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Impact factor for the ionospheric total electron content
response to solar flare irradiation
D. H. Zhang,
1,2
X. H. Mo,
1
L. Cai,
1
W. Zhang,
1,3
M. Feng,
1
Y. Q. Hao,
1
and Z. Xiao
1
Received 3 September 2010; revised 24 December 2010; accepted 1 February 2011; published 15 April 2011.
[1]On the basis of ionospheric total electron content (TEC) enhancement over the
subsolar region during flares, and combined with data of the peak X‐ray flux in the
0.1–0.8 nm region, EUV increase in the 0.1–50 and 26–34 nm regions observed by the
SOHO Solar EUV Monitor EUV detector, also with the flare location on the solar disc,
the relationship among these parameters is analyzed statistically. Results show that the
correlation between ionospheric TEC enhancement and the soft X‐ray peak flux in
the 0.1–0.8 nm region is poor, and the flare location on the solar disc is one noticeable
factor for the impact strength of the ionospheric TEC during solar flares. Statistics indicate
clearly that, at the same X‐ray class, the flares near the solar disc center have much
larger effects on the ionospheric TEC than those near the solar limb region. For the
EUV band, although TEC enhancements and EUV flux increases in both the 0.1–50 and
26–34 nm regions have a positive relation, the flux increase in the 26–34 nm region during
flares is more correlative with TEC enhancements. Considering the possible connection
between the flare location on the solar disc and the solar atmospheric absorption to the
EUV irradiation, an Earth zenith angle is introduced, and an empirical formula describing
the relationship of TEC enhancement and traditional flare parameters, including flare
X‐ray peak and flare location information, is given. In addition, the X‐ray class of the flare
occurring on 4 November 2003, which led the saturation of the X‐ray detector on
GOES 12, is estimated using this empirical formula, and the estimated class is X44.
Citation: Zhang, D. H., X. H. Mo, L. Cai, W. Zhang, M. Feng, Y. Q. Hao, and Z. Xiao (2011), Impact factor for the ionospheric
total electron content response to solar flare irradiation, J. Geophys. Res.,116, A04311, doi:10.1029/2010JA016089.
1. Introduction
[2] As one of the fastest and severest solar events, the solar
flare, which is mainly classified according to the peak flux of
soft X‐rays in the 0.1–0.8 nm region measured on the GOES
X‐ray detector, has a great influence on the earth upper
atmosphere and ionosphere. During a flare, the extreme
ultraviolet (EUV) and X‐rays emitted from the solar active
region ionize the atmospheric neutral compositions in the
altitudes of ionosphere to make the extra ionospheric ioni-
zation that causes many kinds of sudden ionospheric distur-
bance phenomenon (SID), which are generally recorded as
sudden phase anomaly (SPA), sudden cosmic noise absorp-
tion (SCNA), sudden frequency deviation (SFD), shortwave
fadeout (SWF), solar flare effect (SFE) or geomagnetic cro-
chet, and sudden increase of total electron content (SITEC)
[Donnelly, 1969; Mitra, 1974]. The mechanism and the
affected strength about these SID phenomena have been
studied thoroughly from 1960s and summarized by Mitra
[1974] and more recently by Davies [1990]. Although the
increase of electron density during a flare appears in all
ionospheric subregions, it is usually accepted that the SPA
and SCNA are closely related with the increase of electron
density in the ionospheric Dregion which is ionized mainly
by the extra sudden increase of X‐ray during solar flares, and
the strength of the SPA and SCNA have good correlation with
the X‐ray flux [Itkina, 1978; Liu et al., 1996; Thomson et al.,
2005]. Nevertheless, the increase of electron density in the
Fregion mainly due to the flare extra ionization of EUV
radiation is thought to be responsible for a large fraction of the
SITEC, so that the SITEC can be used as an index to represent
the response of the ionospheric Fregion to solar flares
[Mendillo et al., 1974].
[3] Ionospheric variation during solar flares is closely
connected with the flare additional increase of irradiation
flux from X‐ray to EUV. The study of the ionospheric vari-
ation with the flare irradiation is one of the key issues for
better understanding the photochemical process and also for
improving the accuracy of the space weather prediction.
Nevertheless, it was found that the flares exhibit obvious
difference in solar irradiation spectra even for the same level
flares classified according to the peak X‐ray flux in the 0.1–
0.8 nm region observed in GOES detector [Tsurutani et al.,
1
Department of Geophysics, Peking University, Beijing, China.
2
State Key Laboratory of Space Weather, Chinese Academy of Sciences,
Beijing, China.
3
Department of Geodesy and Geomatics Engineering, University of
New Brunswick, Fredericton, New Brunswick, Canada.
Copyright 2011 by the American Geophysical Union.
0148‐0227/11/2010JA016089
JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, A04311, doi:10.1029/2010JA016089, 2011
A04311 1of8
2009; Woods et al., 2004, 2006]. For the flares in the same
X‐ray level, the spectrum in EUV band may be significantly
different. From the point of the flare effect on ionosphere,
this difference in solar flare irradiation will certainly bring
about the obviously different ionospheric TEC response.
This brings about the difficulty for estimating the iono-
spheric response to solar flares. On the basis of the rela-
tionship between the SPA or SCNA phenomenon connected
with the sudden variation of electron density in ionospheric
Dregion and the solar extra irradiation in X‐ray band during
flares, the behavior of the ionospheric Dregion can be
revealed from the temporal variation of the soft X‐ray flux
correctly [Itkina, 1978; Thomson et al., 2005]. As for SITEC,
its complicated dependence on the solar irradiation in the
bands ranged from X‐ray to EUV makes the poor prediction
of the SITEC. Because of the flare to flare difference in
irradiation spectrum, it is difficult to estimate the value of
SITEC just on the basis of solar irradiation flux in some
limited bands [Le et al., 2007; Leonovich et al., 2010;
Tsurutani et al., 2009]. Undoubtedly, the observations in the
fine structure of solar irradiation spectra can promote the
accuracy of SITEC estimation. In recent years, many space
projects focusing on the solar irradiation in different bands
have been put forward and the data in different X‐ray and
EUV bands can be obtained gradually [Brekke et al., 2000;
Judge, 1998; Drobnes, 2005; Woods et al., 2005]. These
data conditions promote the further study of the ionospheric
response to solar flares [Tsurutani et al., 2009].
[4]Matsoukas et al. [1972] studied the correlative rela-
tionship between the solar radio bursts and SITEC by
grouping the flares according to their solar radio flux and flare
locations. They found that the flares near the solar meridian
line (flare longitude) had stronger effect on the SITEC than
the flares near the solar limb region. Furthermore, Donnelly
[1976] proved this relationship and clearly concluded that
the solar flare EUV irradiation had strong center to limb
effects, while there was essentially none for X‐rays. Through
the study of the ionospheric TEC during different solar flares,
the different responses of the ionospheric TEC to the same
level solar flares classified according to the soft X‐ray peak
flux have been noted [Afraimovich et al., 2002; Zhang et al.,
2002]. Case studies showed that besides the solar X‐ray peak
flux, the parameter of the flare location on the solar disc was
also important to reveal the effective strength of the iono-
spheric response to solar flares [Zhang et al.,2001;Zhang and
Xiao, 2003, 2005; Tsurutani, 2005; Tsurutani et al., 2006].
More recently, by comparing the SITEC values and the solar
irradiation flux increase in soft X‐ray and EUV bands during
several extreme solar flares occurred in the last solar cycle,
Tsurutani et al. [2009] examined the flare‐ionosphere con-
nection, and found the strong spectral variability from flare to
flare. Tsurutani et al. [2009] suggested that the continuous full
solar flare spectrum was necessary to understand this con-
nection. Although the concept that ionospheric response is
connected with the solar flare location is accepted, the clear
relationship between ionospheric TEC enhancement and flare
location has not been revealed yet. Undoubtedly, case studies
and statistical analysis are helpful to reveal the relationship
between ionospheric TEC enhancement and the flare location
quantitatively.
[5] During the period from 28 October to 4 November
2003, a large number of extreme solar events occurred and
triggered a nearly continuous series of geophysical dis-
turbances [Gopalswamy et al., 2005]. Particularly, two
flares on 28 October and 4 November 2003 were focused
[Zhang and Xiao, 2005; Tsurutani, 2005; Tsurutani et al.,
2006; Thomson et al., 2004; Brodrick et al., 2005]. Espe-
cially, during the flare on 4 November 2003, the GOES‐12
X‐ray detector was saturated by the extreme X‐ray flux.
Its level was first classified as X 28 by extrapolation
(http://www.swpc.noaa.gov/ftpmenu/warehouse/2003.html).
And then, on the basis of the ionospheric response in Dregion
to this flare, the flare level was estimated using the mea-
surement of SPA and SCNA effect [Thomson et al., 2004;
Brodrick et al., 2005]. The flare’s level was estimated as high
as X 45 (from SPA) and X 36 (from SCNA).
[6] As introduced above, compared with the ionospheric
Dregion response to flares, the response of the ionospheric
SITEC to flares is more complicated owing to the extra
atmospheric ionization ranging in the height from iono-
spheric Dto Fregion by wide bands of irradiation from
X‐ray to EUV. In this study, by choosing the X‐level flares
occurred in the last solar cycle from 1998 to 2006, the
relationships among the ionospheric TEC enhancement
and the flare location, flare X‐ray peak level, the SOHO
Solar EUV Monitor (SEM) EUV flux increase in the 0.1–50
and in 26–34 nm regions will be analyzed statistically.
Furthermore, an empirical formula connected the SITEC,
the flare peak flux in the 0.1–0.8 nm region and its location
on the solar disc will be developed.
2. Data and Methods
[7] The flares occurred from1998 to 2006 are sampled
under the following criteria: (1) the class of the flare is larger
than X1.0; (2) the information of flare peak flux, flare’s
beginning, peak, and ending time, flare’s location on the
solar disc are complete; (3) the time interval between flare’s
beginning and peak time is less than 30 min; and (4) the
corresponding EUV flux measurements from SOHO SEM
during every flare period are available.
[8] Then the increase of EUV flux in the 0.1–50 and 26–
34 nm regions are derived from SEM observation (available
at http://www.usc.edu/). According to the flare peak time,
the subsolar region where solar zenith angle in the earth
surface is less than 10° in the flare peak time is determined,
and the GPS data observed in this region during flare time
are collected from IGS network (available at http://sopac.
ucsd.edu). The temporal resolution of the GPS data is usu-
ally 30 s, All stations use high‐precision, dual‐frequency
GPS receivers, which can provide carrier phase and pseu-
dorange measurements in two Lband frequencies (f
1
=
1575.42 MHz, f
2
= 1227.60 MHz). Using these four mea-
surements, combining the geometrical relation of the satel-
lite, ionosphere, and receiver, a precision TEC can be derived
at every observational epoch [Lanyi and Roth, 1988;
Hofmann‐Wellenhof et al., 1992; Mannucci et al., 1998]. For
each GPS station, at least four vertical TEC values with a
different ionospheric penetration point (IPP), which is the
point of intersection of the line of sight and the ionospheric
shell (usually assumed to be 400 km), can be derived for an
interval of 30 s. Since this study is focused on ionospheric
disturbances caused by flare radiation, only relative TEC
changes or TEC enhancements during the flare are useful,
ZHANG ET AL.: IONOSPHERIC TEC RESPONSE TO SOLAR FLARE A04311A04311
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and the relative accuracy of the TEC is 0.02 total electron
content unit (1 TECU = 10
16
el m
−2
)[Hofmann‐Wellenhof
et al., 1992]. As an example for the procedure of data ana-
lyzing, the ionospheric TEC curves calculated from the GPS
data observed in GALA station (0.74°N, 269.70°E) during
the flare on 6 April 2001 were given in Figure 1. By applying
the TEC deriving method for all collected GPS data corre-
sponding to each flare, the temporal TEC curves during each
selected flare like showed in Figure 1 are obtained. After that,
the TEC enhancement values related to flare extra irradiation
are derived from each ionospheric TEC curve by removing
the influence of the background solar disc irradiation, the
final TEC enhancement value is obtained by averaging all
TEC enhancement value from all ionospheric TEC curves
obtained over the subsolar region [Zhang and Xiao, 2003].
[9] By above procedures, the data set including the
ionospheric TEC enhancement and the corresponding flare
irradiation increases in different spectral band are obtained
and all total 66 flares that meet the above constraint con-
ditions are studied in this paper.
3. Results and Analysis
3.1. Examples of the Different Impact Strength
on Ionospheric TEC for Solar Flares With
the Same X‐ray Class
[10] Figure 2 gives the ionospheric TEC curves derived
from the GPS data during six solar flares that are selected
according to their class and location information. The tri-
angles in Figure 2 mark the local noon of GPS station where
the GPS data observed. The parameters of these flares, the
corresponding ionospheric TEC enhancements and the
information of GPS site are listed in Table 1. These six
flares are classified into three groups on the basis of their
X‐ray peak flux. The two flares in each group have the same
level of the peak X‐ray flux but different solar location. The
different strength of the ionospheric TEC response to the
flares with the same X class can be seen clearly in Figure 2.
On the whole, the flares nearer to the solar center exhibit
stronger impact on the ionospheric TEC. It is known that the
solar irradiation in the band of soft X‐ray and EUV is
responsible for the ionospheric TEC enhancement due to the
sudden increase of the electron density in the whole iono-
spheric height [Donnelly, 1976; Tsurutani et al., 2009]. So
this different response revealed in Figure 2 manifests the
difference in EUV irradiation flux for every flare pair.
3.2. Statistical Relationship Among the Ionospheric
TEC Enhancement, Flares’Irradiation Flux,
and Their Location on the Solar Disc
[11] The dependence of the flare irradiation impact on
ionospheric TEC can be revealed more clearly by analyzing
the flares grouped according to their X‐ray peak level.
According to their X‐ray peak flux, the flares less than X4.0
are divided into four groups: (X1.0–X1.1), (X1.4–X1.8),
(X2.3–X2.8), (X3.0–X3.9). Figure 3 shows the relationship
between the TEC enhancement values (represented as
DTEC) and the flares longitude (relative to solar center
meridian line) during the flares in these four groups. It can
be seen that the TEC enhancement is related to the flares
longitude. Statistically, the larger the longitude, the smaller
the TEC enhancement. This dependence should manifest the
variation of EUV irradiation flux with flare location.
[12] The EUV data from SEM are often used for modeling
solar irradiation spectrum [Judge, 1998]; the data can also
be used in the solar irradiation variation during solar flare
period. Figure 4 shows the irradiation flux measurements of
soft X‐ray in the 0.1–0.8 nm region obtained from GOES
and EUV flux in the 0.1–50 and 26–34 nm regions detected
in SEM during a flare on 15 April 2001. This is another
example which indicates that the increase of the corre-
sponding solar irradiation flux related to the solar flare can
Figure 1. One example of the temporal ionospheric TEC
curves derived from the GALA GPS station during the flare
on 6 April 2001. One TECU = 10
16
el m
−2
.
Figure 2. Ionospheric TEC curves derived from the GPS
data during six solar flares listed in Table 1. Triangles
mark the local noon of the GPS station where the GPS data
were observed.
ZHANG ET AL.: IONOSPHERIC TEC RESPONSE TO SOLAR FLARE A04311A04311
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be derived from this kind of flux curves. Here, it should be
noticed that the postflare increases in the EUV band shown
in Figure 4 is due to the contamination of the flare energetic
particles impinging upon the SEM detector. In the same
way, the relationships between the TEC enhancement and
the EUV enhancement in the 0.1–50 and 26–34 nm regions
during the flares in the group (X1.0–X1.1) are given in
Figure 5, respectively. It shows that the EUV enhancement
during flares in the same X‐ray class varies greatly, ranging
from 1.4 to 3.0 unit for EUV band in the 0.1–50 nm region
and 0.1–0.6 unit for the EUV band in the 26–34 nm region
(1 unit = 10
10
photons m
−2
s
−1
). That illustrates the flare to
flare difference in different solar flare irradiation bands.
Even so, as can be expected, the TEC enhancement caused
by the flares’irradiation is positively correlated with the
EUV flux enhancement.
[13] Figure 6 shows the corresponding relationship
between the flare longitude and the EUV flux enhancement
in group (X1.0–X1.1). Similar to the TEC enhancement, the
EUV flux enhancement related to the same X‐ray class flare
varies with the solar flare longitude, and the nearer to the
solar center, the larger the EUV enhancement. It can also be
seen that the different ionospheric response exists even for
the flares with the same flare longitude and the same X‐ray
class. That also illustrates the variability of the irradiation
spectrum of flare to flare.
3.3. Relationship Among Solar Irradiation in Different
Spectral Bands
[14] From the results illustrated above, we can see that the
positive relationship between the TEC enhancement and the
EUV flux increase exists statistically but is also scattered
obviously. Figure 7 gives the relationship between the EUV
enhancement in the band of 0.1–50 nm and the EUV in the
band of 26–34 nm during the flares in the X1.0–X1.1 class.
That shows the obvious flare to flare difference in solar flare
irradiation spectrum in EUV band. Because the spectrum
that can ionize the atmospheric component ranges from soft
X‐ray to EUV, the solar flare flux in any spectral band
cannot represent the impact strength of the flare to iono-
spheric TEC effectively. So it is meaningful to give a con-
venient flare irradiation proxy by analyzing the relationship
between the TEC enhancement and the corresponding
flare’s parameters.
[15] Figure 8 shows the relationship between the TEC
enhancement and the soft X‐ray peak flux derived from all
selected solar flares in this study. Because the information
of the flares’locations is not taken into consideration, their
correlation is poor, which is understandable based on the
analysis above. Therefore, different from the flare’s effect
on the ionospheric electron density in Dregion, the soft
X‐ray peak flux parameter is not enough to describe the
strength of the sudden increase of ionospheric TEC caused
by the flare extra irradiation.
Table 1. Parameters of Flares, Ionospheric TEC Enhancement, and the GPS Site Information Used in Figure 2
YYMMDD
a
Start Time
(UT)
End Time
(UT)
Peak Time
(UT)
Flare Longitude
(deg)
Flare’sX‐ray
Class DTEC/TECU GPS Site
Site Location
Latitude (deg) Longitude (deg)
981122 1610 1632 1623 89 X2.5 0.25 areq −16.5 288.5
001224 1451 1521 1513 07 X2.3 1.87 braz −15.9 312.1
061206 1829 1900 1847 64 X6.5 2.60 ispa −27.1 250.7
011213 1420 1435 1430 09 X6.2 5.65 braz −15.9 312.1
031028 1951 1124 1110 08 X17.2 15.5 zamb −15.4 28.3
050907 1717 1803 1740 77 X17.0 3.70 bogt 4.6 285.9
a
YYMMDD, year, month, day; read 981122 as 22 November 1998.
Figure 3. Relationship between the TEC enhancement values and the flares’angle distance in four
X‐ray class groups.
ZHANG ET AL.: IONOSPHERIC TEC RESPONSE TO SOLAR FLARE A04311A04311
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[16] Figure 9 gives the relationship between the TEC
enhancement and the increases of the EUV flux in the 0.1–
50 and 26–34 nm regions for all selected flare events. It
has a better correlation than the X‐ray flux that shown in
Figure 8. It can be seen that the correlation between the TEC
enhancement and the extra increase of EUV flux in the 26–
34 nm region is much better than that of the EUV in the 0.1–
50 nm region. This gives a clue that the extra increase of
EUV flux in the 26–34 nm region can be used as a better
parameter to estimate the sudden increase of the TEC than
peak X‐ray flux. Nevertheless, compared with the flare class
represented by the soft X‐ray peak flux in the 0.1–0.8 nm
region, the EUV flux is much less provided to public so is
inconvenient to obtain. Thus, in the practical utility, this
correlative relationship may not be suitable for the mani-
festation of the flare irradiation effect on ionospheric TEC
variation.
[17] The flare class expressed in X‐ray peak flux and
its location on the solar disc are included in the flare list
report that can be easily obtained from Space Weather
Prediction Center (SWPC) web site. Donnelly [1976] owed
the so‐called center‐to‐limb effects of the EUV to the solar
atmosphere absorption. Referring to the theory of the ter-
restrial atmosphere absorption to the solar irradiation ever
used in the Chapman ionizing theory [Chapman, 1931], the
Earth zenith angle (EZA) that represents the angle between
the zenith direction of the flare location in solar surface and
the direction of the line of sight from flare location to Earth
is defined. The cosine of this angle is introduced to describe
the real EUV flux reaching to the earth atmosphere. Then,
the impact of the flare irradiation to the ionospheric TEC
can be described as a factor “ImF”that is the product of
X‐ray peak flux and the cosine of EZA. Figure 10 shows the
relationship of the TEC enhancement and the ImF. It can be
seen that the TEC enhancement exhibits a very good linear
correlation with the factor ImF. The fitting equation to
describe this relationship is as followed:
:DTEC ¼0:89 PXray cos 8ðÞcos ðÞþ0:04 ð1Þ
Figure 4. Irradiation flux measurements of soft X‐ray in the 0.1–0.8 nm region and EUV flux in the
0.1–50 and 26–34 nm regions during a flare on 15 April 2001. The units of the EUV flux are
10
10
photons cm
−2
s
−1
.
Figure 5. Relationship between the TEC enhancement and the EUV enhancement in the (left) 0.1–50
and (right) 26–34 nm regions during the flares in the X1.0–X1.1 class. The units of the EUV flux are
10
10
photons cm
−2
s
−1
.
ZHANG ET AL.: IONOSPHERIC TEC RESPONSE TO SOLAR FLARE A04311A04311
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DTEC represents the TEC enhancement due to flare extra
irradiation, the unit of DTEC is TECU. P
X‐ray
represents the
X‐ray peak flux in unit of the 10
−4
Wm
−2
,’is the solar
flare latitude, and lis the flare longitude usually given in the
SWPC flare list report. The cos(’)cos(l) is the cosine of
EZA derived according to the geometrical relationship
among the Sun, flare and the Earth. Because the flare’s
X‐ray class and its location on the solar disc are the ele-
mentary parameters in the flare list report, the empirical
relationship between the factor ImF and TEC enhancement
is useful for estimating the flare’s impact to ionospheric
TEC enhancement. However, this relationship can also be
used to deduce flare class information under some special
conditions, such as X‐ray detector failure or saturation.
3.4. Estimation of the X‐ray Class for the Flare
on 4 November 2003
[18] As mentioned in section 1, the flare on 4 November
2003 saturates the X‐ray detector in GOES 12. SWPC esti-
mates this flare class as X28 according to the X‐ray flux
tendency before the saturation. Using the long‐range VLF
measurement, Thomson et al. [2004] gave the class of this
flare X45. Also using riometer measurements at 20.1 MHz,
Brodrick et al. [2005] suggested that X38 seems to be more
suitable class for this flare. The similar estimation for this
flare can also be done using equation (1). Figure 11 is the
temporal ionospheric TEC curves obtained near the subsolar
region during this flare. The TEC enhancement value is
4.55 TECU that derived from TEC curves. According to the
SWPC flare list report, the flare location is S19°W83°. The
flare X‐ray class is calculated according to equation (1),
and the class of the flare using this estimation method is X44.
4. Discussion and Conclusions
[19] The behavior of the ionosphere during solar flares is
controlled by many factors. These factors include the solar
zenith angle, the neutral composition distribution, the ioni-
zation and recombination process, the temporal evolution of
flare burst and the solar irradiation spectrum from X‐ray to
EUV. Case studies for the ionospheric response to solar flares
in the sunlit hemisphere show that the value of SITEC is
correlative with local solar zenith angle [Zhang et al., 2002;
Zhang and Xiao, 2003, 2005]. Although the response of the
ionosphere to solar flares sometimes exhibits a little different
strength in the summer‐winter hemisphere owing to the
asymmetrical distribution of atmospheric neutral composi-
tions, on the whole, the smaller the solar zenith angle, the
larger the value of the SITEC. In our study, only the GPS
stations located in the subsolar regions in flare peak time are
selected to derive the SITEC value. This selection ensures the
consistency for the comparison of the TEC enhancement
value for different flares.
Figure 6. Corresponding relationship between the flare longitude and the EUV flux enhancement in the
(left) 0.1–50 and (right) 26–34 nm regions during the flares in the X1.0–X1.1 class. The units of the EUV
flux are 10
10
photons cm
−2
s
−1
.
Figure 7. Relationship between the EUV enhancement in
the 0.1–50 nm region and EUV in the 26–34 nm region dur-
ing the flares in the X1.0–X1.1 class. The units of the EUV
flux are 10
10
photons cm
−2
s
−1
.
Figure 8. Relationship between the TEC enhancement and
the soft X‐ray peak flux derived from all selected solar
flares.
ZHANG ET AL.: IONOSPHERIC TEC RESPONSE TO SOLAR FLARE A04311A04311
6of8
[20] Second, although the atmospheric ionization process
in all ionospheric height caused by the extra flare irradiation
is very quick, the timescale of the recombination process
is very different. The recombination timescale of the elec-
tron in ionospheric Dand Eregions where molecular ions
are predominant is an order of magnitude of minutes, but
the recombination timescale of the atomic ion‐predominant
Fregion is an order of magnitude of hours, much larger than
the timescale for the impulsive flare evolution. Therefore,
the ionospheric TEC change determined by the variation of
electron density in ionospheric Fregion is mainly controlled
by the ionization process during impulsive solar flares.
Certainly, the ionospheric TEC change is a height integra-
tion of ionization plus loss process, and the detailed loss
process is also needed to have a better understanding of the
ionospheric TEC variation caused by full solar irradiation
spectrum [Le et al., 2007; Leonovich et al., 2010; Tsurutani
et al., 2009].
[21] Third, because the current information for solar flare
irradiation is insufficient for fully revealing the ionospheric
response, the flare to flare difference in the temporal evo-
lution and in the irradiation spectrum makes the revealing of
ionospheric variation caused by solar flare irradiation much
difficult. Strictly, for a certain flare, the perfect ionospheric
model combining the real time atmospheric neutral com-
position and the high‐resolution solar irradiation spectrum is
needed to reveal the ionospheric variation during flare
period. Although the data in scattered solar irradiation data
in X and EUV band can be available from some space‐based
observations [Brekke et al., 2000; Judge, 1998; Woods et al.,
2005], it is still impossible to obtain the full spectrum data
with enough resolution to meet the demand for revealing the
ionospheric behavior during flares by ionospheric model
simulation at present. However, by statistical analysis for the
relationships among the ionospheric TEC and currently
available flare information, some clear relationship between
ionospheric TEC enhancement and the flare parameters can
be obtained. In addition, the prediction for the ionospheric
response to flare irradiation can benefit from this relation-
ship. On the basis of the analysis mentioned above, we can
make the following conclusions:
[22] The correlation between ionospheric TEC enhance-
ment and the soft X‐ray peak flux in the 0.1–0.8 nm region
is poor. The flare location on the solar disc is an important
parameter to determine the impact strength of the iono-
spheric TEC response to solar flares. Statistically, for the
flares with the same X‐ray class, the flares near the solar
disc center has stronger effect on the ionospheric TEC than
that near the solar limb region. The relationship between
TEC enhancement and the EUV flux increases in the 26–
34 nm region during a flare is more correlative than that in
the 0.1–0.8 and 0.1–50 nm bands. Given the possible con-
nection between the flare location on the solar disc and the
solar atmospheric absorption to the EUV irradiation, an
Earth zenith angle (EZA) is introduced and an empirical
Figure 9. Relationship between the TEC enhancement and the increases of the EUV flux in the (left)
0.1–50 and (right) 26–34 nm regions for all selected flare events.
Figure 10. Relationship of the TEC enhancement and the
“ImF”that is the product of X‐ray peak flux and the cosine
of the Earth zenith angle of solar flares.
Figure 11. Temporal ionospheric TEC curves obtained
near the subsolar region during the flare on 4 November
2003.
ZHANG ET AL.: IONOSPHERIC TEC RESPONSE TO SOLAR FLARE A04311A04311
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formula describing the relationship of the TEC enhancement
and traditional flare parameters is obtained. The X‐ray class
of the flares occurred on 4 November 2003 has been esti-
mated using this empirical formula, and the estimated class
is X44.
[23]Acknowledgments. The highly precise GPS data are from IGS
network. The EUV data are from the Solar Extreme Ultraviolet Monitor
on board SOHO. This work is jointly supported by the China NSFC (grants
40674089 and 40636032), China NIBRP (grant 2011CB811405), and the
State Key Laboratory of Space Weather.
[24]Philippa Browning thanks Anatoly V. Tashchilin and another
reviewer for their assistance in evaluating this paper.
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