Invited talk presented by V. Andretta at the symposium From Atoms to Stars:the impact of Spectroscopy on Astrophysics, 26th-28th July 2011, Oxford, UK.

1. Helium line emission - Its relation to
atmospheric structure
V. Andretta
INAF - Osservatorio Astronomico di Capodimonte, Italy
Oxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 1

2. Helium line emission - Its relation to
atmospheric structure
Abstract:
The title of this talk reprises the title of a 1980 paper
by Carole Jordan on the anomalously high intensities
of helium lines when compared with lines of other
ions formed at similar temperatures. From that
starting point, I will give a historical overview of an
apparently marginal riddle in Solar Physics, a riddle
nonetheless that has intrigued many in the course of
several decades, inspiring some interesting ideas on
the structure and dynamics of the solar atmosphere.
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4. Actually, that was not the first paper
on the subject...
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6. In the beginning...
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7. Early studies
Goldberg (1939) postulated “...an excess of ultraviolet
radiation in the 500 A region;...” to explain the 1932
eclipse observation of the helium spectrum
Hirayama (1971) analyzed prominence spectra taken
during the 1966 eclipse, concluding that “...the
intensity of neutral helium can be explained in
terms of ionization due to UV radiation even if the
kinetic temperature is as low as 5000 K.”
On the other hand, calculations based on “standard”
collisional excitation at temperatures T>20000 K
(e.g.: Milkey, Heasley, Beebe 1973) typically failed
to reproduce both He and other TR lines.
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8. A paradigm emerges
Zirin (1975, “The helium chromosphere, coronal
holes, and stellar X-rays”, ApJ 199, L63) :
“...We demonstrate how the D3 [He I 5876 A]
emission, as well as the other He I and He II lines,
can be explained quantitatively by photoionization
by coronal back-radiation. A Chapman layer with
N(He)H=5x1017 [cm-2] is formed near τ=1 in the He
I and He II continua. The chromospheric He
emission or absorption is weak in coronal holes
because there is no coronal back-radiation...”
(Note: The relevant photoionization thresholds are at
504 Å for He I, and 228 Å for He II)
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9. A paradigm emerges (some numbers)
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10. Coronal holes
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11. Coronal holes
(From spectra-
spectroheliograms taken at
the Kitt Peak Vacuum Tower
Telescope)
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12. An alternative view
C. Jordan (1975): Mixing of He0, He+ atoms/ions with
“hotter” electrons could enhance the observed
(EUV) lines through sensitivity to Te of the excitation
rates (~exp(-ΔE/kTe)).
C. Jordan (1980) provided the list of ingredients for
the “mixing” recipe (in a certain class of processes),
mainly:
– Non-thermal motions
– Long ionization/recombination times
– Temperature gradient
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13. An alternative view
From the 1980 paper:
“...One can think either of the non-thermal motions
carrying the ions up the steep temperature gradient
or of an intermittent penetration down of hotter
electrons. ...”
An intrinsically “dynamical view”
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14. From the '80s to the '90s
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15. Observations, and more observations...
Daily synoptic observations of magnetic field, He I
10830 from Kitt Peak (spectra and images): 1992-
2003 (then SOLIS, 1993 to the present).
Rocket flights (e.g.: SERTS): EUV line profiles.
SOHO: SUMER, CDS, EIT...
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16. A more detailed, complex picture
emerging
Pure Photoionization- Collisional formation
Recombination [PR] (τ504~1) (T>20000 K)
Mixed formation
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17. A more detailed, complex picture
emerging
The case of the (nearly) optically thin He I 10830 line
[Andretta & Jones 1997]
PR contribution
Collisional contribution
(Further quantitative determinations of the role of EUV
coronal back-radiation, e.g.: Centeno et al. 2008)
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18. Coronal hole boundaries
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19. Coronal hole boundaries
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20. More models...
The FAL (Fontenla, Avrett, and Loeser) series of
papers: diffusion effects (ambipolar diffusion).
Figures from FAL 1993 (“Energy balance in the solar
transition region. III. Helium emission in hydrostatic,
constant-abundance models with diffusion.”)
AR (Plage)
Model FAL C
(QS)
QS
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21. Even more refined models...
Figures from FAL 2002 (“Energy balance in the solar
transition region. IV. Hydrogen and helium mass
flows with diffusion”)
Helium
Hydrogen
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22. Related issues...
FIP effect
Fig. 1 from Geiss
(1998)
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23. ...and more models...
Multifluid models, and the solar wind helium
abundance:
Hansteen, Leer, and Holtzer. (1997): “The role of
helium in the solar outer atmosphere”
Killie, Lie-Svendsen, and Leer (2005): “The helium
abundance of quiescent coronal loops”
Chromosphere
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24. Interlude: A housewarming party
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26. “Helium Line Formation in a Dynamical
Solar Atmosphere”, Naples, April 2000
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27. The last ten years (or so)
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28. Quantifying the extent of the problem
How enhanced are EUV He lines with respect to
rest of the TR spectrum?
Macpherson & Jordan
1999: “The anomalous
intensities of helium
lines in the quiet solar
transition region”
Jordan et al. 2001: “The
anomalous intensities
of helium lines in a
coronal hole”
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29. Quantifying the extent of the problem
Testing the PR
mechanism: Andretta, Del
Zanna, S. Jordan (2003):
“The EUV helium
spectrum in the quiet Sun:
A by-product of coronal
emission?”
Result: the He II 304 Å Note: The CDS radiometric
line alone emits more calibration has been recently
photons than all the revised (Del Zanna et al. 2010,
Del Zanna & Andretta, 2011), but
corona below 228 Å. the above test remains valid, if
somewhat less stringent.
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30. Quantifying the extent of the problem
Reduced network contrast in the He lines: photon
scattering in the cell centers from the boundaries:
Jordan, Smith, and Houdebine (2005), MNRAS 362,
411: “Photon scattering in the solar ultraviolet lines
of He I and He II”
Result: quite possibly
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31. A slightly more detailed version of the
mechanism proposed in the1980 paper
[Andretta et al. 2000] The relevant scaling parameter (the so-called
“velocity redistribution parameter”):
He II 304 Å O III 600 Å
QS
AR
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32. Yet another generalization
From G. R. Smith and C. Jordan, 2002, “Enhancement
of the helium resonance lines in the solar
atmosphere by suprathermal electron excitation – I.
Non-thermal transport of helium ions”:
In this formulation, though, the distinction between
quiet Sun and coronal holes is somewhat lost/hidden.
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33. The other mixing mechanism
From G. R. Smith, 2003, “Enhancement of the helium
resonance lines in the solar atmosphere by
suprathermal electron excitation – II. Non-
Maxwellian electron distributions”:
“...Enhancements of the helium resonance line
intensities are found, but many of the predictions of
the models regarding line ratios are inconsistent
with observations. These results suggest that any
such departures from Maxwellian electron
distributions are not responsible for the helium
resonance line intensities.”
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34. More ways to mix helium and “hot”
electrons
Pietarila & Judge (2004): “On the formation of the
resonance lines of helium in the Sun”
Judge & Pietarila (2004): “On the formation of the
resonance lines of helium in the Sun: Analysis of
SOHO data”:
“...We propose a new enhancement mechanism [...]
in which predominantly neutral species such as
helium diffuse across magnetic field lines into
regions of hot coronal plasma, but charged ions do
not. ...”
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35. More ways to mix helium and “hot”
electrons
Feldman, Ralchenko, and Doschek (2010): “The
effect of hot coronal electrons on EUV spectral
lines of He II emitted by solar TR plasmas”:
“...We show that although the influence of a fraction
as small as 10-4-10-3 of hot electrons on the
intensities of the C and O lines is noticeable, the
effect on the intensities of the He lines is much
larger...”
[Note: “hot”, coronal electrons are modelled here by a
second Maxwellian]
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36. Where the formation of the helium
spectrum is not a (big) problem
Prominces/filaments, for instance.
Examples:
Labrosse & Gouttebroze (2001), A&A 380, 323:
“Formation of helium spectrum in solar quiescent
prominences”
Labrosse et al., (2010), Space Sci. Rev. 151, 243:
“Physics of Solar Prominences: I – Spectra
Diagnostics and Non-LTE Modelling”
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37. Where the formation of the helium
spectrum is not a (big) problem
Active regions? Maybe so, if the enhancement
mechanism is “velocity redistribution”: lower
“turbulent” velocities, higher pressures than in the
QS.
Example: Andretta et al. (2008), ApJ 681, 650:
“Helium line formation and abundance during a C-
class flare”
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38. Where the formation of the helium
spectrum is not a (big) problem
Data set includes:
– From SOHO/CDS:
• He I 584 Å and He II 304 Å (2nd order)
• Various TR and coronal lines
– From the Horizontal Spectrograph at the
NSO/DST (Dunn Solar Telescope) at
Sacramento Peak:
• He I 5876 Å and He I 10830 Å
• Ca II H &K
• Hα
• Na D1 & D2
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39. Where the formation of the helium
spectrum is not a (big) problem
Result: First spectroscopic measurement of AHe in the
solar chromosphere:
6.5%<AHe<8.5%
Oxford, UK, 26th July 2011 From Atoms to Stars:The Impact of Spectroscopy on Astrophysics 39

40. Where the formation of the helium
spectrum is not a (big) problem
Result: First spectroscopic measurement of AHe in the
n!
io
solar chromosphere:
t
ra
lib
ca
6.5%<AHe<8.5%
S
D
C
ld
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O

41. Conclusions
No conclusion yet... because we don't fully
understand the solar atmosphere yet. But: The helium
spectrum is an excellent “stress test” for our
understanding of the chromosphere and transition
region...
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