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GEOPHYSICAL RESEARCH LETTERS, VOL. 36, L15207, doi:10.1029/2009GL039454, 2009
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Ionospheric effects of solar flares at Mars
K. K. Mahajan,1 Neelesh K. Lodhi,1 and Sachchidanand Singh1
Received 5 June 2009; revised 8 July 2009; accepted 14 July 2009; published 15 August 2009.
[1] From an analysis of electron density profiles recorded
aboard Mars Global Surveyor, we report observations of
some new and aeronomically important solar flare effects in
the ionosphere of Mars. We find that all flares result in the
formation of a well defined E layer peak, not always seen on
other days. Further, while majority of flares result in
elevated electron densities in the E region alone, some flares
affect both the E and F1 layers. These altitude - related
effects can provide vital information on the relative
enhancement of photon fluxes in the various wavelength
bands during solar flares. By using the unit optical depth
values at Mars from Fox (2004) and the XUV irradiance
model of Meier et al. (2002) for the Bastille Day solar flare,
we infer that the well defined E peaks could result from
enhancement of photon fluxes in the 10– 13 nm spectral
band. The extension of effect to the F1 layer is due to
hardening of the 26– 91 nm spectral band, as supported by
Solar EUV Monitor measurements on Solar Heliospheric
Observatory. Citation: Mahajan, K. K., N. K. Lodhi, and
Science (RS) experiment aboard Mars Global Surveyor
(MGS), were used.
[3] Several solar flares occurred during the lifetime
(24 Dec. 1998 to 09 June 2005) of RS - MGS experiment
and we found that quite a few electron density profiles, in
addition to the two reported by Mendillo et al. [2006] and the
one reported by Haider et al. [2009], were recorded during
some of these flares. While the events examined by Mendillo
et al and Haider et al exhibited the flare effect as elevated
electron densities in the altitude region 90 to 120 km alone
(i.e., Mars E region), our analysis based on all the flare
affected profiles has shown that there are other vital altituderelated effects in the Martian ionosphere. These effects can
lead to important information about the solar irradiance
during flares, since solar photons create most of the ionization at the altitude of unit optical depth and in a planetary
ionosphere this altitude varies with the wavelength of the
ionizing radiation. In this paper we report these altitude
related solar flare effects in the ionosphere of Mars.
S. Singh (2009), Ionospheric effects of solar flares at Mars,
Geophys. Res. Lett., 36, L15207, doi:10.1029/2009GL039454.
2. Data and Sources
1. Introduction
[2] Large eruptions of photon radiation on the sun, called
solar flares, can cause sudden changes in the plasma
environment of a planet. Effects of these eruptions in the
earth’s ionospheric plasma have been studied by several
workers and most of the earlier works have been described
by Mitra [1974] and by Davies [1990]. Some of the more
recent works on this topic are those of Thomson et al.
[2004], dealing with effects in the D region and those of
Meier et al. [2002], Tsurutani et al. [2005], Liu et al. [2006]
and Liu et al. [2007], dealing with effects in the ionospheric
F region and the Topside. On the other hand, there have
been only three studies dealing with the effects of solar
flares in the ionosphere of other planets, first by Kar et al.
[1986], second by Mendillo et al. [2006] and the third by
Haider et al. [2009]. Kar et al. reported enhanced electron
and ion densities in the Topside (160 to 200 km) ionosphere
of Venus during three low intensity (optical class1) solar
flares by analyzing ionospheric measurements aboard Pioneer Venus Orbiter [Colin, 1980]. Similarly, Mendillo et al.
[2006] observed elevated electron densities in the E region
(90 to 120 km) of Mars ionosphere during two intense solar
flares, while Haider et al noted enhancement of E region
electron content during a violent solar event. In both of
these works electron density profiles measured by the Radio
1
Radio and Atmospheric Sciences Division, National Physical
Laboratory, New Delhi, India.
Copyright 2009 by the American Geophysical Union.
0094-8276/09/2009GL039454$05.00
[4] In our study we have made use of two sets of data,
namely (1) electron density profiles observed by the Radio
Science experiment on the Mars Global Surveyor (MGS),
and (2) EUV and XUV photon fluxes measured by the Solar
EUV Monitor (SEM) experiment on the Solar Heliospheric
Observatory (SOHO). The Radio Science experiment on the
Mars Global Surveyor measured 5600 electron density
profiles in the Martian ionosphere between 24 December,
1998 and 09 June, 2005 on 695 different days and this
experiment has been described by Hinson et al. [1999].
Each electron density profile showed a well defined peak at
about 135 km for the primary layer (called the F1 layer).
The secondary layer (known as the E layer) often appeared
as a shoulder below the main peak between 105 to 120 km.
Several profiles were recorded on any one day at the same
solar zenith angle (SZA), the same local time and about the
same latitude but at different longitudes. There was an
interval of about 2 hours (that is the orbital period of
MGS) between each profile and thus as many as 12 profiles
were recorded on some days, though the average number
was 8. All these profiles were essentially restricted to high
latitudes and corresponded to near terminator conditions
with SZA varying between 71° and 89°. We retrieved the
MGS-RS profiles from the NASA Planetary Data system
website: ftp://pds-geosciences.wustl.edu/mgs-m-rss-sdp-v1,
courtesy of Dr. D. Hinson. These profiles have been used by
several groups to study some important aspects of Mars
ionosphere. A brief summary of these works has recently
been presented by Fox and Yeager [2006] and Mahajan et
al. [2007].
[5] The SEM - SOHO experiment has been described by
Judge et al. [1998]. SEM measures EUV flux integrated in
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effects however, varied from one flare to the other as will
be clear from the following examples.
[7] Figure 1 shows the time series of 15 s - averaged
EUV and XUV fluxes as observed on Earth during three of
the solar flares identified by us from the SOHO/SEM data.
These fluxes continued to be above normal for more than
1.5 hours during all these three flares. The time scale on the
X-axis has been adjusted by 4.5 minutes to compensate for
the extra time taken by solar photons to reach Mars. These
photon fluxes will reduce approximately to 43% of these
values after reaching Mars. The vertical dashed line on Xaxis for each day indicates the time at which electron
density profile was recorded during the XUV flare. To
examine the effect of these solar flares, all the profiles
recorded on each flare day are plotted in Figure 2 (left).
Lines with filled circles indicate the flare-time electron
densities while the thin-full lines exhibit profiles recorded
at other times on the respective day. Universal times, local
times, solar zenith angles and latitudes for these profiles are
also shown. It can be immediately noted that electron
densities were enhanced nearly at all altitudes during these
three flares, thereby indicating that all regions of the
Martian ionosphere including the E (90 to 120 km), F1
(120 to 140 km) and Topside (140 km and above) were
Figure 1. Time series of solar EUV, XUV photon fluxes
for 24 Nov., 2000, 29 May, 2003 and 31 May, 2003 as
measured by SEM-SOHO. The dashed vertical line in each
panel indicates the time when electron density profile was
recorded by MGS during the flare. The time scale on X-axis
has been adjusted to compensate for the extra 4.5 minutes
required for the solar photons to arrive at Mars. The fluxes
at Mars will be about 43% of these fluxes measured at
Earth.
the wavelength band 26 to 34 nm and the XUV flux
integrated in the wavelength band 0.1 to 50 nm. These are
the wavelengths which are indeed responsible for the
creation of a planetary ionosphere [e.g., Bauer, 1973].
The SEM-SOHO fluxes have been used in several studies
dealing with the earth’s ionosphere, especially for solar flare
effects [e.g., Meier et al., 2002; Tsurutani et al., 2005; Liu
et al., 2007]. In our analysis, we used the 15-second
averaged SEM/SOHO fluxes taken from the website: ftp://
pds-geosciences.wustl.edu/dep/space-science/semdata.html,
courtesy of Dr. D.L. Judge and Dr. H.S. Ogawa.
3. Response of Mars Ionosphere to Solar Flares
[6] As stated before, several solar flares occurred during
the lifetime of RS-MGS experiment. We first identified
these flares by surveying the daily plots of 15 s - averaged
XUV fluxes measured by SEM/SOHO on days for which
MGS-RS electron density profiles were available and then
examined all the profiles which were recorded on each of
these flare days. Despite the fact that the MGS data had a
time resolution of about two hours, and that flares occur
occasionally, our search rewarded us with quite a few
profiles which were recorded during some of these XUV
flares. Not surprisingly, we found that all flares, which were
visible both at Earth and at Mars, affected the Martian
ionosphere. The altitude region which exhibited these
Figure 2. (left) Electron density profiles observed by the
MGS on each of the three flare days. Profile recorded
during solar flare, on each of these days, is drawn with filled
circles. Elevated electron densities through out the Martian
ionosphere can be noted for all the three flare-time profiles.
(right) Flare-time profile is also compared with the average
of non-flare profiles for the respective day (along with one
standard deviation). Well formed E peaks can be noted in all
the flare-time profiles. These peaks are not always seen in
the non-flare profiles.
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Table 1. Solar Flares and Martian Ionosphere
S.N.
1
2
3
4
5
6
7
Date
Active
Region
E-S-M
Angle
Class/
Importance
24 Nov. 2000
02 April 2001
10 April 2001
15 April 2001
29 May 2003
31 May 2003
13 May 2005
N21W14
N16W62
S23W09
S20W85
S06W37
S07W65
N12E11
102.6
32.9
28.2
25.6
31.2
30.4
62.5
X18/2N
X11/1F
X2/3B
X14/2B
X12/2B
M9/2B
M8/2B
Region
Affected
E, F1
E
E
E
E, F1
E, F1
E
affected. These effects can be better appreciated by looking
at Figure 2 (right) where the flare affected profiles are
compared with the average of the non-flare profiles for
the respective day. Further, in addition to the elevated
electron densities, we note another feature in the flare
affected profiles. This is the formation of a well defined E
peak. This peak is not always seen in the other non-flare
profiles, as can be observed from Figure 2.
[8] Relevant information about these flares (along with a
few other flares to be discussed subsequently in the text) is
given in Table 1, which contains: (1) Date, (2) Heliocentric
coordinates of active region – taken from NOAA’s Space
Environment Center, (3) Earth-Sun-Mars Angle, with Earth
leading in all cases, (4) Class, X-ray as well as optical and
(5) Ionospheric region affected. It may be noted that the flare
of 24 Nov. 2000 occurred when Earth-Sun-Mars angle was
rather high (=102.6). However, the heliocentric longitude of
the active region was 14°W, so the flare was visible at Mars,
as well. Further, the electron density profile was recorded
when the XUV flux was quite high and was nearly coincident in time with the maximum phase of the flare, as can be
noted from Figure 1. Consequently, for this particular event,
the flare effect was quite prominent at all the altitudes.
[9] Another profile which was recorded on 24 Nov. 2000
at 04:23 U.T. showed flare-like features though the altitudes
of E and F1 peaks for this profile were displaced downward
by about 8 to 10 km. This profile is also plotted in Figure 2.
No flare, that was visible both at Earth and Mars, occurred at
that time. We assume that this profile could have resulted
from a flare which was visible at Mars alone. The Earth-SunMars angle was quit high on this day. The downward
displacement of the peak altitudes of E and F1 layers could
possibly be due to the zonal variations of these altitudes in
response to the migrating and non-migrating tides in the
Martian atmosphere [e.g., Bougher et al., 2001].
[10] The EUV and XUV fluxes for the flare affected
profiles of 29 May, 2003 and 31 May, 2003 were not as
large as those for the flare affected profile of 24 Nov. 2000.
In fact the XUV fluxes were barely around 10% and 25%
respectively above the normal values at the time when the
flare time profiles of 29 May, 2003 and 31 May, 2003 were
recorded. The absence of strong fluxes, perhaps, did not
result in any significant increase in the electron density in
the small altitude range from 120 to 130 km on these two
days. This region is the base of the Martian F1 layer and is
formed by solar X-rays and EUV photons.
[11] We found that in majority of cases, solar flares
affected only the E region of the Martian ionosphere.
Figure 3 shows four such cases and Table 1 contains
relevant information about these flares. Figure 3 (left)
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exhibits electron density profiles observed on these four
days. As in Figure 2, electron densities observed during
flares are shown by filled circles, while the non-flare time
profiles for the respective day are shown by thin-full lines. It
can again be observed that each of the flare-time profile
exhibits a well formed E peak at about 115 km, not always
seen in the non-flare profiles. Further, whole of the E
region, covering the altitude range 90 to 120 km, shows
elevated electron densities, as also observed by Mendillo et
al. [2006] in the two events (i.e., 15 April, 2001 and 26
April, 2001) examined by them. The data for 15 April, 2001
is also shown in Figure 3. The EUV and XUV fluxes for
each of these flare days are shown in Figure 3 (right). The
vertical dashed line on the X-axis for each day indicates the
time at which the electron density profile was recorded
during the XUV flare. It may be noted that all the flare-time
profiles were recorded during the decay phase, and the
XUV and the EUV fluxes for all these profiles were well
above the preflare values. It is perhaps necessary to mention
that EUV and XUV fluxes measured by SEM/SOHO have
sometimes been corrupted due to the impinging of energetic
particles upon the SEM detectors [Tsurutani et al., 2005].
Figure 3. (left) Electron density profiles observed by the
MGS on each of the four flare days. Profile recorded during
the flare, on each of these days, is drawn with filled circles.
Elevated electron densities in the altitude range 90 to
120 km, alone for all the four flare time profiles, can be
noted. Majority of flares affected this region alone. (right)
Further, well formed E peaks can be noted in all the flaretime profiles. These peaks are not always seen in the nonflare profiles.
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Figure 4. A comparison of relative changes in N2 (N is
electron density) and solar ionizing flux in the relevant
spectral bands. Subscripts f and 0 are respectively for flare
time and preflare values.
This phenomenon can be noted in the EUV and XUV plots
of 15 April, 2001, in Figure 3.
[12] The day time E and F1 layers of the Martian
ionosphere are expected to be in photochemical equilibrium
[Bauer, 1973], given by the equation Q = a N2, where Q is
the electron (ion) production rate, a the dissociative recombination coefficient of the dominant ion O+2 and N the
electron (ion) density. Since Q is directly proportional to
the solar ionizing flux (=F), then the flare-time increase in
electron density should be related to the flare-time increase
in the ionizing flux, F. We have examined this relationship
in Figure 4 by plotting Ff/F0 against (Nf/N0)2 at 115 and
135 km (the respective altitudes of E and F1 peaks). The
subscripts f and 0 stand respectively for the flare time and
preflare values of the parameters concerned. Although the
sample is small, yet one sees a positive correlation between
the flare time increase in electron density and the ionizing
flux in the relevant spectral band. It is also seen, that small
increases in the integrated fluxes, result in large changes in
electron density at the E and F1 peaks. This feature can only
be explained if photon fluxes at some specific wavelengths
(like He 304) get relatively more enhanced than at the rest
of the wavelengths.
4. Discussion
[13] A unique feature noted in the flare affected profiles
was the formation of a well defined E region peak. This
peak can show up either due to the formation of a minimum
between the E and F1 peaks or due to the creation of
relatively more ionization at the altitude of E peak [see also
Fox, 2004]. Since most of the energy of the solar photons is
deposited at the altitude where the optical depth is unity, the
wavelengths responsible for the ionization at the E peak can
be inferred from a knowledge of unit optical depth. Fox
[2004] has calculated the altitude ranges as a function of
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wavelength for unit optical depth at Mars with her standard
60° SZA model. She found that for the altitude ranges 105–
110 km, 110– 115 km and 115– 120 km, these spectral
ranges would be 5.5 to 7.5 nm, 7.5 to 10.0 nm and 10.0
to 13.0 nm respectively. These altitude ranges are expected
to be somewhat lower for higher solar zenith angles (say
for example, 110– 115 km for 10.0 to 13.0 nm). The flareaffected profiles were observed at SZA between 71° and
78°. Thus if photon flux for wavelengths longer than
10.0 nm gets relatively less enhanced during solar flares,
a minimum in electron density can be formed above 110 km
thereby creating an E peak. Alternately, a well defined E
peak may be formed, if the photon flux in the wavelength
range 10.0 to 13.0 nm gets relatively more enhanced during
a solar flare.
[14] Meier et al. [2002], by analyzing the effects of
Bastille Day flare of July 14, 2000 on the dayglow and
ionosphere data at Earth, generated a solar irradiance model
with a spectral resolution of 1 nm. In this model the
irradiance during this flare increased from a factor of 1.5
at 10.0 nm to a factor of about 10 at 14.0 nm. Since the
major enhancement in the solar ionizing flux during the
Bastille Day flare according, to Meier et al’s model was in
the spectral range 10.0 to 13.0 nm, it seems to us that the
formation of a well defined Martian E peak could be due to
production of relatively more ionization at altitudes between
110 and 115 km. However, this argument is only valid if
Meier et al’s model is uniformly applicable to all the flares.
[15] The second important effect observed by us is the
presence of elevated electron densities nearly at all altitudes
in the Martian ionosphere, covering the E, F1 and Topside,
during some flares. While the ionosphere E region at Mars
is created by soft X-ray, in the wavelength band 2 to15 nm,
the F1 and Topside are formed due to ionization by wavelengths in the band 26 to 91 nm [Bauer, 1973]. The EUV
fluxes in the wavelength band 26 to 34 nm have indeed
shown values, somewhat above the non-flare values when
the flare-time profiles were recorded and thus are responsible for the effect in F1 layer and Topside. It has been
observed that during solar flares fluxes in some wavelengths
get relatively more enhanced than at others and Meier et al’s
model indeed shows that.
5. Conclusions
[16] An analysis of electron density profiles measured at
Mars during solar flares shows three major effects;
(1) formation of well defined E peaks, not always seen on
other days (2) elevated electron densities in the E region
alone in majority of the flares and (3) elevated electron
densities both in the E and F1 regions during some flares.
By using the unit optical depth values at Mars from Fox
[2004] and the XUV irradiance model of Meier et al. [2002]
for the Bastille Day solar flare, we infer that the well
defined E peaks could result from enhancement of photon
fluxes in the 10– 13 nm spectral band. The extension of
effect to the F1 layer is due to hardening of the 26– 91 nm
spectral band, as supported by SEM - SOHO measurements.
[17] Acknowledgments. Authors are grateful to the MGS and SOHO
teams for making the data from Radio Science and Solar EUV Monitoring
experiments respectively available at their websites. K.K. Mahajan is
thankful to the Indian National Science Academy for the award of INSA
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Senior Scientist scheme. N.K. Lodhi acknowledges with thanks financial
assistance from a project funded by ISRO’s PLANEX programme, coordinated by PRL, Ahmedabad.
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N. K. Lodhi, K. K. Mahajan, and S. Singh, Radio and Atmospheric
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