphere, as fusing of individual grains and aggregates into larger single particles will increase the settling velocity of these lightning-induced particles relative to that of the original constituent grains (Genareau et al., 2019). Some of the smaller affected ash grains may even be destroyed based upon their proximity to the discharge channel axis, removing them completely from the total grain size distribution (Genareau et al., 2017) and potentially impacting an accurate quantification of vol- canic explosivity using distal ashfall deposits. The shock wave produced by the lightning will not only contribute to structural alteration of ash grains and aggregates but may potentially alter fluid dynamic processes (e.g., turbulent eddies) occurring in the convecting column and/or the laterally spreading plume. This effect is evidenced by high-speed video footage of current impulse experiments (Suppl. Video 3), which show expulsion of glowing ash particles tens of centimeters from their original location following discharge of a plasma Fig. 12. Experimental fulgurites. Micro- (a) and nano-tomographic (b, c) images of an experimental fulgurite showing melting features. The volcanic analogue starting material is composed of micrometric glass fibers (short axis 14 μm). Voxel in (c) has 60 μm side (after Cimarelli et al., 2017). (d) Secondary electron image showing the globular shape of an experimental fulgurite generated by an impulsive (500 ms) electrical discharge (227 A) on 63–90 μm volcanic ash from Laacher See volcano (Germany). (e) Backscattered electron image revealing the internal glassy texture of the fulgurite in (d). The bimodal population of larger and smaller vesicles (black circular structures) suggests the entrapment of external air and the degassing of volatile elements during the instantaneous melting of the ash. C. Cimarelli and K. Genareau Journal of Volcanology and Geothermal Research 422 (2022) 107449 15
  • 16. channel only a few millimeters in diameter. Although lightning dis- charge deep within the convective column may not impact fluid dy- namic processes, lightning at the column and plume margins may arrest or enhance the incorporation of ambient atmosphere (Genareau et al., 2020). Modelling the fluid dynamic effects of shock fronts gener- ated by volcanic lightning would be an interesting topic for future study as intense lightning activity may alter ash dispersion. 9. Challenges and future perspectives The electrification of volcanic plumes and the generation of volcanic lightning are common phenomena during explosive eruptions. After centuries of anecdotal reports and observations we are now in posses- sion of the appropriate technology to characterize and quantify these phenomena. However, the use of volcanic lightning detection as a rou- tine monitoring method is today still limited by three main factors: 1) a poor understanding of the electrification mechanisms at play in vol- canic plumes, including the microphysical dynamics between volcanic ash and hydrometeors; 2) the absence of systematic quantitative obser- vations relating electrical activity to the plume dynamics; and 3) the lack of sensors specifically tailored for the electrical monitoring of volcanoes. Microphysical observations derived from laboratory experiments should address the different mechanisms of particle charging under Fig. 13. Secondary electron images of ash particles following high-current (7–100 kA) impulse experiments. The various textures produced include (a), (b) spherules; (c), (d) hair-like particles; (e), (f) plate-like particles. The starting materials were ash grains <32 μm in diameter, indicating that lightning can melt and fuse multiple particles together. Of these experimentally produced textures, only spherules have been observed in natural ashfall samples in addition to spherule aggregates and pumiceous particles, which are not shown here (Genareau et al., 2020). C. Cimarelli and K. Genareau Journal of Volcanology and Geothermal Research 422 (2022) 107449 16