This document discusses planetary and tidal wave oscillations observed in ionospheric sporadic E layers over Tehran, Iran. Wavelet analysis was used to examine time series data of critical frequency (foEs) and virtual height (h'Es) of sporadic E layers from July 2006 to June 2007. The results show: 1) Diurnal (24-hour) and semidiurnal (12-hour) oscillations were present in all seasons for both foEs and h'Es. Terdiurnal (8-hour) variations were also observed. 2) Planetary wave oscillations of around 2, 4, 6, and 10 days were observed, with maximum activity during equinox seasons. 3)

1. Planetary and tidal wave-type oscillations in the ionospheric
sporadic E layers over Tehran region
K. Karami,1
S. Ghader,1
A. A. Bidokhti,1
M. Joghataei,1
A. Neyestani,1
and A. Mohammadabadi1
Received 19 December 2011; revised 26 February 2012; accepted 2 March 2012; published 17 April 2012.
[1] It is believed that in the lower ionosphere, particularly in the ionospheric sporadic
E (Es) layers (90–130 km), the planetary and tidal wave-type oscillations in the ionized
component indicate the planetary and tidal waves in the neutral atmosphere. In the present
work, the presence of wave-type oscillations, including planetary and tidal waves in the
ionospheric sporadic E layers over Tehran region is examined. Data measured by a
digital ionosonde at the ionospheric station of the Institute of Geophysics, University of
Tehran, from July 2006 to June 2007 are used to investigate seasonal variations of
planetary and tidal waves activities. For the purpose of accurate comparison between
different seasons, wavelet transform is applied to time series of foEs and h′Es, namely,
the critical frequency and virtual height of Es layers, respectively. The results show that
the sporadic E layers over Tehran region are strongly under the influence of upward
propagation of waves from below. More specifically, among diverse range of
periodicities in the sporadic E layers, we found that diurnal (24 hours) and semidiurnal
(12 hours) oscillations in all seasons for both parameters. Moreover, terdiurnal (8 hours)
tide-like variation is observed during spring and summer for foEs parameter and summer
and winter for h′Es. Furthermore, the results show that diurnal tidal waves obtain their
maximum activities during autumn and winter seasons, and their activities decrease
during the late spring and summer. In addition, periods of about 2, 4, 6, 10, 14, and
16 days in our observation verifies the hypothesis of upward propagation of planetary
waves from lower atmosphere to the ionosphere. Moreover, planetary waves have their
maximum activities during equinox.
Citation: Karami, K., S. Ghader, A. A. Bidokhti, M. Joghataei, A. Neyestani, and A. Mohammadabadi (2012), Planetary and
tidal wave-type oscillations in the ionospheric sporadic E layers over Tehran region, J. Geophys. Res., 117, A04313, doi:10.1029/
2011JA017466.
1. Introduction
[2] Understanding the effects of different waves coming
from lower atmosphere into the ionosphere improves our
physical and dynamical knowledge of energy and structure
of the middle and upper atmosphere. Additionally, such
knowledge will improve the schemes for modeling the
structure of the ionosphere and prediction of its evolution for
communication applications such as radio telecommunica-
tion and electronic systems in satellites and spacecrafts.
Moreover, study of the atmosphere from the planetary
boundary layer to the upper ionosphere from the viewpoint
of wave propagation, give us this perception that we can
consider the atmosphere as a whole, in which each layer
contributes to the evolutions in other layers, rather than
classifying the atmosphere into different disconnected layers
[Nappo, 2002; Andrews, 2010].
[3] In spite of the contemporary progress in understanding
the coupling between activities of the lower atmosphere and
ionospheric parameters, the physical details of such coupling
is not yet well understood [Kazimirovsky et al., 2003].
[4] The ionosphere is formed and evolved chiefly by solar
activity. However, variability of the ionosphere due to various
phenomena in the mesosphere and stratosphere and particu-
larly in the troposphere should not be neglected. As a matter of
fact, the overall variability of the ionosphere is fairly under
control of the meteorological activities [Laštovička, 2006].
[5] Ionosondes and other ionospheric measurement tools
often detect a densely ionized layer in the E region which
does not seem to be related to the typical E layer. This
unexpected phenomenon is called sporadic layer (Es),
because the formation and strength of Es does not show any
regular and predictable behavior. The Es layers are narrow
layers (0.6–2 km thick) of dense plasma developed in the
mid latitude E region and generally in the altitude range of
90–130 km [Pancheva et al., 2003]. In spite of the other
1
Department of Space Physics, Institute of Geophysics, University of
Tehran, Tehran, Iran.
Copyright 2012 by the American Geophysical Union.
0148-0227/12/2011JA017466
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 117, A04313, doi:10.1029/2011JA017466, 2012
A04313 1 of 6

2. regular ionospheric layers, the Es cannot be explained via
Chapman theory. The apparent chaos in occurring of the Es
layers, confirms that complex mechanisms control the
behavior of these layers [Šauli and Bourdillon, 2008].
[6] Most of the studies to explain the formation of Es
layers involve a redistribution of existing charged particles
rather than producing extra ionization. On the other hand, It
is accepted that long-lasting metallic ions such as Mg+
, Al+
,
Fe+
and Mn+
form the Es layers which are frequently
observed in the ionosphere, rather than typical ionospheric
ions such as NO+
and O+
[Brasseur and Solomon, 2005].
The mathematical equation of the movement and dynamics
of charged particles in the Earth’s ionosphere is presented by
Kelly [2009]. The first plausible physical description of the
formed Es relies on the so-called “wind shear theory”
[Whitehead, 1961]. This theory proposes that the vertical
wind shear in association with ion-neutral collision and
Lorentz electromagnetic force would cause long-lasting
metallic ions convergence in the dense plasma layers. A vast
variety of phenomena originating in the troposphere can
affect the ionospheric E layer by forcing different types of
waves from below and prepare such wind shear which is
essential for Es layers formation [Haldoupis et al., 2007].
These waves can be generated by thunderstorms, typhoons,
hurricanes and tornadoes [Holton, 2004; Kazimirovsky et al.,
2003].
[7] In fact, many investigators have proven that meteo-
rological activities significantly contribute to the Es forma-
tion [Pancheva et al., 2008; Laštovička, 1997; Mayr et al.,
2001; Haldoupis and Pancheva, 2006; Manzano et al.,
1998]. This close relationship is justified mainly through
two channels: electromagnetic phenomena and upward
propagation of waves from neutral atmosphere to the iono-
sphere [Laštovička, 2006]. For example Shalimov et al.
[1999] and Shalimov and Haldoupis [2002] introduced a
mechanism in which a planetary wave is considered as a
successive high (anticyclonic) and low (cyclonic) vortices in
neutral wind . In this model the metallic ions converge and
accumulate inside areas of positive planetary wave vorticity
associated with the cyclonic neutral wind shears. In addition,
plasma depletion would be a result of anticyclonic planetary
wave winds.
[8] Different meteorological processes and periodic solar
heating and radiation cooling excite waves in the lower
atmosphere. These waves are planetary, gravity, tidal and
almost neglected infrasonic waves [Salby, 1996]. Generally,
there are two types of Rossby waves in the atmosphere:
stationary Rossby waves, and westward propagating global
free Rossby waves [Andrews et al., 1987]. Both may con-
tribute substantially to dynamic of middle and upper atmo-
sphere by preparing upward transferring of heat and
momentum from troposphere into the middle and upper
atmosphere [Salby, 1996; Holton, 2004]. The sun and moon
produce tidal forces in the atmosphere, the periods related to
the solar day (24 hours) and lunar day (24.8 hours) [Salby,
1996]. In the stratosphere, periodic absorption of solar
ultraviolet energy by ozone is major cause of tidal waves.
However, infrared energy (l > 700 nm) absorption by water
vapor in the troposphere is the key cause of tidal waves
[Brasseur and Solomon, 2005]. The effects of tides coming
from below are playing major role in the process of ion
convergence in the ionospheric E layer [Šauli and
Bourdillon, 2008]. In the present work, we focus on plane-
tary and tidal waves. These waves transport energy and
momentum from the source region to the lower ionosphere
and deposit their energy at critical layer (where phase speed
of wave is equivalent to the average speed of layer) and not
only considerably change the structure of the ionosphere, but
also change the conductivity of these layers [Andrews et al.,
1987; Kelly, 2009].
[9] It is an irrefutable fact that some phenomena have
unpredictable physical properties in nature. In order to
understand the sophisticated behavior of these phenomena,
wavelet transformation is really an appropriate tool. It
enables the detection of strength and frequency of temporal
and spatial oscillations present in the time and space series.
In other words, the wavelet transform decomposes series
into time-frequency space and enables the identification of
both dominant modes of variability and time variation of
those modes. For these reasons, wavelet techniques in the
atmospheric science and particularly in the ionospheric
research have become increasingly popular [Šauli et al.,
2007; Domingues et al., 2005; Abdu et al., 2006]. Gener-
ally speaking, wavelet analysis is applied to signals to obtain
further information about the signal that is not already
available in the raw signal [Grossman and Morlet, 1984].
2. Data Analysis, Results and Discussion
[10] In this study, data measured by a digital ionosonde
system (IPS71 of KEL Aerospace., Australia) at the iono-
spheric station of the Institute of Geophysics, University of
Tehran (35.4
N, 51.2
E) from July 2006 to June 2007 is
used to investigate the seasonal variation of critical fre-
quency of sporadic E layers ( foEs) and virtual height of
sporadic E layers (h′Es). To measure the data, the regular
vertical sounding has been done once an hour. The data are
displayed as ionograms, and to avoid the possible error due
to automatic scaling, manual scaling has been applied.
Figure 1 shows a typical form of Es layer formation that are
recorded at ionospheric station of the Institute of Geophys-
ics, University of Tehran.
[11] In the present research, the continuous wavelet
transform is applied to time series of foEs and h′Es in the
diverse range of periodicities including tidal and planetary
waves. In spite of vast variety of periodicities in both para-
meters, our results are based on those periodicities with
confidence level above 90 percent.
[12] The power spectrum of the wavelet presented in
Figures 2 and 3 illustrate different periods of tidal waves in
the time series of foEs and h′Es. Additionally, in the period
range corresponding to tidal waves the most obvious fluc-
tuation within both foEs and h′Es is diurnal (24 hours) and is
almost clear in all seasons. Also, among diverse range of
periodicities in the sporadic E layers, we found that diurnal
(24 hours) and semidiurnal (12 hours) oscillations are pres-
ent in all seasons for both parameters. However, as seen in
Figure 2 the diurnal oscillation is attenuated in late spring
and summer season. Moreover, terdiurnal (8 hours) tide-like
variation is observed during spring and summer for foEs
parameter and during winter for h′Es.
[13] In order to examine accurately the different periodi-
cities in the time series of foEs and h′Es, planetary waves are
divided into two ranges: 2–6 day period and 6–18 day
KARAMI ET AL.: WAVE OSCILLATIONS IN Es OVER TEHRAN A04313A04313
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3. period. Figures 4 and 5 show the wavelet power spectra of
the time series of foEs and h′Es indicating that their period
ranges are 2–6 days corresponding to planetary wave
activities. The most distinctive characteristics of Figures 4
and 5 are the maximum activities of planetary waves in
equinox. However, these activities are negligible during
Figure 1. A typical Es layer recorded at ionospheric station, Institute of Geophysics, University of
Tehran.
Figure 2. Wavelet power spectrum of critical frequency of
Es layers in the period of tidal wave-type oscillations.
Figure 3. Wavelet power spectrum of virtual height of Es
layers in the period of tidal wave-type oscillations.
KARAMI ET AL.: WAVE OSCILLATIONS IN Es OVER TEHRAN A04313A04313
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4. winter season. Furthermore, 5 day and nearly 2 day periods
in the wavelet power spectrum of both foEs and h′Es are
stronger than other periodicities. Figures 6 and 7 demon-
strate the power spectrum of the observed foEs and h′Es in
period range from 6–18 days, respectively.
[14] It is observed that diverse ranges of periodicities have
not continuous trend during different seasons. As seen in
Figures 6 and 7 planetary waves activities obtain their
maximum activities during equinox. Also, planetary waves
are more active in spring than autumn season. In addition,
the most pronounced planetary waves activities in this range
are 8–10 day and 14–16 periods. Table 1 presents a sum-
mary of different observed periods in foEs and h′Es para-
meters, covering planetary and tidal waves spectrum in
sporadic E layers over Tehran region. These results are
based on spectral analysis of time series of foEs and h′Es.
There is a wide range of different periods, but our results
are based on those periodicities with confidence level above
90 percent. Such oscillations have also been observed in
ionospheric F2 layer by others as Apostolov et al. [1998]
who attributed mainly to the planetary waves activities.
Figure 4. Wavelet power spectrum of critical frequency of
Es layers in the period of 2–6 day oscillations.
Figure 5. Wavelet power spectrum of virtual height of Es
layers in the period of 2–6 day oscillations.
Figure 6. Wavelet power spectrum of critical frequency of
Es layers in the period of 6–18 day oscillations.
Figure 7. Wavelet power spectrum of virtual height of Es
layers in the period of 6–18 day oscillations.
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5. These oscillations are also modulated by solar cycle while
influenced by geomagnetic activities.
[15] Our observations show that sporadic E layers over
Tehran region is strongly under the control of tidal and
planetary waves originated from lower atmosphere which
propagate energy and momentum to the lower ionosphere
and cause significant change both in formation and evolution
of sporadic E layers. There are 3 modes observed in time
series of Es layers that are connected to tidal wave effects.
These modes are diurnal (24 hours), semidiurnal (12 hours)
and terdiurnal (8 hours). However, terdiurnal variation is
missing in some cases. Moreover, planetary wave-type
oscillations are present with periodicities of nearly 2, 4, 6
and 9 days in the time series of Es parameters in different
seasons.
[16] The first basic explanation of vertical propagation of
Rossby waves is given by Charney and Drazin [1961]. For
vertically propagating Rossby waves we must have
0 u À c b
k2þl2, where u is background flow velocity, c is
zonal phase speed of Rossby waves, k is zonal wave number
and l is meridional wave number and b is the variation of
Coriolis force with latitude. For stationary Rossby waves
(c = 0) , we obtain the so-called Charney-Drazin criterion
for vertical propagation of stationary Rossby waves as
0 u b
k2þl2 which means long waves propagate vertically
under broad range of eastward flows than short waves
actually do. Moreover, during summer season stratospheric
easterlies prohibit all horizontal scales from propagating
vertically because Rossby waves encounter critical levels,
where u = c and wave activity is absorbed. It is worth to
mention that strong westward stratospheric winter flows also
prevent vertical propagation of stationary Rossby waves
[Andrews et al., 1987] and this can be an explanation of
maximum wave activity during equinox which are present in
our results.
[17] Both the occurrence and strength of Es layers have a
strong seasonal dependence by a noticeable summer maxi-
mum. This result agrees with what found by Pietrella and
Bianchi [2009], who observed a much higher percentage of
appearance and strength of Es layer over Rome during
summer months. The cause of such seasonal dependence
cannot be explained by the wind shear theory of Es layers
formation. Recently, Haldoupis et al. [2007] have given a
reasonable explanation for the seasonal dependence of mid
latitude Es layers. Their research is based on strong corre-
lation between meteoric influx and occurrence and the
strength of Es layers parameters. A meteor shower may
occur when the Earth passes near the orbital path of a comet.
It is believed that ions in the Es layers are meteor debris.
Thanks to the huge accumulation of electrons in the Es lay-
ers, the rate of recombination of O2
+
and NO+
increases. The
consequence of such high rate of recombination leads to
reduces the amount of O2
+
and NO+
. On the other hand, due
to the fact that metallic ions have relatively low recombi-
nation coefficient, their densities are relatively higher than
typical ions in the Es layers [Brasseur and Solomon, 2005].
However, in spite of recent progresses, the physical details
of such maximum are still open to question.
3. Conclusions
[18] The primary goal of this research is to investigate the
contribution of planetary and tidal waves, originated from
lower atmosphere activities, to the formation of Es layers
over Tehran region. The results show that the sporadic E
layers are very dynamic and strongly variable in their struc-
ture and intensity. In addition, there are vast variety of peri-
odicities in the foEs and h′Es including planetary and tidal
wave-type oscillations. The most distinctive periodicities are
diurnal and semidiurnal variabilities which are evident in all
seasons for both foEs and h′Es. Furthermore, terdiurnal var-
iations are present in the time series of foEs for summer and
spring seasons and in summer and winter seasons for h′Es.
In addition, planetary waves activities are present with
period ranges between 2–18 days. The most significant
planetary wave activities are 5, 10 and nearly 16 day periods.
Furthermore planetary wave activities are more noticeable
during equinox, specially, during spring season. It is
worthwhile mentioning that the results are statistically reli-
able. These results strongly support the hypothesis of forcing
the ionosphere by upward propagation of waves from below
which are mostly originated from lower atmosphere activi-
ties and slightly perturb the Es layers.
[19] Acknowledgments. The authors would like to thank University
of Tehran for supporting this research work. We also thank the anonymous
reviewers for their helpful comments on the manuscript.
[20] Robert Lysak thanks the reviewers for their assistance in evaluat-
ing this paper.
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A. A. Bidokhti, S. Ghader, M. Joghataei, K. Karami, A. Mohammadabadi,
and A. Neyestani, Department of Space Physics, Institute of Geophysics,
University of Tehran, North Kargar Avenue, Tehran, Iran. (bidokhti@ut.
ac.ir; sghader@ut.ac.ir; karami.met@ut.ac.ir)
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