Ionospheric solar flare effects monitored by the ground-based GPS receivers: Theory and observation
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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 109, A01307, doi:10.1029/2003JA009931, 2004
Ionospheric solar flare effects monitored by the ground-based GPS
receivers: Theory and observation
J. Y. Liu1 and C. H. Lin2
Institute of Space Science, National Central University, Jungli City, Taoyuan, Taiwan
H. F. Tsai3
Radio Science Institute for Space and Atmosphere, Kyoto University, Uji, Japan
Y. A. Liou4
Center for Space and Remote Sensing Research, National Central University, Jungli City, Taoyuan, Taiwan
Received 11 March 2003; revised 4 August 2003; accepted 7 October 2003; published 24 January 2004.
[1] The ionosphere responses to a solar flare observed by using ground-based receivers of
the global positioning system (GPS) are investigated in this paper. Two quantities, the total
electron content (TEC) and its time rate of change (rTEC), can be derived from the
receivers. The theoretical studies show that the rTEC is related to the frequency deviation
of the GPS signals. Meanwhile, worldwide ground-based GPS receivers are employed to
derive the TEC and associated rTEC to monitor the ionospheric solar flare effect on
14 July (Bastille Day) 2000. It is found that ionospheric solar flare effects can be observed
from predawn to postdusk regions, and the most pronounced signatures appear in the
midday area. The agreement between theoretical predications and observations
demonstrates that the TEC is suitable to monitor the overall variations of flare radiations
while the rTEC is capable to detect sudden changes in the flare radiations. INDEX TERMS:
2435 Ionosphere: Ionospheric disturbances; 2479 Ionosphere: Solar radiation and cosmic ray effects; 7519
Solar Physics, Astrophysics, and Astronomy: Flares; 2423 Ionosphere: Ionization mechanisms; 2494
Ionosphere: Instruments and techniques; KEYWORDS: ionosphere, solar flare, GPS, TEC, Bastille Day
Citation: Liu, J. Y., C. H. Lin, H. F. Tsai, and Y. A. Liou (2004), Ionospheric solar flare effects monitored by the ground-based GPS
receivers: Theory and observation, J. Geophys. Res., 109, A01307, doi:10.1029/2003JA009931.
1. Introduction 1972], sudden enhancement/decrease of atmospherics [Sao
et al., 1970], and sudden increase in total electron content
[2] A solar flare is a sudden brightening in an active (TEC) [Mendillo et al., 1974; Davies, 1980]. Meanwhile,
region usually near a complex group of sunspots of the Ohshio [1964] studied geomagnetic field strengths response
photosphere, which produces immediate increases in the to solar flares, which are termed the geomagnetic solar flare
ionospheric ionization of varying degrees at different effects, by global ground-based magnetometers. Liu et al.
heights, together called the Sudden Ionospheric Disturban- [1996a] further estimated ionospheric electron density
ces (SIDs) or the ionospheric solar flare effects [Dellinger, changes at about 90 km altitude by examining simultaneous
1937]. The disturbances have important effects on radio measurements of ground-based geomagnetic field strengths
communications and navigations over the entire radio and space based flare X-ray radiations during solar flares.
spectrum [Davies, 1990]. Davies [1990] reviewed that SIDs [3] To simultaneously monitor a large area of the iono-
were generally recorded as the short wave fadeout sphere, the global positioning system (GPS) is ideal to be
[Stonehocker, 1970], sudden phase anomaly [Jones, 1971; employed. The system consists of more than 24 satellites,
Ohshio, 1971] sudden frequency deviation (or frequency distributed in six orbital planes around the globe at an altitude
shift; Doppler shifts) [Donnelley, 1971; Liu et al., 1996a], of about 20,200 km. Each satellite transmits two frequencies
sudden cosmic noise absorption [Deshpande and Mitra, of signals ( f1 = 1575.42 MHz and f2 = 1227.60 MHz).
1 Since the ionosphere is a dispersive medium, scientists are
Also at Center for Space and Remote Sensing Research, National
Central University, Jungli City, Taoyuan, Taiwan. able to evaluate the ionospheric effects with measurements
2
Also at High Altitude Observatory, National Center for Atmospheric of the modulations on carrier phases and phase codes
Research, Boulder, Colorado, USA. recorded by dual-frequency receivers [Sardón et al., 1994;
3
Now at National Space Program Office, Hsinchu, Taiwan. Leick, 1995; Liu et al., 1996b]. From recorded broadcast
4
Also at Institute of Space Science, National Central University, Jungli
City, Taoyuan, Taiwan.
ephemeris and given subionospheric height, the slant TEC
along the ray path can be converted into the vertical TEC at
Copyright 2004 by the American Geophysical Union. its associated longitude and latitude [cf., Tsai and Liu,
0148-0227/04/2003JA009931$09.00 1999].
A01307 1 of 12A01307 LIU ET AL.: IONOSPHERIC SOLAR FLARE EFFECTS MONITORED BY GPS A01307
[4] In this paper, in addition to the vertical TEC derived [Ohshio, 1964; Matsushita and Campbell, 1967; Liu et al.,
from ground-based GPS receivers, we introduce the time rate 1996a]. On the other hand, TEC derived from ground-based
of its change (rTEC) as a new quantity to simultaneously satellite receivers have been used to estimate the upper and/
monitor ionospheric responses to solar flares. The TEC and or integrated ionospheric N changes during soar flares.
rTEC derived from worldwide GPS receivers are employed Owing to limited satellites and their receivers available, in
to monitor the ionosphere response to a large solar flare event early years the most common technique to study the iono-
on 14 July (Bastille Day) 2000 and some other smaller events spheric solar flare effects is to examine Doppler shift f in
(see Appendix A). Finally, the flare signatures of the two signals transmitted by Doppler sounding systems. On the
quantities in dawn, daytime (midday), dusk, and nighttime basis of a collisionless Appleton formula [e.g., Budden,
(midnight) regions are examined and discussed. 1985] and neglecting the contribution of the geomagnetic
field, and dropping the transport term, during the flare
2. Theory and Model occurrence the shift f can be approximated by [cf., Liu
et al., 1996a]
[5] In this section, we not only examine the physical
presentations and meanings of the GPS TEC and rTEC but Z Rx
f @m
also develop links between the two quantities and previous f ¼ ðQ LÞds; ð4Þ
cm Tx @N
observations. In the ionosphere, the rates of change of the
electron density N can be expressed by the continuity
where Tx and Rx denote the transmitter and receiver
equation [Davies, 1990, p. 65],
antennas, f is the transmitted radio wave frequency, c is light
speed in free space, m is the refractive index in the
@N
¼ Q L r ð N vÞ; ð1Þ ionosphere, and s denotes the integration along the radio
@t wave path. Note from equations (1) and (4) that the f is
proportional to the time rate of change of electron density
where Q and L represent the rates of the electron production
N/t. Owing to the high frequency (HF) used, the
and loss of the photochemical processes, v is the electron
Doppler sounding system observation, however, suffers
velocity, and therefore the divergent term is due to the
from the short wave fadeout, and often no data can be
transport. The rate of production is mainly a function of the
recorded even during the midway of the flare occurrence
solar X-ray and EUV radiations [Ratcliffe, 1972]. The rate
[e.g., Davies, 1990].
of loss is determined by the recombination constant, which
[7] The transmitted frequencies in UHF (f1 = 1575.42 MHz
can be generally given by
and f2 = 1227.60 MHz) are much greater than the iono-
spheric collision frequencies, and therefore the ionospheric
L ¼ aN 2 þ bN ; ð2Þ absorption (signal fadeout) effects for the GPS signals are
minor. Note that if let Tx be the satellite onboard transmit-
where a and b are the recombination constants in the ting antenna, equation (4) can be fully adopted by the GPS
ionospheric E and F1 regions, respectively. Since the observation. Thus, scientists can use the TEC and f
photochemical process is much faster than the transport obtained from ground-based GPS receivers to continuously
during the occurrence of a solar flare, the divergent term in monitor the ionosphere response to solar flares. To further
equation (1) can be neglected and the change of the electron understand the two quantities, we examine their physical
density at a certain altitude can be expressed as meanings.
Z t
[8] The TEC between a GPS satellite (Tx) and a receiver
N ¼ ðQ LÞdt; ð3Þ (Rx) can be expressed as
t0
Z Rx
where t0 denote a certain time before the solar flare. It can STEC ¼ Nds; ð5aÞ
Tx
be seen that equation (2) is a second-order polynomial,
which results in the integration of equation (3) being where s denotes the integration path along the ray, and
nonlinear and very complex [Liu et al., 1996a]. therefore the TEC change STEC during a solar flare event
[6] Although the change of electron density in equation (3) is given by
is the most direct index showing the ionospheric solar flare
effect, very limited observation instruments can be routinely Z Rx
employed and operated [Mitra, 1974]. For instance, STEC ¼ Nds: ð5bÞ
incoherent scatter soundings are suitable to monitor both Sat
lower and upper ionospheric electron density variations,
however, most of these observations are not continuously For practical data reduction, a simple way detecting the
operated. Thus three observations, magnetic field fluctua- vertical TEC change during a solar flare is to use STEC(t0),
tions, total electron content (TEC) changes, and radio which is observed slightly before the flare onset at time t0,
wave frequency shifts, have often been used to evaluate as a reference to offset its after. It thus can be written as
ionospheric electron density variations during solar flares.
For the lower ionosphere, ground-based magnetometer TEC ðt Þ ¼ ½STEC ðt Þ STEC ðt0 Þ M ð6aÞ
measurements have been employed to estimate the N at
about 90 km altitude [Liu et al., 1996a]. However, this where M = h/s is the projection factor or mapping
estimation often suffers from other geophysical disturbances function, and h and s are the altitude of the ionospheric
2 of 12A01307 LIU ET AL.: IONOSPHERIC SOLAR FLARE EFFECTS MONITORED BY GPS A01307
Figure 1. Locations of the subsolar point (star symbol) and 60 GPS receivers (solid triangle).
point and distance between the ionospheric point and the be well correlated to each other. Meanwhile, for data
ground-based GPS receiver, respectively. Since the Earth’s processing, the rTEC has been defined by subtracting each
rotation and GPS satellite orbit are one sidereal day (1 sd = TEC (or TEC in equation (6a) or (6b)) from its previous
23 hours 56 min) and one-half sidereal day (1/2 sd), 30-s value, which is a simple 2-point differentiation. The
respectively, Hernandez-Pajares et al. [1997] developed the simple 2-point differentiation meets physical sprit of
high-resolution TEC monitoring method to derive the equation (7a), which builds a connection (physical equiva-
difference in TEC along the same geometric path. Thus lence) between current rTEC study and previous f works
an alternative way obtaining the vertical TEC change can be (see the papers listed in the works of Mitra [1974] and
written as Davies [1990]). Based on equations (6) and (7), the TEC
monitors the overall time variations of the flare X ray and
TEC ðtÞ ¼ ½STEC ðt Þ STEC ðt þ k sd Þ M : ð6bÞ EUV radiations (or integrated ionospheric electron density)
and the rTEC instantaneously registers their time rate
where k is an integer. changes (or sudden changes).
[9] Liu et al. [1996a] study the ionospheric solar flare
effects observed by a Doppler sounding system and find
that the change rate of the flare radiations dramatically 3. Observation
affects the ionospheric frequency deviation (i.e., Doppler [10] The solar flare originated near the center of the solar
shift) f. For the GPS signals, the Doppler shift f is made disk, and its brightness started at 1003 UT, peaked at 1024
up of two parts: (1) a part due to the motion of the satellite UT, and ended at 1043 UT on 14 July (the Bastille Day)
with respect to the receiver, and (2) a part due to the rate of 2000. The X57 flare has been categorized as an X-class
change of the total electron content dTEC/dt (or rTEC) flare, a classification reserved for the most powerful flares.
along the path, which can be expressed as (for detail see Sixty ground-based receivers of the international GPS
Davies [1990]) service (IGS) are subdivided into four tracking networks
to globally observe the ionospheric TEC variations in the
vl 40:3 dTEC 40:3 dTEC 40:3 TEC dawn (4 42N, 269 324E), daytime (17 62N,
f ¼ f ms þ ffi ffi
c cf dt cf dt cf t 324 97E), dusk (14 36N, 97 144E), and
40:3 nighttime (36 40N, 144 269E) regions (Figure 1).
¼ rTEC; ð7aÞ
cf The subsolar point of the Earth’s ionosphere (denoted by the
star symbol) is located at about (23.5N, 22.5E geographic).
i.e., Note that all the GPS quantities TEC (or TEC) and rTEC
in this study have been properly converted into their
cf f vertical component and location [cf. Liu et al., 1996b; Tsai
rTEC ffi ; ð7bÞ and Liu, 1999]. Since no time series data of EUV radiations
40:3
are available, solar X-ray radiations in the 1-min time
where ms is the refractive index at the satellite and vl is the resolution recorded by the geosynchronous operational
line-of-sight component of the GPS satellite velocity. With a environmental satellite GOES-10 are examined. Figure 2a
relatively slow and constant speed of the GPS satellite, the illustrates the solar X-ray flux intensities in 1 8 Å on 13
first term in equation (7a) is about a small constant and a and 14 July 2000, the one on the Bastille Day reaches a
sudden Doppler shift is mainly caused by temporal changes maximum at 1024 UT. To visualize the global flare
in TEC. Since c and f are constant, the rTEC and f should responses, Figures 2b and 2c display the sum of the all
3 of 12A01307 LIU ET AL.: IONOSPHERIC SOLAR FLARE EFFECTS MONITORED BY GPS A01307
Figure 2. The X-radiations 1 8 Å recorded by the GOES-10 on 13 (the reference day), and 14 (the
Bastille Day) July 2000 (a), and the sum of the TEC (b), and rTEC (c) on each day and their differences
from the sixty GPS receivers.
recorded (about 5 10 satellites 60 receivers) total and 5 illustrate the TEC(7/14) and TEC(7/14-7/13)
electron content TEC(7/13) on 13 and TEC(7/14) on 14 together with their associated averages over all the receivers
July 2000, and the difference between the two days, TEC(7/ from the five GPS satellites during 1000 1100 UT,
14– 7/13) as well as the associated rTEC, respectively (1 respectively. It is found that the averaged TEC(7/14) of
TECu = 1016 el m2). Recall that the TEC is deduced from satellite PRN 20 and 29 (PRN 11, 15, and 31) reach their
GPS signals recorded by a receiver every 30 s, while the maximum (saturated or flatting) values at about 1029 UT
rTEC is obtained by subtracting each TEC from its previous (Figure 4), while almost all the averaged TEC(7/14-7/13)
value, i.e., a simple 2-point differentiation. It can be seen of the five satellites yield clear maximum features at about
that during 1000 1100 UT the flare signatures in the 1029 UT. Since the five curves yield similar and consist
TEC(7/14) and TEC(7/14– 713) are slightly different in features, TEC(7/14-7/13) obtained by applying the high
shape but generally have similar tendencies which reach resolution method of Hernandez-Pajares et al. [1997] has a
their maxima around 1029 UT. Note that the flare signa- better chance than the simple offset TEC(7/14) to remove
tures in the two TEC measurements are relatively small and background contributions for monitoring flare features in
somewhat difficult to be identified. By contrast, the asso- the vertical GPS TEC variations.
ciated rTEC(7/14) and rTEC(7/14 – 713) yield nearly iden- [12] Figure 6 presents the solar X-ray flux intensities, and
tical and rather clear spike flare signatures at about 1014 the averaged TEC(7/14-7/13) and rTEC(7/14) per satellite
1031 UT. Thus to avoid unwanted features from 13 July, we and per receiver (for simplicity, TEC and rTEC, hereafter)
simply use rTEC(7/14) to monitor the ionosphere response of the four regions during 1000 1100 UT. The most
to the solar flare. pronounced solar flare effects in TEC and rTEC appear in
[11] To understand the solar flare signatures in TEC in the daytime region but no signature in the nighttime region.
further detail, we investigate the vertical TEC(7/14) and In the dawn, daytime, and dusk regions, TECs yield only
TEC(7/14– 7/13) (defined by equations (6a) and (6b), one peak at about 1029 UT, while each associated rTEC
respectively) observed by the dayside receivers from five reveals three spikes at 1019, 1024, and 1027 UT, respec-
GPS satellites, PRN 11, 15, 20, 29, and 31. Figure 3 tively. Variations in the daytime TEC are similar to those
displays traces of the ionospheric points of the five GPS of the solar X-ray flux intensity, while the dawn and dusk
satellites observed by the dayside receivers. To remove the TECs yield a ledge and a maximum at about 1029 UT and
background contributions, each vertical TEC has been have increase and decrease trends, respectively. Based on
offset at 1000 UT (see equation (6a) and let t0 = 1000 UT). equations (5) and (7), the rTEC value is related to the
For example, each high resolution TEC(7/14 – 7/13) is associated TEC/t (or integrated N/t). Thus due to
obtained by offsetting TEC(7/14) and TEC(7/13) first, the increase and decrease of the flare radiations, the daytime
and then carry out the one-to-one subtraction. Figures 4 rTECs yield positive and negative values before and after
4 of 12A01307 LIU ET AL.: IONOSPHERIC SOLAR FLARE EFFECTS MONITORED BY GPS A01307
Figure 3. The dayside satellite traces of PRN 15, 29, 31, 11, and 20 during 1000 1045 UT on the
reference day or the Bastille Day. The circles show the ionospheric points of satellite-receiver ray paths at
1000 UT.
1029 UT, respectively. Similarly, gradual increases and the rTEC and associated rTEC/TECt at 1024 UT. Note that
decreases of the solar radiations during the dawn and dusk TEC and rTEC reach their greatest values at 1029 and
period result in rTEC values being positive and negative 1024 UT, respectively. Figures 7a and 7d reveal that the
during 1000 1100 UT, respectively. Note that slow and signatures in TEC and TEC/TECo are rather complex.
small oscillations near zero values of both TEC and rTEC Figures 7b and 7e yield a similar feature that the noontime,
indicate no solar (flare) radiation contribution in the night- dawn and dusk TEC yield pronounced flare signatures.
time region. Nevertheless, it is obvious that the very Although the signatures have the greatest values in the
pronounced flare features in the daytime rTEC appear at noontime region, we find no clear and simple relationship
1019, 1024, and 1027 UT. between them and the hour or zenith angle (see Figures 7b
[13] The global distributions of the GPS receivers allow and 7e or Figures 7c and 7f). The complex relationships
us to further examine the TEC and rTEC as well as their imply that the ionospheric background electron density and/
associated percent changes at various latitudes, longitudes, or other geomagnetic variabilities can heavily and easily
local time, and zenith angles. The percent changes of the disturb the flare TEC observations. In contrast, Figure 8
two quantities are defined as TEC/TECo and rTEC/TECt, displays that the solar flare effects can be seen even in the
where TECo and TECt denote the TEC value observed at predawn (0400 LT) and postdusk (2000 LT) regions and the
1000 UT before the solar flare occurrence and the instant most pronounced feature appears near the subsolar point
TEC value when rTEC is derived, respectively. The com- (1200 LT), which is in the midday area (Figure 8a).
parison between the two quantities and their associated Moreover, it is interesting to find that the rTECs flare
percent changes allows us to further understand the con- signatures are symmetry to the hour angle (Figure 8b) while
tributions from the ionospheric ambient (or background) their percent changes yield the greatest values during
condition. Figures 7a, 7b, and 7c (7d, 7e, and 7f) display predawn (0500 LT) (Figure 8e). The quasi-cosine relations
TEC and (TEC/TECo) distributions at 1029 UT at shown in Figures 8b and 8c confirm that the rTEC flare
various latitudes/longitudes hour angles, and zenith angles, signatures are functions of the solar hour angle and zenith
respectively. Figure 8 illustrates the same distributions of angle. The comparisons between Figures 8b– 8c and 8e – 8f
5 of 12A01307 LIU ET AL.: IONOSPHERIC SOLAR FLARE EFFECTS MONITORED BY GPS A01307
Figure 4. The offset TECs and the associated averages of the GPS satellite PRN 11, 15, 20, 29, and
31 to the dayside receivers on the Bastille Day.
show that it is the rTEC but not its percentages to be sounding systems for studying the ionospheric solar flare
functions of hour angles and zenith angles. effects [Donnelley, 1971] can be used to examine the time rate
of changes of ionospheric electron densities at certain alti-
tudes below the F-peak [Liu et al., 1996a]. In this paper based
4. Discussion and Conclusion on equations (5a) and (7a), we introduce the TEC and rTEC
[14] Numerous techniques have been employed to monitor (or TEC) derived from measurements of the ground-based
the ionospheric solar flare effects (see the papers listed in the GPS receivers to simultaneously investigate variations of
works of Mitra [1974] and Davies [1990]). However, most of electron densities and the associated time rate of their changes
the techniques simply observe flare features at certain alti- in the whole ionosphere ranging from 90 to 20,200 km
tudes. For instance, magnetic filed strengths recorded by altitude during occurrences of solar flares.
ground-based magnetometers for studying the geomagnetic [15] Liu et al. [1996a] found that the temporal variations
solar flare effect [Ohshio, 1964] can be employed to evaluate of the ionospheric electron density N derived from ground
ionospheric electron density changes at about 90 km, and based magnetometer data and those of the solar X-ray
frequency shifts in radio signals probed by HF Doppler radiations yield similar tendencies. Equations (3) and (5a)
6 of 12A01307 LIU ET AL.: IONOSPHERIC SOLAR FLARE EFFECTS MONITORED BY GPS A01307
Figure 5. The high-resolution TECs and the associated averages of the GPS satellite PRN 11, 15, 20,
29, and 31 to the dayside receivers derived with the Bastille Day and the reference day.
also show that the variations of the TEC are proportional ing systems. The theoretical derivation of equation (7)
to those of the electron production rate of the flare radia- shows the rTEC and f derived from ground based GPS
tions. Based on Liu et al. [1996a] and the theoretical receivers to be nearly proportional. Therefore we expect to
derivations of this paper, we conjecture that the temporal observe rTEC spike features appearing when the flare
variations of TEC and those of the flare radiations yield radiations sharply (or suddenly) increase. Figure 6 illus-
similar tendencies. Similar tendencies in the daytime TEC trates three rTEC maxima appearing around 1019, 1024,
and the flare X-ray radiations shown in Figure 6 (for more and 1027 UT but one maximum in the flare radiations at
examples also see Appendix A) confirm that TEC is 1027 UT. To search the moments of sharp increases, we
suitable to monitor the overall temporal variations of the differentiate the X-ray 1 8 Å radiations of the GOES-10
solar flare radiations. shown in Figure 6a. Figure 9 illustrates that the rTEC and
[16] Liu et al. [1996a] demonstrate that not only the the differentiated X-ray radiations simultaneously appear at
magnitude but also the time rate changes of flare radiations 1019 UT. Recently, Masuda et al. [2000] report that the
affect the frequency shifts f observed by Doppler sound- hard X-ray observation of the Yohkoh satellite starts the early
7 of 12A01307 LIU ET AL.: IONOSPHERIC SOLAR FLARE EFFECTS MONITORED BY GPS A01307
Figure 6. The solar 1 – 8 Å X-ray radiations (a), as well as the averaged high-resolution TECs (b), and
their associated rTECs (c) in the dawn, daytime, dusk, and nighttime regions observed during 1000 –
1100 UT on the Bastille Day.
phase around 1011 – 1013 UT, which has some of the impul- changes in the hard X-ray shown by Masuda et al. [2000]
sive phase at about 1019 UT, yields sudden increase at coinciding with the three maxima of the rTEC observed in this
1024 UT, reaches its peak at 1027 UT, and finally goes back paper at 1019, 1024, and 1027 UT (see Figure 6c) confirms
to the previous intensity level at 1030 UT. The three sudden that the rTEC variation is highly sensitive to the change rate of
Figure 7. The spatial, solar hour angle, and zenith angle distributions of the high-resolution TECs and
their percent changes at 1029 UT derived with the Bastille Day and the reference day.
8 of 12A01307 LIU ET AL.: IONOSPHERIC SOLAR FLARE EFFECTS MONITORED BY GPS A01307
Figure 8. The spatial, solar hour angle, and zenith angle distributions of the rTECs and their percent
changes at 1024 UT on the Bastille Day.
the solar flare radiations. It can be found that there are no spheric gravity waves, etc., result in the flare features in
obvious X-ray radiation increases when the rTEC maxima the TEC (or TEC) being relatively difficult to be ob-
appear at 1024 and 1027 UT (Figure 9). We consider that the served. There are two types of the flare radiations, which are
discrepancies in the sharp increases between the GOES-10 the sudden and the gradual. When the duration of a solar
and Yohkoh X-ray radiations might result from that the flare flare is relative long, variations of other geophysical effects
X-ray flux has different temporal features depending on start to contaminate and burry the TEC flare features (for
wavelengths. Although no EUV data is available and pre- more examples see Appendix A). Nevertheless, the theoret-
sented in this study, it has been well known by ionospheric ical derivations and observational results demonstrate the
scientists that not only the solar X-ray but also EUV radiations suitability and detectability of the TEC and rTEC to be
are responsible to the ionospheric ionizations [see, e.g., different.
Ratcliffe, 1972]. Therefore the rTEC can be used to detect [18] To reduce the contaminations from other geophysical
the sudden increases in the X-ray and EUV flare radiations. effects, the high resolution TEC monitoring method pro-
[17] Figure 2 illustrates that the flare feature of the TEC posed by Hernandez-Pajares et al. [1997] are adopted and
(or TEC) is not as obvious as that of the rTEC. It can be tested. Relatively observable features in TEC(7/14 –7/13)
seen that many large wave-like fluctuations in the sum of (see Figures 2 and 5) indicate the high-resolution method to
TEC(7/13) and TEC(7/14) shown in Figure 2, which might be a better way to monitor the ionospheric TEC response to
be caused some other geophysical effects, such as geomag- the solar flare. Figure 4 reveals that TEC(7/14) of satellite
netic storms, traveling ionospheric disturbances, atmo- PRN 20 and 29 reach maximum while those of PRN 11, 15,
Figure 9. The differentiated solar 1 8 Å X-ray radiations and the sum of the rTECs during 1000
1100 UT on the Bastille Day.
9 of 12A01307 LIU ET AL.: IONOSPHERIC SOLAR FLARE EFFECTS MONITORED BY GPS A01307
Figure A1. The flare radiations, the averaged rTEC, and the averaged TEC of the three M solar flares
occurred on 1/10, 3/8, and 4/5 2001. The three downward arrows in the third panel respectively denote
the starting, max, and the ending times of the 1 8 Å X-ray flare radiations.
and 31 yield saturated values at about 1029 UT. Note that the 1029 UT. This once again confirms that the high-resolution
footprints (i.e., ionospheric point traces) of the satellites in the TEC monitoring method developed by Hernandez-Pajares et
daytime region are around south of the ionospheric midlat- al. [1997] partially remove the ambient effect and is more
itude trough, 52N geomagnetic, where electron densities suitable to observe solar flare TEC features.
have minimum values [Ratcliffe, 1974]. Thus the ionospheric [19] The TEC observations show no obvious flare signa-
points of GPS satellites moving away from the trough ture observed in the nighttime region, and clear flare signa-
latitudes result in the TEC increase. It is interesting to see tures at dusk but relatively unclear at dawn. It might be
that when the satellite PRN 20 and 29 move toward the interesting to find possible mechanisms causing the differ-
trough, their TEC(7/14) differences between 1000 and ence between the dawn and dusk regions. Figure 6b displays
1029 UT are relatively small, and the two TECs reach at dawn that the ambient TECs yield an increase tendency due
maximum values at 1029 UT. Meanwhile, for the ionospheric to the sunrise ionization, which superimposes with the TECs
points of satellites PRN 11, 15, and 31 moving away from the increase due to the flare radiations around 1030 UT. The
trough, their TEC(7/14) differences between 1000 and mixture of the two increases causes the flare features in the
1029 UT are relatively large and right after 1029UT tend to TECs (or TECs) to be difficultly identified at dawn. By
have saturated features. This indicates latitudinal (or ambient) contrast, at dusk, the ambient TEC has a decreasing trend
effects in the TEC(7/14) to be significant. By contrast, the while the TEC temporarily increase due to the flare radiations
flare features in all five averaged TEC(7/14 –7/13) shown the TECs increase. The opposite variations in the TECs in
in Figure 5 are rather similar and reveal maximum values at practice enhance the visibility of flare signatures (see
10 of 12A01307 LIU ET AL.: IONOSPHERIC SOLAR FLARE EFFECTS MONITORED BY GPS A01307
Figure A2. The flare radiations, the averaged rTEC, and the averaged TEC of the three X solar flares
occurred on 4/21, 7/3, and 7/23 2002.
Figure 6b). Nevertheless, the dawn and dusk results indicate that the duration of the flare radiation on the 2000 Bastille
day-to-day variations and background of the ionospheric Day is rather long. During such a long period, many other
electron densities can easily affect observations of the iono- geophysical viariabilities, such as the wave-like variations in
spheric solar effects in the GPS TECs. TEC(7/13) and TEC(7/14) shown in Figure 2b, could affect
[20] Figure 7 shows the relationships between the associ- the TEC flare observation (for more examples see
ated percentage changes in the TEC and the geographic Appendix A). By contrast, rise times between the start and
latitude/longitude, local time (or solar hour angle), and zenith the maximum of flare radiations usually are rather short, often
angle are rather complex. It can be seen from Figures 2 and 6 less than 10 min (see Figures 2, 6, A1, and A2). Owing to a
short time interval, other geophysical effects become less
important, and therefore the rTEC flare signatures are gen-
Table A1. Parameters of the Six Flare Events
erally obvious. This explains that the rTEC is a nice function
Start, Maximum, Flare Increase Rate, Duration,
Date hhmm hhmm End Class Watt/m2-min min Table A2. GPS Receivers Used for the M Class Flares
1/10, 2001 1012 1016 1018 M3.5 9 106 6
3/8, 2001 1113 1118 1121 M5.7 1.2 105 8 Receiver Geographic Latitude Geographic Longitude
4/5, 2001 0837 0922 0954 M8.4 1 106 77 BRUS 50.61N 4.36E
4/21, 2002 0043 0151 0238 X1.5 2.4 106 115 GOPE 49.72N 14.79E
7/3, 2002 0208 0213 0216 X1.5 3.4 105 8 MATE 40.46N 16.70E
7/23, 2002 0018 0035 0047 X4.8 3 105 29 WTZR 48.95N 12.88E
11 of 12A01307 LIU ET AL.: IONOSPHERIC SOLAR FLARE EFFECTS MONITORED BY GPS A01307
Table A3. GPS Receivers Used for the X Class Flares [25] Acknowledgments. Data used in this paper retrieve from IGS
and Ministry of the Interior of Taiwan. This research was partially
Receiver Geographic Latitude Geographic Longitude supported by the Ministry of Education grant 91-N-FA07-7-4 and the
AUCK 36.42N 174.83E Office of Naval Research project N00014-00-0528 to the National Central
CHAT 43.76N 176.57E University. The authors wish to thank A. D. Richmond at the High Altitude
KSMV 35.77N 140.66E Observatory for useful comments and suggestions. The manuscript was
MIZU 38.95N 141.13E originally submitted to Journal of Geophysical Research-Space Physics for
publication in March 2001 (2001JA007519).
[26] Lou-Chuang Lee thanks M. J. Keskinen and another reviewer for
of the hour angle (or zenith angle) shown in Figures 8b and their assistance in evaluating this paper.
8c. Long-term observations show that solar flare radiations
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TEC and rTEC can be observed. For those with the obvious
rTEC features, the time rate of increase in Table A1 show C. H. Lin, High Altitude Observatory, National Center for Atmospheric
that the 7/3 event are the greatest value, followed by the Research, Boulder, CO 80301, USA. (clin@ucar.edu)
7/23, 3/8, and 1/10 events. Through the sequence of the Y. A. Liou, Center for Space and Remote Sensing Research, National
Central University, 300 Jungda Road, Jungli City, Taoyuan 320, Taiwan.
increase rates and that of the magnitude of the rTEC, we (yue-ian@csrsr.nce.edu.tw)
discover in Figures A1 and A2 that sudden increases in flare J. Y. Liu, Institute of Space Science, National Central University, 300
radiations result in obvious flare features of the rTEC. Jungda Road, Jungli City, Taoyuan 320, Taiwan. (jyliu@jupiter.ss.nce.
Meanwhile, we find for the obvious that the magnitude of edu.tw)
H. F. Tsai, Radio Science Center for Space and Atmosphere, Kyoto
the TEC flare features and the flare classes seem to be University, Gokasho, Uji, Kyoto 611-0011, Japan. (hftsai@ss.nce.
highly correlated. edu.tw)
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