Journal of Atmospheric and Solar-Terrestrial Physics 75–76 (2012) 133–140

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Journal of Atmospheric and Solar-Terrestrial Physics

1364-68

doi:10.1

n Corr

E-m

journal homepage: www.elsevier.com/locate/jastp

Study of ionospheric TEC during space weather event of 24 August 2005at two different longitudes

Shweta Sharma a, P. Galav a, N. Dashora b, R.S. Dabas c, R. Pandey a,n

a Department of Physics, M. L. S. University, Udaipur 313 001, Indiab National Atmospheric Research Laboratory, Gadanki 517 512, Indiac National Physical Laboratory, New Delhi 110 012, India

a r t i c l e i n f o

Article history:

Received 14 October 2010

Received in revised form

29 March 2011

Accepted 6 May 2011Available online 20 May 2011

Keywords:

Geomagnetic storms

Low latitude ionosphere

TEC

GPS

26/$ - see front matter & 2011 Elsevier Ltd. A

016/j.jastp.2011.05.006

esponding author. Tel.: þ91 98281 99781; fa

ail address: pandey.rj@gmail.com (R. Pandey)

a b s t r a c t

Response of low latitude ionosphere to the geomagnetic storm of 24 August 2005 has been studied

using total electron content (TEC) data obtained from the global positioning system (GPS) receivers

located at Udaipur (Geog. Lat. 24.671N, Geog. Long. 73.71E) and Yibal (Geog. Lat. 22.181N, Geog. Long.

56.111E), near the northern crest (�161N Mag. Lat.) of the equatorial ionization anomaly. This study has

also been substantiated with the ground based ionosonde observation at New Delhi, a low latitude

station. The TEC on 24 August reveals two well defined humps. The enhancements in TEC have been

attributed to the prompt penetration electric fields, inferred from the height variations of the F-layer

peak using the ionosonde observations. In addition to these fields, contribution of the abnormal

equatorial plasma fountain during the late afternoon hours has been proposed to explain the second

hump in TEC.

& 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Satellite based navigation and ground positioning is now beingcarried out entirely by the global positioning system (GPS)satellites. This is due to the wide spatial and temporal coverageprovided by the GPS satellites under the all-weather conditions.The GPS comprises of a constellation of satellites positioned in sixorbital planes that have a relative inclination of 551 with respectto the equatorial plane. There are at least four satellites in eachplane, situated in a circular orbit at a nominal altitude of20,200 km. The constellation is such that at least six satellitesare in view at any instant of time from any location on the earth.The GPS satellites rely on trans-ionospheric communication ontwo L-band frequencies, L1¼1575.42 MHz and L2¼1227.6 MHz.During the passage of the signal from the satellite to the receiverit passes through the intervening ionosphere that is dispersive innature. This results in the degradation of the received signal. Thusthe success of the GPS based navigation and communication isgreatly dependent on the state of the ionosphere, which is knownto vary drastically during events like geomagnetic storms andequatorial spread F. The total electron content (TEC) has beenknown as an important parameter for various ionospheric studies(e.g., Rastogi and Klobuchar, 1990). Various studies have shownthat the TEC may undergo sudden and large variations during

ll rights reserved.

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geomagnetic storms (Ho et al., 1998; Jakowaski et al., 1999; Basuet al., 2001a, b; Maruyama et al., 2004; Foster and Rideout, 2005;Lin et al., 2005; Dashora and Pandey, 2007; Dashora et al., 2009).The TEC of the ionosphere is the parameter upon which the GPSbased navigation and positioning is heavily dependent. This isbecause the range errors in the GPS signals are directly propor-tional to the TEC between a satellite and receiver pair. While avariation of merely 1 TECU (1 TECU¼1�1016 electron/m2) pro-duces a range error of 0.16 m, large, sudden changes in TEC mayintroduce intolerable range errors that pose a great threat to theGPS based navigation. This underlines the importance of the studyof TEC variations during the geomagnetic storms.

The earth directed coronal mass ejections (CMEs) from the sun,which result in increased ram pressure characterized by suddenincreases in solar wind velocity, temperature and density as wellas large changes in the interplanetary magnetic field (IMF)specially its north–south component (Bz), trigger the geomag-netic storms. These storms not only affect the ionosphere but avast region of space from Earth’s magnetosphere down to theatmosphere could equally be affected and such studies areimportant from the point of view of space weather relatedprocesses. A geomagnetic storm may lead to a positive or anegative ionospheric storm (e.g., Buonsanto, 1999; Maruyamaet al., 2004; Kumar et al., 2005) resulting due to storm timemodifications in electric fields, neutral winds and chemicalcomposition (e.g., Sastri et al., 2000). The quiet time equatorialand low latitude ionosphere is mainly governed by the zonal(east–west) electric field, which leads to the formation of

S. Sharma et al. / Journal of Atmospheric and Solar-Terrestrial Physics 75–76 (2012) 133–140134

equatorial ionization anomaly (EIA) during quiet geomagneticconditions. Thus it is reasonable to believe that the disturbanceelectric fields are the most important factor, responsible for thestorm time modification of the equatorial and low latitudeionosphere.

The storm time disturbance electric fields have been classifiedin to two categories (1) prompt penetration (PP) electric fields,associated with the solar wind-magnetosphere dynamo (Seniorand Blanc, 1984; Spiro et al., 1988) and (2) the disturbancedynamo (DD) electric fields arising due to the changes in globalthermospheric wind circulation, resulting from storm time jouleheating at high latitudes (Blanc and Richmond, 1980).

By correlating the interplanetary electric field variations withthe geomagnetic variation in the equatorial region, Nishida et al.(1966) were the first to provide the evidence for the PP electricfields. These PP fields are short-lived in nature, with a rise anddecay time of about 15 minutes and last for about an hour(Gonzalez et al., 1979; Fejer and Scherliess, 1997). Large south-ward turning of IMF Bz from its steady northward course maylead to the under shielding conditions that result in the genera-tion of the PP electric fields, which are eastward/westward duringthe daytime/nighttime (e.g., Fejer, 2002). PP fields can be experi-enced near simultaneously over a range of latitudes from mid tolow and equatorial latitudes (Kikuchi et al., 1996, 2000; Fejeret al., 2007). Similarly, northward turning of IMF Bz from itssteady southward course could also generate a PP electric field(Kelley et al., 1979), which would be westward (eastward) duringthe daytime (nighttime). The storm time prompt penetration ofelectric fields can significantly modify the dynamics of equatorialand low latitudes. This is because the E�B vertical drift, which isthe main contributing factor for the formation and developmentof EIA at the day side of the globe, gets drastically modified bysuch PP fields. During the daytime these eastward directed PPfields enhance the vertical E�B drift that results in the upliftingof the plasma to higher altitudes where it survives for longer timedue to slower recombination rates.

In contrast to the PP fields, the storm induced DD fields areobserved over much larger time scales, from a few to severalhours (Fejer et al., 2007) and are generated by the excessiveenergy dumping in to the high latitude ionosphere (Blanc andRichmond, 1980). DD fields of various time scales have beenattributed to different types of equatorward winds (Fejer et al.,2007). In particular, the combined effect of storm inducedequatorward winds and the conductivity variations have beenshown to be mainly responsible for the DD electric fields thatoccur after one day from the geomagnetic storm (Scherliess andFejer, 1997), resulting from storm driven ionospheric compositionchanges (Fejer et al., 2007). At the equator, the direction of DDfields is westward during the daytime and eastward during thenighttime (Scherliess and Fejer, 1997). Hence such DD fields areopposite to the ambient electric field by the day and night as well.

Equatorward fast traveling atmospheric disturbances (TADs)have also been reported by several workers, which are believed tobe generated due to the impulsive energy dumping at the highlatitudes during the geomagnetic storms (Balthazor and Moffett,1997; Prolss, 1997; Fuller-Rowell et al., 2002). The TADs drag theionization along the geomagnetic field lines (Kirchengast et al.,1996; Sastri et al., 2000) and lead to a positive ionospheric storm.TADs are seen even up to the low and equatorial latitudes(Bruinsma and Forbs, 2007) and modulations up to 25% in theF-region density can be brought about by these.

There have been a number of studies related to the ionosphericbehavior during the geomagnetic storms (e.g., Spiro et al., 1988;Fejer et al., 1990; Abdu et al., 1995; Fejer, 1997; Sastri et al., 1997,2002; Basu et al., 2001a, b). With the advent of GPS basednavigation, during the recent past, such studies have been carried

out in terms of TEC, derived using the GPS receivers. During thegeomagnetic storms, poleward expansion of the EIA along withabnormally enhanced TEC (Tsurutani et al., 2004; Mannucci et al.,2005; Zhao et al., 2005) has been reported. In the aftermath of thegeomagnetic storm, Tsurutani et al. (2004) have also reportedsignificant reduction in the day side TEC and have attributed it tothe DD electric fields.

The disturbed space weather conditions are known to producelarge variations in ionospheric electron density such that the TECis known to increase (e.g., Kutiev et al., 2005; Lin et al., 2005; Zhaoet al., 2008; Dashora et al., 2009) as well as decrease (Kutiev et al.,2005; Dashora and Pandey, 2007) substantially. The situation is ofparticular concern near the crest of the equatorial ionizationanomaly where large variations in TEC have been observed. Sincethe TEC is directly proportional to the range errors in the receivedGPS signals, large changes in TEC due to severe space weatherconditions are a cause of great concern to the GPS based naviga-tion in the low latitude regions, near the crest of the EIA. Sincegeomagnetic storms may occur at any time of the day, theireffects on the ionospheric TEC would be varied and need to beinvestigated in detail to mitigate errors in the GPS based naviga-tion and ground positioning.

It may be pertinent to note here that there have been only afew geomagnetic storms during the declining phase of the solarcycle 23, characterized by a large southward Bz, that occurredduring the daytime in the region under study. As was reported byus earlier (Dashora et al., 2009), the present work reinforces therole of PP electric fields that causes local ionospheric uplift due tothe E�B drift, and the super plasma fountain to give a two-humpstructure in TEC. In the present work, however, modulations inTEC, which were attributed to the TADs earlier, are not seen. Wehave also shown that, around the northern crest of the EIA, theionospheric response to a geomagnetic storm at two differentdaylight longitudes is nearly similar. Such studies employing theTEC data from the region are probably the first of its kind.

2. Data sets

To show the variation of solar wind parameters namely, protondensity (NP), proton temperature (TP) and solar wind speed (VSW),we have made use of 64 s data of level 2 of ACE satellite. Whereasper 16 s data points have been used of the same satellite for theIMF Bz (GSM coordinates) in the present study. Using the formulaEy(mV/m)¼�VSW Bz, the zonal component of the interplanetaryelectric field (IEF Ey) has been calculated. From the Hill–Siscoepolar cap formula (Siscoe et al., 2002; Ober et al., 2003; Fejeret al., 2007), we have calculated the polar cap potential drop FP

FPðkVÞ ¼30þ57:6ERP�1=6

SW

1þ0:20SPP�1=3SW þ0:036ERP�1=2

SW SP

Here, ER (mV/m)¼VSWB sin2(y/2), is the reconnection electricfield, B (nT) is the Y–Z plane component of the IMF in GSMcoordinates and y is the IMF clock angle in the Y–Z plane, with 01and 1801 corresponding to northward and southward, respec-tively. A 30 kV potential has been included to account for viscousmerging and predicts saturation for large solar wind reconnectionelectric fields. PSW (nPa)¼NPmpVSW

2 , is the solar wind pressure,NP is the solar wind number density, and mp is the proton mass, andSP¼0.77 (F10.7)1/2 is the height integrated Pedersen conductivity.

We have used the SYM-H, ASY-H and AE indices, whichrepresent the strength of the symmetric ring currents, long-itudinal symmetry of the ring currents and the strength of theauroral electrojet currents, respectively.

TEC variations during the geomagnetic storm of 24 August2005 have been studied at two stations, Udaipur and Yibal, that

S. Sharma et al. / Journal of Atmospheric and Solar-Terrestrial Physics 75–76 (2012) 133–140 135

have nearly same magnetic latitude (�161N) but different geo-graphic longitudes, 561E and 73.71E. These stations are situatedaround the anomaly crest in the northern hemisphere. Yibal is anIGS (International GNSS Service) station, whose data has beendownloaded from the site ftp://garner.ucsd.edu/. The TEC fromthe IGS GPS data, which is available in public domain in the RINEX(Receiver INdependent EXchange) format, has been computedthrough the computer codes, developed by us (Sharma et al.,2011). Hence, data from a GPS receiver is used to get the line ofsight TEC (slant TEC) between a satellite and receiver pair, whichsuffers from the satellite and receiver bias errors. For the presentstudy these errors have been taken care of during post processingof data. The bias values have been downloaded from the siteftp://ftp.unibe.ch/auib/. Using the formula of Ma and Maruyama(2003), the VTEC has been computed from the slant TEC afterremoving the bias errors. For this purpose a thin ionospheric shellat 350 km height has been assumed. Thus, the computed VTECcorresponds to the coordinates of the ionospheric pierce points.

VTEC ¼ STEC 1�Re cosyRehmax

� �2" #1=2

The TEC data near the northern crest of the equatorial anomalyhas been substantiated by ionosonde observations from NewDelhi (Geog. Lat. 28.641N, Geog. Long. 77.171E, and Geomagneticlatitude 19.941N). For this, the variations of hmF2 and foF2 havebeen used.

3. Results and discussion

3.1. Solar wind, IMF Bz and geomagnetic data

Top to bottom of Fig. 1 depicts the temporal variations ofvarious solar wind parameters like NP, TP, VSW, and the IMF Bz, FP,IEF Ey along with various geomagnetic indices, such as AE, SYM-Hand ASY-H for the period 0400 UT to 1400 UT on the storm day,i.e., 24 August 2005. To account for the shock arrival time fromthe location of the ACE satellite to the magnetosphere, it iscustomary to shift the solar wind parameters and IMF Bz tomatch with the sudden positive impulse in SYM-H. Hence thesolar wind parameters and IMF Bz have been shifted in time byabout 35 min. On the storm day, an abrupt increase in VSW is seenat 0610 UT when the solar wind velocity increased from 440 toabout 550 km/s. This sudden increase in VSW was accompanied bysimilar increases in TP and NP, which lead to an enhanced polarcap potential to about 200 kV. At the same time, the IEF Eyincreased to 8 mV/m for a short duration. The increased FP is dueto the increased ram pressure that caused sudden compression inthe magnetosphere (Gonzalez et al., 1992, 1999), and manifesteditself as a sudden impulse or SI, seen in SYM-H as a positive jump.

Around 0630 UT the IMF Bz turns southward for a shortduration to a value of about �15 nT. At around 0835 UT, theIMF Bz suddenly increased to a northwardly value of �30 nT.Concurrently, the FP dropped near zero and the IEF Ey decreasedsharply to a value of ��13 mV/m. At 0900 UT, a sudden north-ward rise in IMF Bz, up to about 50 nT has been observed. At thesame time, the solar wind velocity also increased abruptly toabout 600 km/s resulting in positive jerks in SYM-H. Thus theperiod 0830 to 0910 UT could be taken as an initial phase of thestorm. It can be seen from Fig. 1 that the IMF Bz turned southwardat about 0910 UT, and reached its maximum value of about�55 nT at around 1000 UT. Consequently the SYM-H reached itsminimum value of about �179 nT. This southward turning of IMFBz at 0910 UT created strong eastward interplanetary and recon-nection electric fields, IEF Ey and ER (not shown here though).

These electric fields reached a maximum value of �35 mV/m.Resultantly, the under shielding conditions built up and a strongeastward IEF Ey is likely to have penetrated up to the equatorialionosphere, as a PP field. From 1000 UT onwards the IMF Bzstarted recovering from its maximum southward value. At thesame time, the AE and ASY-H indices showed marked increaseand reached up to 3500 and 225 nT, respectively. The IMF Bzagain turned southward at 1145 UT and remained southward forover an hour past 1145 UT signifying an eastward IEF Ey (max-imum value �20 mV/m), which could again penetrate to the lowand equatorial latitudes as a PP field.

3.2. Ionosonde observation

The storm time behavior of ionosphere has also been exam-ined using ionosonde observations from New Delhi (Geog. Lat.28.421N, Geog. Long. 77.211E, Geomagnetic Lat. 19.361N), a lowlatitude Indian station north of the EIA. For this purpose theprofiles of hmF2 and foF2 on the storm day, i.e., 24 August 2005,have been shown, respectively, in the upper and lower panel ofFig. 2. The observational data points have been marked with reddots that have been fitted with a spline to fill the gaps. Storm dayvariation of hmF2 and foF2 has been compared with a correspond-ing mean profile, computed by taking the average of internationalquiet days of the same month. To show the day-to-day variabilityin both the parameters, the 2s variations have also been markedon the mean quiet day’s profile. In Fig. 2, the quiet time profile ofhmF2 is seen to increase after the sunrise and attains a maximumheight of about 325 km around 0700 UT. After attaining itsmaximum height, the hmF2 is seen to decrease to reach an altitudeof about 250 km by the local evening hours (�1400 UT). Simi-larly, the foF2 rises after the local dawn and is seen to maximize at�1000 UT with a value of about 10 MHz. It falls graduallythereafter.

In contrast to their quiet day variations, on the storm day bothhmF2 and foF2 reveal two peak structures that stand out beyondthe 2s variation in each parameter. The height of the F2 peak isseen to rise gradually from about 0630 UT and reached up to350 km around 0800 UT. It stays there about for some time and at0915 UT, hmF2 rose sharply to a height of about 475 km. This risein height coincides with the time of generation of a large east-ward IEF Ey, which points to the possibility of transmission of aneastward PP electric field. An average vertical drift speed of nearly48 m/s can be estimated by the sudden uplift of the F-region; by130 km within 45 min due to the storm time penetration of theelectric field. This drift speed would correspond to an eastwardelectric field of about 1.9 mV/m. Since the value of IEF Ey aroundthis time was about 20 mV/m, it implied that about 10% of themagnetospheric electric field could reach the low latitude iono-sphere. After 1000 UT hmF2 decreased rapidly and was at 320 kmby 1130 UT. Thereafter, it started rising again and reached analtitude of 475 km. This rise of hmF2 nearly coincides with thesouthward turning of IMF Bz at 1145 UT, resulting in an eastwardIEF Ey of 20 mV/m. The sudden rise of hmF2 by 150 km within aperiod of 45 min corresponds to an average vertical drift speed of56 m/s. This drift speed implies an eastward PP electric field of2.2 mV/m in the low latitude ionosphere. It may be noted herethat the storm generated equatorward neutral winds are alsocapable of producing positive ionospheric storm (Prolss, 1995;Lin et al., 2005). But these could not be considered in the presentcase as the rise time of hmF2 is very small. Whereas storm inducedplasma upwelling should be a much slower process. Thus theeastward PP electric field could be used to explain the variation ofhmF2 on the storm day.

Before discussing the storm time variations of foF2 over a lowlatitude station like New Delhi, it is quite in order to know first

0

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012345

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-60-40-20

0204060

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Fig. 1. The solar wind density, temperature, speed, IMF Bz, cross polar potential drop, zonal interplanetary electric field, AE index, SYM-H and ASY-H indices on

24 August 2005.

S. Sharma et al. / Journal of Atmospheric and Solar-Terrestrial Physics 75–76 (2012) 133–140136

about its quiet time variation and the mechanism thereof. On anormal day, the foF2 is seen to increase monotonically and reachesa maximum by about 1000 UT. This is a well known feature of EIAand has been explained in terms of plasma fountain caused by theupward vertical E�B drift of plasma at the geomagnetic equatorand its subsequent dumping at the low latitudes resulting in theformation of two crests at about 715–201 dip latitude (Martyn,1955; Hanson and Moffett, 1966).

Returning back to the lower panel of Fig. 2, it is seen that thevariation of foF2 also has two peaks that occurred at 1130 and1315 UT. Since the first episode of the eastward PP field occurredat 0910 UT, it resulted in the local uplift of plasma. This uplifted

plasma sustained for a longer time due to the smaller recombina-tion rate at higher heights and resulted in enhanced foF2 at�1130 UT at a low latitude station like New Delhi. The foF2 againshowed an increasing trend from 1215 UT and peaked at 1315 UT.This peak can be attributed to two factors. The first of which is theeastward PP electric field at 0910 UT, whose effect has beenobserved in hmF2 at New Delhi. The same eastward PP electricfield is also expected to reach the geomagnetic equator. At thegeomagnetic equator, this eastward PP electric field gave rise toan abnormal plasma fountain in the late afternoon hours. Due tothis abnormal fountain the equatorial plasma rose to very highaltitudes in the F-region where the recombination rates were

0 1 2 3 4 5 6 7 8 9 10 11 12 13 142

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foF2, August 24

spline fit on foF2 of August 24

mean foF2

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m)

hmF2, August 24

spline fit on hmF2 of August 24

mean hmF2

Fig. 2. Variation of hmF2 and foF2 at New Delhi on 24 August (red dots are the data values with a spline fit in green). Curve in magenta is the mean quiet day variation along

with the 2s bounds. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

0

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VTEC

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Yibal (LT ~ UT + 03:45 Hrs)

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Fig. 3. Variation of storm day VTEC (in red) for 24 August over Yibal and Udaipur. These stations have nearly same magnetic latitude (16 N) but differing longitudes. The

quiet day mean VTEC is plotted in black with 2s variations in green. (For interpretation of the references to color in this figure legend, the reader is referred to the web

version of this article.)

S. Sharma et al. / Journal of Atmospheric and Solar-Terrestrial Physics 75–76 (2012) 133–140 137

slower than at the region from where it was uplifted. Thus theuplifted plasma could survive longer. Its diffusion along the fieldlines led to enhanced dumping of plasma at the low latitudes inthe late afternoon hours. This process is being termed as adelayed effect of eastward PP electric field. This dumpingincreased the plasma density abnormally at a place like NewDelhi and contributed to the formation of the second peak in foF2.The other factor is the second episode of eastward PP electric fieldthat occurred at 1145 UT. This PP electric field again resulted inlocal uplifting of plasma in the low latitudes. Thus the combinedeffect of the abnormal fountain and the local uplift of plasma due

to the eastward PP electric field at 1145 UT can be attributed forthe formation of second peak in foF2 at New Delhi. The foF2

started decreasing after 1315 UT and arrived at its normal valueby about 1400 UT.

3.3. Variations in TEC at two different longitudes

Fig. 3 gives variations in TEC at Yibal and Udaipur, that havenearly same geographic (magnetic) latitude, �241N (�161N), butdifferent longitudes, 56.111E and 73.71E, respectively. The curvein black gives the mean quiet day variation in TEC and the band in

S. Sharma et al. / Journal of Atmospheric and Solar-Terrestrial Physics 75–76 (2012) 133–140138

green over it marks its 2s variability. The curves in red give theTEC variations for 24 August 2005 at these stations. The meanquiet day variations in TEC at Yibal show a broad daytimemaximum centered around 1200 UT corresponding to a localtime of 1545 h. The mean quiet day TEC variation for Udaipurshow a peak around 1000 UT, corresponding to the local time of1500 h. The largest quiet time TEC at both the stations is about40 TECU. It can be seen from Fig. 3 that prior to the episode ofsouthward turning of IMF Bz at 0910 UT, the TEC at both thestations is within its day-to-day variability (2s bounds). Fromabout 1000 to 1400 UT, TEC variations on 24 August show amarked deviation from its mean quiet day pattern. Two wellseparated peaks, or humps in TEC exist at both the stations. Thedifference in peak value of TEC from its quiet day mean is about25 TECU. Such drastic variations of TEC within a short period are acause of grave concern for GPS based ground positioning andnavigation. Since a change of 1 TECU corresponds to a range errorof 0.16 m, observed changes in storm time TEC would producelarge range errors.

The first peak in VTEC is observed around 1115 UT whereasthe time of occurrence of second peak is around 1315 UT. Thetime of occurrence of the first peak in VTEC is nearly same as thatin Fig. 2 for the foF2. The fact that the first peak in VTEC is seennearly simultaneously at both the stations lying in the samelatitude, but different longitudes, implies a common mechanismfor their formation. It strengthens our earlier contention, dis-cussed in Section 3.2, that it is due to the eastward PP electricfield at 0910 UT.

The second peak in VTEC could be explained in terms of thecombined effect of the same two factors, which were invoked toexplain the second peak in foF2 in Section 3.2. The first of these isthe local uplifting of the F-layer due to the second episode ofeastward PP electric field around 1145 UT. The second contribut-ing factor is the delayed effect of eastward PP electric field at0910 UT resulting in abnormal equatorial fountain. Thus theincrease in density due to the local uplifting was further rein-forced by the dumping of plasma at low latitudes due to theabnormal equatorial fountain. Here it is to be noted that theamplitudes of the peaks in TEC for both the stations are different.However, the exact reasons for these differences are to be found.

An interesting aspect of the present observations is that thepeaks in foF2 and TEC are seen to occur nearly at the same times.Kutiev et al. (2005) also report a similar variation of TEC and foF2

during the storm time. They further report a local time depen-dence of the ionospheric response to the geomagnetic storms.

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Fig. 4. Variation of VTEC over Udaipur on 25 August (in red), a day after the storm. The c

the quiet day mean, the VTEC on 25 August is seen to be reduced during the daylight ho

the reader is referred to the web version of this article.)

3.4. Disturbance dynamo effect:

Electrodynamic effects of geomagnetic storms are observedover various time scales from an hour up to almost 24 h andbeyond. During the recovery phase of the geomagnetic storms,effects of disturbance dynamo fields could be discernible pro-vided the wind circulation patterns are changed to the extent toproduce fields of magnitude that are comparable to the ambientfield at the low and equatorial latitudes. The disturbance dynamofields are always in opposition to the ambient electric field. Sincethe ambient field at the equator is eastward during the day, theDD field would then be westward. An eastward electric field atthe equator is known to be the main electrodynamic force(vertical EXB drift at the equator) responsible for the formationof EIA in the low latitudes. Hence, during the recovery phase ofthe storm, if the DD fields are produced, they would tend tomodify the formation of EIA, either by suppressing it completelyor making its crests move equatorward depending upon thestrength of the DD fields because the vertical drift of plasma isdirectly proportional to the magnitude of the net eastwardelectric field.

Although not shown here, the net eastward electric field on 25August was indeed reduced as was inferred through the observa-tions of ground based magnetometer data recorded at Tirunelveli(an equatorial station) and Alibag (a low latitude station outsidethe electrojet belt). By eliminating the nighttime level for eachstation, the difference profile of the horizontal component of themagnetic field provides an estimate of the strength of theequatorial electrojet. Such a profile for 25 August 2005 hasrevealed a very subdued equatorial electrojet.

We have looked for the effect of such DD fields near thenorthern crest of the EIA in terms of changes in TEC. Fig. 4 gives aplot of TEC on 25 August over Udaipur. Unfortunately, the TECdata at Yibal is not available for most of the day on 25 August. It isclearly observed from the plot that the VTEC on 25 August, from0200 to 1300 UT (corresponding to the local time 0700 to 1800 h)are substantially reduced compared to the mean day quiet values.The maximum reduction is to the tune of about 15 TECU around0900 UT, the hour when the TEC normally peaks on a quiet day.Thus, it may be concluded that in the aftermath of the geomag-netic storm, strong DD fields were produced, which had profoundeffect on the low latitude ionosphere.

However, modification of F-region plasma density could alsobe partly related to changes in thermospheric [O/N2] ratio result-ing due to storm induced neutral winds (e.g., Lin et al., 2005;

2 14 16 18 20 22 24e (UT)

Udaipur (LT ~ UT + 05:00 Hrs)

urve in black is the quiet day mean VTEC with 2s variations in green. Compared to

urs over Udaipur. (For interpretation of the references to color in this figure legend,

S. Sharma et al. / Journal of Atmospheric and Solar-Terrestrial Physics 75–76 (2012) 133–140 139

Goncharenko et al., 2007). We have verified (but not shown here)that, compared to a quiet day, the thermospheric [O/N2] ratio on25 August was somewhat reduced. Thus the lower thermospheric[O/N2] ratio could also be a minor contributing factor for theobserved reduction in TEC on 25 August over a low latitudestation, like Udaipur.

4. Conclusions

We have studied the response of low latitude ionosphere tothe space weather event of 24 August 2005 in terms of variationsin foF2, hmF2 and TEC. For this study, we have made use of solarwind parameters, interplanetary magnetic field and ground basedSYM-H/ASY-H index. Salient features of this study are as follows:

1.

Episodes of sharp rise in hmF2 at New Delhi have been shownto be due to the eastward PP electric field at 0910 and1145 UT.

2.

The response of low latitude ionosphere to the geomagneticstorm has been characterized by enhancements in foF2 andTEC in the form of two peaks (humps). The first peak has beenattributed to the local response of the low latitude ionosphereto the eastward PP electric field. The combined effect ofeastward PP electric field and enhanced equatorial plasmafountain at the equator has been proposed to be the cause ofthe second peak in the observed ionospheric parameters.

3.

The responses of foF2 and TEC to the PP electric fields aresimilar in nature. The TEC variation at the two stations (withnearly same magnetic latitude but differing longitudes) isfound to exhibit local time dependence, the origin of whichremains to be understood.

4.

Based on the height variations of hmF2, the magnitude ofeastward PP electric fields transmitting to the equator havebeen estimated to be about 10% of the IEF Ey.

5.

Effect of DD field near the EIA crest at Udaipur was observable,as the TEC on 25 August was significantly reduced.

Acknowledgments

The GPS receiver at Udaipur was purchased through the grantsfrom the University Grants Commission, New Delhi under theSpecial Assistance Program. This work is partially supportedunder the ISRO-RESPOND program. Shweta Sharma is thankfulto the UGC for the UGC-NET-SRF fellowship. IGS data was down-loaded from the site ftp://garner.ucsd.edu. The solar wind and IMFBz data were downloaded from the site http://www.srl.caltech.edu/ACE/ASC/. The geomagnetic indices were downloaded fromthe site http://wdc.kugi.kyoto-u.ac.jp/.

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