Response of low‐latitude ionosphere of the Indian regionduring the supergeomagnetic storm of 31 March 2001

Sneha Yadav,1 Rupesh M. Das,1 R. S. Dabas,1 P. Subrahmanyam,1 and A. K. Gwal2

Received 12 December 2010; revised 20 April 2011; accepted 25 May 2011; published 27 August 2011.

[1] The study explores the response of ion composition (H+ and O+ ion) and iontemperature at the F2 region altitude during the intense geomagnetic storm of31 March 2001 by using retarding potential analyzer onboard SROSS‐C2 satellite dataover the low‐latitude region. The simultaneous enhancement of H+ ion and iontemperature along with depletion of O+ ion concentration is observed at the altitudeof about 350 km during the beginning of storm recovery phase. These results are uniqueof their kind and attributed to the energetic particle (H+/H atoms) originating fromring current and the consequent charge exchange reaction. In addition, the ionosondeobservations show the short‐term positive storm effect during the main phase andbeginning of recovery phase (RP) over the low‐mid latitude station, Delhi. The resultsclearly show that such effect is due to the passage of storm‐induced traveling atmosphericdisturbances. The dominance of storm‐induced disturbance circulation is also observedduring the second day of RP in terms of enhanced peak density heights. The rise in heightdue to storm‐induced meridional wind is effectively seen in the nighttime ionosphereand is not associated with any considerable variation in F2 region critical frequency.

Citation: Yadav, S., R. M. Das, R. S. Dabas, P. Subrahmanyam, and A. K. Gwal (2011), Response of low‐latitude ionosphere ofthe Indian region during the supergeomagnetic storm of 31 March 2001, J. Geophys. Res., 116, A08311,doi:10.1029/2010JA016373.

1. Introduction

[2] The behavior of the low‐latitude ionosphere‐thermospheresystem to the geomagnetic storms is one of the active researchtopics in the space weather studies. Geomagnetic storms arecapable of generating large scale perturbations in neutralcomposition, electric fields and the global circulation of thethermosphere. The effect of these perturbations depends onthe geographic location of observing station. Over equatorialand low‐latitude ionosphere, the most important contributorto the storm time response is the electric field disturbancesof magnetospheric origin [Fejer and Emmert, 2003]. Thedisturbance electric fields at low latitudes fall into two cate-gories: (1) prompt penetration and (2) disturbance dynamoeffects [Blanc and Richmond, 1980]. Prompt penetration isthe immediate response and is associated with the southwardturning of the IMF Bz [Fejer et al., 1979; Sastri et al., 1992].On the contrary, the disturbance dynamo fields are linkedwith the extensive amount of energy deposition into the highlatitude thermosphere during geomagnetically active periodsand the time scales associated with the transmission of thesefields are much longer [e.g., Scherliess and Fejer, 1997].

Usually, some of these mechanisms work together to producethe observed positive and/or negative ionospheric stormeffects [Prölss, 1995; Lu et al., 2001; Kil et al., 2003; Meieret al., 2005]. During geomagnetic storms, impulsive amountof energy is injected to the high latitude regions in the formof particle precipitation and joules heating. On being heated,the air expands and the upwelling of gas in the high lati-tudes causes the increase in molecular to atomic ratio in theF region. The storm‐induced circulation developed which isdirected equatorward and thus brings the heated gas awayfrom the source region. In the descending part of the circu-lation, at middle and low latitudes, the air undergoes adi-abatic compression, and thus causing rise in the thermospherictemperature at these latitude regions [Rishbeth et al., 1975].The energy deposition at high latitude triggers travelingatmospheric disturbances (TADs) which carry equatorwarddirected meridional winds of moderate magnitude. TADsdissipate their energy by heat conduction, viscosity, and iondrag [Richmond, 1978; Hunsucker, 1982]. Due to this energydissipation, temperature at low‐latitude rises and is believedto be responsible for geomagnetic activity effect at these latitude[Klostermeyer, 1973; Mayr and Volland, 1974; Richmond,1979; Prolss, 1982; Forbes et al., 1987; Burns and Killeen,1992]. The storm‐induced effect over equatorial and low‐latitude region is very complicated owing to the presence ofequatorial ionization anomaly phenomenon and many otherfactors. Several studies have been conducted to understandthe response of the geomagnetic storms on low latitudinalionosphere that have significantly advanced the knowledge

1Radio and Atmospheric Sciences Division, National PhysicalLaboratory, Council of Scientific and Industrial Research, New Delhi,India.

2Department of Physics, Barkatullah University, Bhopal, India.

Published in 2011 by the American Geophysical Union.

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 116, A08311, doi:10.1029/2010JA016373, 2011

A08311 1 of 11

on this subject [Kelley et al., 1979; Abdu et al., 1990, 1995;Fejer, 1991; Lakshmi et al., 1997; Sobral et al., 1997; Abdu,1997; Sastri et al., 2000; Abdu, 2001; Sastri et al., 2002;Pavlov et al., 2004; Lynn et al., 2004; Dabas et al., 2006;Sreeja et al., 2009]. The geomagnetic storm produces variousdistinct global processes in the ionosphere/thermosphere/magnetosphere system and the effect of magnetic stormdepends on the season, geographic location as well as onthe severity, time of occurrence, and duration of the storm[Buonsanto, 1999]. Therefore, the responses of ionosphereat different geographic locations to the geomagnetic stormsmay be quiet different even for the same geomagneticstorm.[3] One of the important aspects which need to investigate

is the effect of ring current particle precipitation at theF2 region altitude during the geomagnetically disturbedcondition. It is well known that the ring current becomessignificantly more intense during geomagnetic storms. Theenergetic protons present in the ring current undergo acharge exchange reaction with the ambient hydrogen atom.These energetic neutralized hydrogen atoms are no longer ina constraint to follow the magnetic field and can directlyprecipitate in to the Earth’s denser atmosphere [Dessleret al., 1961] which is in turn is responsible for producingheating in the upper atmosphere [Krassovsky, 1968; Evans,1970; Prolss, 1971, 1973]. The direct insertion of energyfrom the ring current in the form of particle precipitationinto the upper thermosphere is also believed to play a crucialrole over the equatorial and low‐latitude region [Tinsley,1981].[4] In line of the above studies, the purpose of present

work is to study the anomalous features of the low‐latitudeionosphere/thermosphere system during the severe geo-magnetic storm of 31 March 2001. The main phase of thisevent coincides with the local daytime period, which pro-vides the opportunity to explore the role of geomagneticstorm over the low‐latitude region in the presence of usualdaytime phenomenon. The study is aimed to investigate therole of H+ ions, O+ ions and ion temperature at the altitudeof about 350 km during the geomagnetically disturbedconditions. The work also explores the response of bottomside ionosphere to this intense storm over the low‐midlatitude station, Delhi.

2. Data Analysis

[5] The present study is based on the two types of exper-imental observations: (1) satellite based and (2) groundbased. The ion and electron F region plasma measurementsmade by the ion and electron retarding potential analyzers(RPAs) onboard the Indian satellite SROSS‐C2, have yiel-ded excellent data set over the Indian region for more thanhalf a solar cycle. The absolute ion density, ion temperatureand ion composition parameters are derived from these insitu measurements by using the relationship between I‐Vcurves as explained by Garg et al. [2003]. The satellite wasorbiting at the altitude of about 350 km during the consideredtime period (March 2001) which is close to the peak densityheight of the low latitude ionosphere. This provides thedistinct chance to investigate the role of H+ ions, O+ ions andion temperature during geomagnetically disturbed conditions

at the altitude range of 350 km over the low‐latitude region.The selection of data utilized for the present study is basedupon the pass path and also on the best quality of availabledata. The RPA experiment onboard the SROSS‐C2 consistsof an electron RPA, ion RPA and a potential probe formaking the measurements of different ionospheric para-meters. The raw data obtained from RPA are processed to getthe basic data in the form of I‐V curve for electron and ionRPAs by incorporating the payload constants, scaling fac-tors, corrections, etc. and by interpolating the orbital para-meters corresponding to each sample of measurements.These characteristic I‐V curves yield information regardingthe plasma parameters, i.e., ion density, electron and iontemperatures, ion composition, etc. More information aboutthe SROSS experiment and data analysis technique is dis-cussed in detail by Garg et al. [2003]. Also, only thosepasses have been considered which have the longitudinaldifference of less than 2° from the ground based ionosondestation, Delhi.[6] KEL made IPS‐71 advanced digital ionosonde system

operated at the National Physical Laboratory; New Delhi(28.6°N, 77.2°E, dip 42.44°N) is used as a ground basedmonitoring system. The system provides the critical fre-quency and base height of the different ionospheric layers.At Delhi, the regular vertical sounding is carried out sys-tematically in every 15 min, round the clock and thus thedata is available at the resolution of 15 min. Althoughsystem has automated computerized scaling capability butall the required data were scaled manually to achieve thehigh accuracy level. The storm‐induced effect on the iono-spheric parameters has been examined by comparing it withfive quiet day mean value of the month. In addition, the 2sstandard deviation is also calculated in order to evaluate anyirregular behavior against the day‐to‐day variability. Theheight wise electron density profile is deduced from Iono-sonde data by using the POLAN algorithm. These profilesare further used to estimate the height wise electron densityvariation.[7] The different geophysical parameters e.g., Bz, Dst, Ap

and AE indices were downloaded from the Website of WDCat Kyoto (Japan) and UK. The parameters Bz, Dst and AEare used at the time resolution of one hour.

3. Results

[8] The present work explores the distinct features of thelow‐latitude ionosphere during the supergeomagnetic stormof 31 March 2001. Figure 1 shows the variation of magneticactivity indices Dst, AE, Ap and the z component of theInterplanetary Magnetic Field (Bz). The storm suddencommencement (SSC) was occurred at around 0600 EMTand is indicated by the arrow in Figure 1. The suddencommencement of the storm coincides with the local dawnhours and the main phase (MP) continues till the localafternoon hours. It can be seen from Figure 1 that the fall ofDst index is very sharp during the main phase and attains themaximum negative excursion of about −387 nT at 1300EMT. The clean southward orientation of Bz index duringthe above mentioned time period is responsible for pro-ducing such sharp decrease in the Dst index. However,it can be seen that the duration of recovery phase (RP) is

YADAV ET AL.: GEOMAGNETIC STORMS A08311A08311

2 of 11

longer and takes almost 2 days to attain the normal level.The sudden southward orientation of Bz component duringthe recovery phase of the storm causes the onset of anotherstorm which further extended the recovery phase. The dawntime sudden commencement of the storm provides anopportunity to study the ionospheric response in the pres-ence of ambient rates of production, chemical loss, andmotion of ionization. For better understanding, the study isdivided into two parts. The first part deals with the geo-magnetic storm effect on the ion concentration (H+ and O+

ion) and ion temperature while the second part explores thestorm‐induced effect on the bottom side ionosphere.

3.1. Variation of Ion (H+ and O+) Concentrationand Ion Temperature

[9] The orbital height of the SROSS‐C2 satellite wasabout 350 km during the high solar activity period (2001)which is well within the 100 km of the peak density height.This provides a unique opportunity to investigate theresponse of ion composition (H+ and O+ ion) and ion tem-perature at the F2 region height during the geomagneticallydisturbed period. The variation of H+ and O+ ion concen-tration for 31 March 2001 and chosen five reference dayshas been shown in Figures 2a and 2b, respectively. Suchvariations are shown during the time interval 1500–1600 EMT, which coincides with the beginning of stormRP. The reference days has been chosen on the basis of bestavailable data set around the same time period. The elevated

values of H+ ions on the storm day as compared to thereference day for all the latitudinal range can be easilyidentified from Figure 2a. It is seen from the latitudinalprofile of H+ ion density that normally, the values of H+ ionremain around 106 ions per m3 over equatorial and lowlatitude region under the normal geophysical conditions.However, on the storm day, the observed values becomeshigher than 1010 ions per m3 for the all the latitude range.It can be seen from Figure 2a that the increase in H+ ion ismore apparently seen at latitudes less than 16 degree ascompared to the reference day. On the contrary, a significantreduction in the O+ ion concentration during the beginningof storm RP in comparison to the reference days can beeasily seen from Figure 2b. Unlike H+ ion concentration, thedecrease in O+ ion concentration is seen throughout thelatitudinal belt (4–24 degree). This simultaneous enhancedH+ ion and depletedO+ ion concentration during the stormRPremains one of the ambiguous features of the present results.[10] Figure 3 shows the variation of ion temperature

during the time interval 1500–1600 EMT for 31 March 2001and the chosen reference days. The sharp rise in the iontemperature for all the latitude range during the beginningof storm RP as compared to the reference day can beeasily seen from Figure 3. It is observed that the increasein ion temperature is more evidently seen at latitudes above16 degrees. It can be seen from Figure 3 that the ion tem-perature (Ti) remains less than 1500 K on all the referencedays; however, on the storm day Ti exceeds 1500 K.

Figure 1. The variation of magnetic activity indices during 30 March to 1 April 2001.

YADAV ET AL.: GEOMAGNETIC STORMS A08311A08311

3 of 11

3.2. Storm‐Induced Effect on the Bottom SideIonospheric Parameters

[11] The study is further extended to explore the storm‐induced effect on the bottom side ionospheric parameters.Figure 4 shows the temporal variation of F region para-meters, i.e., F2 region critical frequency (foF2) and height ofits peak density (hmF2) along with the AE and Dst indexduring the three consecutive days, i.e., from 30 March to1 April 2001. The five quiet days mean value along with 2sstandard deviation is also plotted in Figure 4. The storm‐induced signature over low‐latitude ionosphere is observedin terms of enhancement in hmF2 values. Although, hmF2appears to be outside the 2s standard deviation boundaryfrom ∼0700 EMT but the abrupt enhancement is seen from∼0900 EMT. It can be seen from Figure 4 that on 31 March2001 the hmF2 values exhibits the oscillatory behaviorduring the storm MP and RP. The two prominent wave‐likefluctuations are observed during the end of MP and begin-ning of storm RP. It is noted that the hmF2 values remainwell outside the 2s standard deviation boundary for theentire MP and RP of the storm. On 31 March 2001 themaximum increase in hmF2 of about 180 km with respect tomean is observed during the MP of the storm (i.e., at around1200 EMT). It is seen that the foF2 value exhibits thesimilar trend as observed in the hmF2 values but with sometime delay. The first peak of foF2 has been observed afterthe 60 min. of the first peak of hmF2 while the second peakhas a time delay of around 90 min. The foF2 values alsoshow the considerable enhancement (positive storm effect)during the storm MP and beginning of RP and are slightlyoutside the 2s standard deviation boundary. The maximumincrease in foF2 of about 2.5 MHz as compared to the meanvalue is observed during the end of MP and beginning ofRP. Then foF2 values attain the normal levels as the RPproceeds, i.e., after around 1600 EMT. No considerable

storm‐induced effect has been observed in the foF1 values.The presence of additional stratification is also seen duringthe considered three days which is a topic of furtherresearch. Thus, it can be inferred that the storm‐inducedeffects are significant at the F2 region altitudes whereas nosuch effects are observed in the F1 region.[12] In the premidnight hours of the 31 March 2001, when

the recovery phase of the storm was in progress anotherstorm sets in. The AE index rise abruptly and the maximumvalue is seen at around 2100 EMT whereas the Dst indexreached a maximum negative excursion of −284 nT at 0200EMT on 1 April 2001. The onset of another storm causes thefurther lifting of the F layer which persists up to the sunrisehours. Further, it can be seen from Figure 4 that hmF2follows the behavior of AE index and begins to increasefrom around 2300 EMT. The maximum deviation in hmF2values of about 240 km with respect to mean is observedat around 0000 EMT, i.e., with the time delay of about 3 hfrom the maximum value of AE index. Dabas et al. [2006]by studying the space weather impact on F region overDelhi during the events Nov. 2003 and Oct., Nov. 2004 alsoshows the drastic increases in h’F and hmF2, with a devi-ation reaching up to a maximum of 200 to 300 km in themid night hours which persists until about sunrise time.Thus, the peak density height of the F layer is significantlyaffected in the nighttime ionosphere as compared to that indaytime during the geomagnetic storms. However, the foF2values show no considerable variation corresponding to thisabrupt height rise observed during the nighttime period ofstorm RP as can be seen from Figure 4. During the daytimehours of 1 April 2001 which is associated with the recoveryphase of the storm, the foF2 values achieve the value ofabout 16 MHz during the early morning hours, i.e., at about0800 EMT which is about 4 MHz more than the quiet daysmean value. Then as the day proceeds, the peak plasmadensity of the F2 layer does not show any significant

Figure 2a. The variation of H+ ion concentration duringthe time interval 1500–1600 EMT for the storm day(31 March 2001) and the five chosen reference days at thealtitude of around 350 km.

Figure 2b. Same as Figure 2a but for O+ ion.

YADAV ET AL.: GEOMAGNETIC STORMS A08311A08311

4 of 11

variation and the constant value of about 16 MHz isobserved up to about evening hours, i.e., up to 1700 EMT.After 1700 EMT, the foF2 values follow the usual diurnalpattern and start decreasing and then again show the

enhancement in the late evening hours, i.e., from 1900 to2000 EMT. The daytime hmF2 values also show the con-siderable increase from the usual quite day pattern and areseen outside the 2s standard deviation boundary.

Figure 3. The variation of ion temperature during the time interval 1500–1600 EMT for the storm day(31 March 2001) and the five chosen reference days at the altitude of around 350 km.

Figure 4. The temporal variation of critical frequency of F2 region (foF2) and height of its peak density(hmF2) along with AE and Dst index during the three consecutive days, i.e., from 30 March to 1 April2001. The five quiet days mean along with 2s standard deviation is also plotted in Figure 4.

YADAV ET AL.: GEOMAGNETIC STORMS A08311A08311

5 of 11

[13] Figure 5a shows the plot of electron density asfunction of time and altitude for 31 March 2001. In order toexamine the perturbations of electron density on the stormday against the day‐to‐day variability, the averaged fivequiet day electron density profile is also plotted and isshown in Figure 5b. This enables us to explore rigorouslythe changes in the altitude and electron density throughout

the bottom side F region. The significant departure ofelectron density profile on the storm day from the quietmean day pattern can be easily seen from Figures 5a and 5b.It can be seen from the normal day pattern that the peakelectron density appears at around 300 km. However, onthe storm day, it appears around 400 to 450 km range. Thesubstantial enhancement in electron density above 350 km

Figure 5a. The plot of electron density as function of time and altitude for 31 March 2001.

Figure 5b. Same as Figure 5a but for five global quiet day average.

YADAV ET AL.: GEOMAGNETIC STORMS A08311A08311

6 of 11

duringMP and beginning of RP (i.e., from 1100 to 1600 EMT)is one of the interesting features of the present results. Thewave‐like fluctuations in the electron density variations areobserved during the MP and beginning of RP. The depletionof O+ ion concentration in the SROSS‐C2 data during theinterval 1500–1600 EMT at 350 km is also reflected in theelectron density variation in terms of depletion in electrondensity (see Figure 5a).[14] The wave‐like fluctuations in foF2 and hmF2 sug-

gests the existence of Traveling Atmospheric Disturbances(TAD) on 31 March 2001 triggered by the geomagneticstorms. To verify the presence of TADs, the altitudinalvariations of different sounding frequencies are analyzed.Figure 6 shows the temporal variation of virtual height atdifferent sounding frequencies from 31 March to 1 April2001. The wave‐like fluctuations in the virtual height var-iations for all the sounding frequencies can be easily iden-tified from Figure 6. However, the signatures of TADs areobserved only during the MP and RP of the storm occurredon 31 March 2001 which is shown in the inset panel ofFigure 6. It can be seen that there exists the time lag in theheight variations of the frequencies which is marked by thebroken line in Figure 6. The maximum and minimum virtualheight variations occur first at higher frequencies and thenafter a few minutes later at the lower frequencies and thusrepresent downward phase propagation velocities. Thiswavefront tilt of the disturbance is the signature of TADs[Ruster, 1965; Richmond and Matsushita, 1975; Millwardet al., 1993]. The signatures of TAD are not observed dur-ing storm RP occurred on 1 April 2001. Further, Figure 6shows that the perturbations in different sounding frequen-

cies increase with the altitude and almost no perturbationsare seen at the lower altitude range.

4. Discussion

[15] The present study describes the of role of H+ and O+

ion concentration and ion temperature at the altitude ofabout 350 km during the intense geomagnetic storm of31 March 2001. The behavior of bottom side ionosphereduring this intense geomagnetic storm is also studied by usingthe ionosonde observation over low‐mid latitude station,Delhi. Results show that there is a substantial enhancement inthe H+ ion concentration and ion temperature along withthe depletion of O+ ion concentration at the F2 region alti-tude (i.e., at 350 km) during the beginning of storm RP.These results are unique and have been reported for the firsttime to the best of our knowledge and resources. Theenhanced H+ ion concentration which appears during thebeginning of storm RP could be attributed to the directprecipitation of energetic particle from the ring current inthe upper ionosphere/thermosphere system. During thegeomagnetic storms, a significant amount of energy getsinjected in the ring current. The energetic H+ ions present inthe ring current may directly precipitate along the magneticfield line threading the ring current and outer plasmasphere.On the other hand, the energetic protons (p+) are removedfrom the ring current by charge exchange collision with theambient neutral hydrogen atom (H) according to the rela-tionship p+ + H → p + H+. The neutralized energetichydrogen atoms so produced are no longer restricted tofollow the magnetic field lines and they continue their travelalong a straight line path. The part of the resulting hydrogen

Figure 6. The temporal variation of virtual height at different sounding frequencies from 31 March to1 April 2001. The inset panel shows the enlarge picture of the variation during the time interval whenTADs type perturbations was present.

YADAV ET AL.: GEOMAGNETIC STORMS A08311A08311

7 of 11

flux will impinge upon the Earth’s denser atmosphere[Dessler et al., 1961; Krassovsky, 1968; Evans, 1970;Prolss et al., 1973]. This energetic H atom can again getionized and could be responsible for the appearance ofenhanced values of H+ ion. It is well known that the con-centration of H+ and O+ ions is controlled by the chargeexchange reaction:

Oþ þ H $ Oþ Hþ ð1Þ

[16] The decrease in the O+ ion concentration along withthe simultaneous increase in the H+ ion provides the evi-dence that the charge exchange reaction between directprecipitated H atoms and O+ ions plays a very crucial roleand might be responsible for depleted O+ ion concentrationduring the geomagnetic storm event. However, the compo-sition disturbance also plays a very important role indetermining the electron concentration and ion composition;therefore, its contribution has to be taken in account. It iswell known that during the geomagnetic storms, the tran-sient excessive amount of energy is deposited over the highlatitude region by joules heating and particle precipitation.On being heated, the air expands upwards, creating hori-zontal pressure gradient that causes the air to flow outwardsand away from the heated region. The up flow of air in thehigh latitude region lifts in to the F region air with a greatermolecular/atomic ratio (i.e., N2/O ratio) than in normalfound there. This molecular enriched air is transported tolower latitude with in a few hours by storm time globalcirculation wind [Rishbeth et al., 1975]. This compositiondisturbance is known to be responsible for generating thenegative F region storm effect at middle latitude byincreasing the recombination rate. Unfortunately, due to theunavailability of ionospheric data of a midlatitude station inthe same longitude zone we could not assess the possiblecandidature of the composition disturbances for the decreasein O+ density. However, the work reported by Yuan et al.[2009] which deals with the response of daytime F regionionosphere to the event of 31 March 2001 significantly addsup to the present work. They observed a short‐durationpositive ionospheric storm effect which they attributed tothe equatorial meridional wind associated with TADs. Theshort duration positive storm effects are also observed overDelhi during the end of MP and beginning of RP as can beseen from Figure 4. It is reported that the storm‐inducedequatorward wind often takes the form of equatorwardsurges or traveling atmospheric disturbances (TADs) whenthe heating events are impulsive [e.g., Richmond andMatsushita, 1975; Roble et al., 1978; Burns and Killeen,1992; Hocke and Schlegel, 1996]. The TADs are the wavelike atmospheric perturbations moving away from thesource region, i.e., equatorward with a velocity of a fewhundred meters per second. The equatorward wind associ-ated with TAD causes the vertical motion of ionized layerover low‐latitude region along the slope geomagnetic fieldlines and thus leads to the increase in the height of iono-spheric layers, as observed over Delhi. If this process takesplace in the sunlit period of the day then it causes theincrease in electron density if no disturbances (or underweak disturbances) in the neutral composition is present.Jones [1971] reported that the positive storm effect occur

only when the main phase of the storm takes place in thetime interval from 0800 to 1200 LT. Jones and Rishbeth[1971] attributed the equatorward wind due to the auroralheating as the probable mechanism for such F layer heightrise where ion loss rate are smaller and thus leads to thepositive storm effect. The pulse like fluctuations in thevirtual height variations of different sounding frequencieswith the wavefront tilt of the disturbance shown in Figure 6confirms the existence of TAD during the end of MP andbeginning of RP of the storm. Yuan et al. [2009] also showthe presence of storm‐induced TAD having a meridionalvelocity of 622 m/s. Thus, the short duration positive stormeffect observed during the end of MP and beginning of RP isassociated with the existence of TADs. These facts suggestthat during the event of 31 March 2001, the equatorwardwind associated with TAD does not cause the disturbance orproduces very weak disturbance in the neutral compositionduring the MP and beginning of RP. Thus, the significantdepletion in the O+ ion concentration during the beginningof the storm RP cannot be fully associated with the distur-bance in the global circulation and can be attributed tocharge exchange reaction. It can be inferred from the abovediscussion that during the geomagnetic storm event, theconcentrations of O+ ions are also controlled by the chargeexchange reaction even at the altitude of 350 km. Theappearance of decreased values of O+ ion concentrationalong with the simultaneous increase of H+ ion concentra-tion provides the evidence that the H+ ions produced at theexpense of O+ ions by the charge exchange reaction.However, the increase in H+ ion concentration which is seenmore evidently at latitudes less than 16 degree unlike thedecrease in O+ density which is observed throughout theconsidered latitudinal belt remains one of the equivocalfeatures of the present results.[17] Further, the significant increase in ion temperature

during the storm RP can be explained by two mechanisms:(1) the precipitation of energetic ring current particle and(2) the storm‐induced thermospheric circulation. It is sug-gested that the precipitation of highly energetic ring currentparticle in the earth’s atmosphere is responsible for heatingin the upper atmosphere [Krassovsky, 1968; Evans, 1970;Prolss et al., 1973]. The appearance of simultaneouslyenhanced values of H+ ion concentration along with iontemperature provides the strong evidence that the energeticparticles originating from ring current is responsible forproducing heating at low latitude region. Denton et al.[2002] by studying the SROSS‐C1 observations duringthe severe geomagnetic storm of 24 March 1991 alsoshowed the increase in the electron and ion temperature.The second mechanism involves the storm‐induced distur-bance circulation. The excessive amount of heating at highlatitude region during the storms triggers the disturbancecirculation which is directed equatorward and thus bringsthe heated gas away from the source region. In the des-cending part of the circulation, at middle and low latitudes,the air undergoes adiabatic compression, and thus causingrise in the thermospheric temperature at these latituderegions [Rishbeth et al., 1975]. In addition, it is also knownthat TADs dissipate their energy by heat conduction, vis-cosity, and ion drag during their propagation [Richmond,1978; Hunsucker, 1982]. Therefore, TADs might also be

YADAV ET AL.: GEOMAGNETIC STORMS A08311A08311

8 of 11

responsible for rise in thermospheric temperature at low‐latitude region. However, the appearance of substantialincrease in ion temperature at latitudes above 16 degreessuggests the crucial role of disturbance circulation/TADsat low‐latitude region in enhancing the thermospherictemperature.[18] Results show that the effect of storm‐induced dis-

turbance thermospheric circulation is more effectively seenin the nighttime ionosphere as compared to the daytime. It iswell known that the direction of meridional wind is equa-torward during the nighttime. Thus, during nighttime thetwo circulations, i.e., the background and storm‐inducedcoincides as they both are directed equatorward. This resultsin the amplification of the storm‐induced circulation whichin turn leads to the stronger lifting of the ionosphere. On thecontrary, during daytime, the poleward wind offers theresistive force to the storm‐induced equatorward wind andthus leads to the comparatively reduced lifting of the iono-spheric layer. The frequent occurrence of height rise duringnighttime as compare to daytime on the basis of the twocirculations (the background and storm‐induced ones) wassuggested by Danilov [2001].[19] The virtual height variation of different sounding

frequencies shows that the intensity of storm‐induced per-turbations enhances with increasing altitude. The appear-ance of significant storm‐induced effect in the virtual heightof higher sounding frequencies as compared to lower fre-quencies can be ascribed to the differential role of photo-chemical and transport processes at different altitude. It iswell known that the photochemical processes and solarcontrol plays a predominant role at the heights of F1 layerwhere the peak production layer lies during the daytimehours whereas the behavior of F2 region and above iscontrolled by the transport and diffusion processes whichare significant at these heights. The results strongly supportthe dominance of photochemical processes at the lowerregion and the dynamical processes at the altitude range ofF2 region and above, during the daytime hours.[20] The significant effect of storm RP is observed

during the daytime hours of 1 April 2001. The appearanceof considerably high hmF2 values during the daytime of1 April 2001 suggests the dominance of disturbed thermo-spheric equatorward meridional wind even during thesecond day of storm RP. These equatorward winds causesthe vertical motion of ionized layer over the low‐latituderegion along the slope geomagnetic field lines and thus leadsto the increase in the height of ionospheric layers. It is wellknown that the electron density in the ionosphere can beincreased either by increase in the ionization rate or byreduction in the recombination rate. The ionization processjust initiates during the early morning hours and theincreased height provides the reduction in recombinationprocess. The simultaneous presence of these two processesduring the early morning hours of 1 April 2001 could beresponsible for the anomalous enhancement of foF2 duringthis time period. It has been suggested that the positiveionospheric storm effects (increase in electron concentration)are also caused by neutral composition changes [Rishbeth,1991; Fuller‐Rowell et al., 1996; Field and Rishbeth,1997] and by the transport of ionization [Prölss, 1995]

which could be introduced either by electric fields [Reddyet al., 1990] or by thermospheric winds [Prolss, 1991].

5. Conclusion

[21] The present study emphasized the variation in H+

and O+ ion concentration along with the ion temperatureat the altitude of around 350 km by using SROSS‐C2observations during the major geomagnetic storm of31 March 2001 over the equatorial and low‐latitude regionof the Indian sector. The response of bottom side ionosphereto this intense storm has also been studied over the low‐midlatitude station, Delhi. Results reveal some striking and newaspects of the storm time ionospheric behavior which aresummarized as follows:[22] 1. The simultaneous enhancement of H+ ion and

depletion of O+ ion concentration is observed at the altitudeof about 350 km during the beginning of storm RP. Resultssuggests that the energetic particle (H+/H atoms) originatingfrom ring current and the consequent charge exchangereaction plays a crucial role even at the altitude range of350 km during intense geomagnetic storm.[23] 2. Significant enhancement in ion temperature is

observed during the beginning of storm RP.[24] 3. The signatures of storm‐induced TADs is seen in

terms of wave‐like fluctuations in the virtual height varia-tions of different sounding frequencies and also in electrondensity profile during the main and recovery phase ofstorm. These TADs are responsible for the observed shortduration positive storm effect during the end of MP andbeginning of RP.[25] 4. The dominance of storm‐induced equatorward

thermospheric wind is seen even during the second day ofstorm RP observed on 1 April 2001 which in turn leads tothe lifting of hmF2.[26] 5. The results strongly support the dominance of

photochemical processes at the lower altitude region and thedynamical processes at the F2 region heights and above,during the daytime hours.[27] Thus, the present study highlights the role of ener-

getic particle originating from ring current and the conse-quent charge exchange reaction at the F2 region altitudeduring the geomagnetically disturbed conditions. However,some more experimental evidences are required to confirmthese findings.

[28] Acknowledgments. One of the authors (Sneha Yadav) acknowl-edges the Council of Scientific and Industrial Research (CSIR), India, forproviding the financial support for conducting research work at theNational Physical Laboratory, New Delhi.[29] Robert Lysak thanks the reviewers for their assistance in evaluat-

ing this paper.

ReferencesAbdu, M. A. (1997), Major phenomena of the equatorial ionosphere‐thermosphere system under disturbed conditions, J. Atmos. Sol. Terr.Phys., 59, 1505–1519, doi:10.1016/S1364-6826(96)00152-6.

Abdu, M. A. (2001), Outstanding problems in the equatorial ionosphere–thermosphere electrodynamics relevant to spread F, J. Atmos. Sol. Terr.Phys., 63, 869–884, doi:10.1016/S1364-6826(00)00201-7.

Abdu, M. A., G. O. Walker, B. M. Reddy, J. H. A. Sobral, B. G. Fejer,T. Kikuchi, N. B. Trevedi, and E. P. Szusxczewicz (1990), Electric fieldversus neutral wind control of the equatorial anomaly under quiet and

YADAV ET AL.: GEOMAGNETIC STORMS A08311A08311

9 of 11

disturbed conditions: A global perspective from SUNDIAL 86, Ann.Geophys., 8, 419–430.

Abdu, M. A., I. S. Batista, G. O. Walker, J. H. A. Sobral, N. B. Trivedi, andE. R. de Paula (1995), Equatorial ionospheric electric fields during mag-netospheric disturbances: Local time longitude dependences from EITScampaigns, J. Atmos. Terr. Phys., 57, 1065–1083, doi:10.1016/0021-9169(94)00123-6.

Blanc, M., and A. D. Richmond (1980), The ionospheric disturbancedynamo, J . Geophys . Res . , 85 , 1669–1686, do i :10 .1029/JA085iA04p01669.

Buonsanto, M. J. (1999), Ionospheric storms—A review, Space Sci. Rev.,88, 563–601, doi:10.1023/A:1005107532631.

Burns, A. G., and T. L. Killeen (1992), The equatorial neutral thermosphericresponse to geomagnetic forcing, Geophys. Res. Lett., 19, 977–980,doi:10.1029/92GL00522.

Dabas, R. S., R. M. Das, V. K. Vohra, and C. V. Devasia (2006), Spaceweather impact on the equatorial and low latitude F region ionosphereover India, Ann. Geophys., 23, 1–9.

Danilov, A. D. (2001), F2‐region response to geomagnetic disturbances,J. Atmos. Sol. Terr. Phys., 63, 441–449, doi:10.1016/S1364-6826(00)00175-9.

Denton,M. H., G. J. Bailey, C. R.Wilford, A. S. Rodger, and S. Venkatraman(2002), He+ dominance in the plasmasphere during geomagneticallydisturbed periods: 1. Observational results, Ann. Geophys., 20, 461–470,doi:10.5194/angeo-20-461-2002.

Dessler, A. J., W. B. Hanson, and E. N. Parker (1961), Formation of thegeomagnetic storm main‐phase ring current, J. Geophys. Res., 66,3631–3637, doi:10.1029/JZ066i011p03631.

Evans, J. V. (1970), F region heating observed during the main phase ofmagnetic storms, J. Geophys. Res., 75, 4815–4823, doi:10.1029/JA075i025p04815.

Fejer, B. G. (1991), Low latitude electrodynamic plasma drifts, J. Atmos.Terr. Phys., 53, 677–693, doi:10.1016/0021-9169(91)90121-M.

Fejer, B. G., and J. T. Emmert (2003), Low‐latitude ionospheric distur-bance electric field effects during the recovery phase of the 19–21 October1998 magnetic storm, J. Geophys. Res., 108(A12), 1454, doi:10.1029/2003JA010190.

Fejer, B. G., C. A. Gonzalez, D. T. Farley, and M. C. Kelley (1979), Equa-torial electric fields during magnetically disturbed conditions, J. Geo-phys. Res., 84, 5797–5802, doi:10.1029/JA084iA10p05797.

Field, P. R., and H. Rishbeth (1997), The response of the ionospheric F2–1ayer to geomagnetic activity: An analysis of worldwide data, J. Atmos.Sol. Terr. Phys., 59, 163–180, doi:10.1016/S1364-6826(96)00085-5.

Forbes, J. M., R. G. Roble, and F. A. Marcos (1987), Thermosphericdynamics during the March 22, 1979 magnetic storm: 2. Comparisonsof model predictions with observations, J. Geophys. Res., 92,6069–6081, doi:10.1029/JA092iA06p06069.

Fuller‐Rowell, T. J., M. V. Codrescu, H. Rishbeth, R. J. Moffett, andS. Quegan (1996), On the seasonal response of the thermosphere andionosphere to geomagnetic storms, J. Geophys. Res., 101, 2343–2353,doi:10.1029/95JA01614.

Garg, S. C., et al. (2003), RPA experiment onboard the Indian satelliteSROSS‐C2–Some important aspects of the payload and satellite, IndianJ. Radio Space Phys., 32, 5–15.

Hocke, K., and K. Schlegel (1996), A review of atmospheric gravity wavesand travelling ionospheric disturbances, Ann. Geophys., 14, 917–940.

Hunsucker, R. D. (1982), Atmospheric gravity waves generated in thehigh‐latitude ionosphere, Rev. Geophys., 20, 293–315, doi:10.1029/RG020i002p00293.

Jones, K. L. (1971), Storm time variation of F2‐layer electron concentra-tion, J. Atmos. Terr. Phys., 33, 379–389, doi:10.1016/0021-9169(71)90143-7.

Jones, K. L., and H. Rishbeth (1971), The origin of storm increases ofmid‐latitude F‐layer electron concentration, J. Atmos. Terr. Phys., 33,391–401, doi:10.1016/0021-9169(71)90144-9.

Kelley, M. C., B. G. Fejer, and C. A. Gonzales (1979), An explanation foranomalous equatorial ionospheric electric fields associated with a north-ward turning of the interplanetary magnetic field, Geophys. Res. Lett., 6,301–304, doi:10.1029/GL006i004p00301.

Kil, H., L. J. Paxton, X. Pi, M. R. Hairston, and Y. Zhang (2003), Casestudy of the 15 July 2000 magnetic storm effects on the ionosphere‐driver of the positive ionospheric storm in the winter hemisphere, J. Geo-phys. Res., 108(A11), 1391, doi:10.1029/2002JA009782.

Klostermeyer, J. (1973), Thermospheric heating by atmospheric gravitywaves,J. Atmos. Terr. Phys., 35, 2267–2275, doi:10.1016/0021-9169(73)90142-6.

Krassovsky, V. I. (1968), Aurorae, Planet. Space Sci., 16, 47–59,doi:10.1016/0032-0633(68)90044-5.

Lakshmi, D. R., B. Veenadhari, R. S. Dabas, and B. M. Reddy (1997), Sud-den post‐ midnight decrease in equatorial F region electron densities

associated with severe magnetic storms, Ann. Geophys., 15, 306–313,doi:10.1007/s00585-997-0306-9.

Lu, G., A. D. Richmond, R. G. Roble, and B. A. Emery (2001), Coexis-tence of ionospheric positive and negative storm phases under northernwinter conditions: A case study, J. Geophys. Res., 106, 24,493–24,504,doi:10.1029/2001JA000003.

Lynn, K. J. W., M. Sjarifudin, T. J. Harris, and M. Le Huy (2004), Com-bined TOPEX/Poseidon TEC and ionosonde observations of negativelow‐latitude ionospheric storms, Ann. Geophys., 22, 2837–2847,doi:10.5194/angeo-22-2837-2004.

Mayr, H. G., and H. Volland (1974), Magnetic storm dynamics of thermo-sphere, J. Atmos. Terr. Phys., 36, 2025–2036, doi:10.1016/0021-9169(74)90190-1.

Meier, R. R., G. Crowley, D. J. Strickland, A. B. Christensen, L. J. Paxton,D. Morrison, and C. L. Hackeert (2005), First look at the 20 Novembersuperstorm with TIMED/GUVI: Comparisons with a thermosphericglobal circulation model, J. Geophys. Res., 110, A09S41, doi:10.1029/2004JA010990.

Millward, G. H., R. J. Moffett, S. Quegan, and T. J. Fuller‐Rowell (1993),Effects of an atmospheric gravity waves on the midlatitude ionosphericF layer, J. Geophys. Res., 98, 19,173–19,179, doi:10.1029/93JA02093.

Pavlov, A. V., S. Fukao, and S. Kawamura (2004), F region ionosphericperturbations in the low‐latitude ionosphere during the geomagneticstorm of 25–27 August 1987, Ann. Geophys., 22, 3479–3501,doi:10.5194/angeo-22-3479-2004.

Prolss, G. W. (1971), A theoretical study of the heating of the low latitudeupper atmosphere by the decaying magnetic storm ring current, Rep. 71‐4,Radioscience Lab., Honolulu, Hawaii.

Prolss, G. W. (1973), Radiation production and energy deposition by ringcurrent protons dissipated by the charge exchange mechanism, Ann.Geophys., 29, 503–508.

Prolss, G. W. (1982), Perturbation of low‐latitude upper atmosphere duringmagnetic substorm activity, J. Geophys. Res., 87, 5260–5266,doi:10.1029/JA087iA07p05260.

Prolss, G. W. (1991), Thermosphere‐ionosphere coupling during disturbedconditions, J. Geomag. Geoelectr., 43, 537–549.

Prölss, G. W. (1995), Ionospheric F region storms, in Handbook of Atmo-spheric Electrodynamics, vol. 2, edited by H. Volland, pp. 195–248,CRC Press, Boca Raton, Fla.

Prolss, G. W., K. Najita, and P. C. Yuen (1973), Heating of the low‐latitudeupper atmosphere caused by the decaying magnetic storm ring current,J. Geophys. Res., 35, 1889–1901.

Reddy, C. A., S. Fukao, T. Takami, M. Yamamoto, T. Tsuda, T. Nakamura,and S. Kato (1990), A MU radar‐based study of mid‐latitude F regionresponse to a geomagnetic disturbance, J. Geophys. Res., 95,21,077–21,094, doi:10.1029/JA095iA12p21077.

Richmond, A. D. (1978), Gravity wave generation, propagation and dis-sipation in the thermosphere, J. Geophys. Res., 83, 4131–4145,doi:10.1029/JA083iA09p04131.

Richmond, A. D. (1979), Thermospheric heating in magnetic storm:Dynamic transport of energy from high to low latitudes, J. Geophys.Res., 84, 5259–5266, doi:10.1029/JA084iA09p05259.

Richmond, A. D., and S. Matsushita (1975), Thermospheric response to amagnetic substorm, J. Geophys. Res., 80, 2839–2850, doi:10.1029/JA080i019p02839.

Rishbeth, H. (1991), F region storms and thermospheric dynamics, J. Geo-mag. Geoelectr., 43, 513–524.

Rishbeth, H., T. J. Fuller‐Rowell, and A. S. Rodger (1975), F region stormsand thermospheric circulation, J. Atmos. Terr. Phys., 37, 1055–1064,doi:10.1016/0021-9169(75)90013-6.

Roble, R. G., A. D. Richmond, W. L. Oliver, and R. M. Harper (1978),Ionospheric effects of the gravity wave launched by the September 18,1974, sudden commencement, J. Geophys. Res., 83, 999–1009,doi:10.1029/JA083iA03p00999.

Ruster, R. (1965), Height variations of the F2‐layer above Tsumeb duringgeomagnetic bay‐disturbances, J. Atmos. Terr. Phys., 27, 1229–1245,doi:10.1016/0021-9169(65)90084-X.

Sastri, J. H., K. B. Ramesh, and H. N. Rangnath Rao (1992), Transientcomposite electric field disturbances near dip equator associated withauroral substorms, Geophys. Res. Lett., 19, 1451–1454, doi:10.1029/92GL01447.

Sastri, J. H., N. Jyoti, V. Somayajulu, H. Chandra, and C. Devasia (2000),Ionospheric storm of early November 1993 in the Indian equatorialregion, J. Geophys. Res., 105(A8), 18,443–18,455, doi:10.1029/1999JA000372.

Sastri, J. H., J. Niranjan, and K. S. V. Subbarao (2002), Response ofthe equatorial ionosphere in the Indian (midnight) sector to the severemagnetic storm of July 15, 2000, Geophys. Res. Lett., 29(13), 1651,doi:10.1029/2002GL015133.

YADAV ET AL.: GEOMAGNETIC STORMS A08311A08311

10 of 11

Scherliess, L., and B. G. Fejer (1997), Storm time dependence of equatorialdisturbance dynamo zonal electric fields, J. Geophys. Res., 102(A11),24,037–24,046, doi:10.1029/97JA02165.

Sobral, J.,M.Abdu,W.González, B. Tsurutani, I. Batista, andA. deGonzález(1997), Effects of intense storms and substorms on the equatorial iono-sphere/thermosphere system in the American sector from ground‐basedand satellite data, J. Geophys. Res., 102, 14,305–14,313, doi:10.1029/97JA00576.

Sreeja, V., C. V. Devasia, S. Ravindran, T. K. Pant, and R. Sridharan(2009), Response of the equatorial and low‐latitude ionosphere in theIndian sector to the geomagnetic storms of January 2005, J. Geophys.Res., 114, A06314, doi:10.1029/2009JA014179.

Tinsley, B. A. (1981), Neutral atom precipitation—a review, J. Atmos.Terr. Phys., 43, 617–632, doi:10.1016/0021-9169(81)90124-0.

Yuan, Z., B. Ning, and X. Deng (2009), Effects of TADs on the F region ofthe mid‐latitude ionosphere during an intense geomagnetic storm, Adv.Space Res., 44, 1013–1018, doi:10.1016/j.asr.2009.05.022.

R. S. Dabas, R. M. Das, P. Subrahmanyam, and S. Yadav, Radio andAtmospheric Sciences Division, National Physical Laboratory, Council ofScientific and Industrial Research, New Delhi 110012, India. (sneha.yadav84@gmail.com)A. K. Gwal, Department of Physics, Barkatullah University, Bhopal

462026, India.

YADAV ET AL.: GEOMAGNETIC STORMS A08311A08311

11 of 11