TEC response during severe geomagnetic storms near the ...nopr.niscair.res.in/bitstream/123456789/7387/1/IJRSP 39(1) 11-24.pdf · TEC response during severe geomagnetic storms near

Indian Journal of Radio & Space Physics Vol. 39, February 2010, pp. 11-24

TEC response during severe geomagnetic storms near the crest of equatorial ionization anomaly

Amit Jain1,$,*, Sunita Tiwari1, Sudhir Jain2 & A K Gwal1 1Space Science Laboratory, Department of Physics, Barkatullah University, Bhopal 462 026, India

2Department of Physics, Govt M L B Girls’ P G College, Bhopal, India E-mail: [email protected]

Received 26 May 2009; revised 17 November 2009; accepted 1 December 2009

The responses of the ionosphere near equatorial anomaly crest as observed at Bhopal (geographic 23.2°N, 77.4°E, and magnetic latitude 14.2°N), India for geomagnetic storms during 15 May 2005 (SSC: 0239 hrs UT, SymH: -305 nT) and 24 August 2005 (SSC: 0615 hrs UT, SymH: -179 nT) has been studied using total electron content (TEC). TEC data is recorded by GPS Ionospheric Scintillation and TEC Monitor (GISTM) GSV4004A at Bhopal. The temporal variations of vertical total electron content (VTEC) during these storms were analysed. Fifteen minute average of VTEC is considered for analysis. More than +68% and +70% deviation in VTEC was observed for 15 May 2005 and 24 August 2005 storms, respectively, relative to average of five quiet days of the respective months. Significant deviations in VTEC during the main phase of the storms were found to be associated with the prompt penetration of electric fields which originate due to the under-shielding and over-shielding conditions. Peaks like structure in EIA region were observed for both the storms. Peaks like structure well before the fountain effect in EIA region were observed for 15 May 2005. Latitudinal enhancement of anomaly peak in EIA region was also observed for both the storms. Suppressed equatorial electrojet strength and consequently VTEC, observed on 25 August 2005 during recovery phase of the storm, are mainly due to the thermospheric composition changes and also possibly due to the delayed disturbance dynamo electric fields in this region. The range error is also discussed in the present paper.

Keywords: Total electron content, Ionospheric disturbance, Prompt penetration electric fields (PPE), Travelling atmospheric disturbances (TAD), Disturbance dynamo electric fields (DDE)

PACS Nos: 94.20.Vv; 94.30.Lr

1 Introduction The Sun is the major source that controls the near

earth space and drives its weather and climate. Earth’s magnetosphere, thermosphere and ionosphere are driven mostly by the energy source of the Sun. Coronal mass ejections (CMEs) from the Sun interact with the solar wind and the interplanetary magnetic field (IMF) during their propagation, disrupt the flow of the solar wind. Fast CMEs (faster than ~ 500 km s−1) eventually drive shocks on the earth’s magnetosphere and trigger geomagnetic storms. Large geomagnetic storms, among other things, cause electrical power outages and damage communications satellites. Since every geomagnetic storm has unique character and therefore, forecasting the response of ionospheric parameters like total electron content (TEC) is one of the major issues in space weather studies. In this context, the response of the ionosphere over low latitudes has been the matter of research across the globe. The severe space weather events on 15 May 2005 and 24 August 2005 were such

geomagnetic storm events when prominent changes in the ionosphere were observed over the low and equatorial latitudes and have been reported in this paper.

The ionospheric response to geomagnetic storms at low and equatorial latitudes in the Indian longitude sector has been observed since last few decades and reported by researchers using different techniques like ionosonde, radar and satellites measurements for ionospheric parameters, viz. h′F, foF2, VHF/UHF scintillations, electron density, zonal electric fields, and total electron content (TEC)1-16. Recently, the response of TEC to the magnetic storm has been reported for the station near the crest of equatorial anomaly17-19 using GPS data. The response of the equatorial and low latitude ionospheric region in Indian sector has been carried16,20 using GPS and ionosonde data.

During the geomagnetic storms, it has been observed that ionospheric disturbances cause data loss in satellite communication links and loss of signal for

INDIAN J RADIO & SPACE PHYS, FEBRUARY 2010

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GPS satellite21-23. This can also reduce availability of satellites for GPS navigation, which may affect GPS positional accuracy, navigational accuracy and the GPS tracking performance21,22,24.

The interaction between solar wind and magnetosphere under southward interplanetary magnetic field (IMF) Bz causes a change in the region-1 current (under-shielding) leading to a sudden increase in the dawn-to-dusk convection electric field at high latitudes6,25-28. This results in instantaneous penetration of an electric field from high latitude to the middle and equatorial ionosphere8,26-31. These transient electric fields have typical rise and decay times, shorter than about 15 min and lifetime of about an hour (ref 32 and references therein) or more than an hour in some cases27,28. These prompt penetrating electric (PPE) fields have eastward (westward) polarity on the day (night) side26,33. In the recovery phase (northward IMF Bz), the electric field, due to the shielding layer that remains, penetrates to equatorial latitudes as an over-shielding electric field with opposite polarity33,34.

The ion convection under large high-latitude electric fields can cause accelerations of the neutrals leading to equator ward disturbance winds while the continuing energy input causes heating of the high-latitude thermosphere-ionosphere system whereby atmospheric disturbances propagate to lower latitude in the form of gravity waves (TIDs) or travelling atmospheric disturbances (TADs) including disturbances in thermospheric winds7,35,36. They produce longer-lasting disturbance dynamo electric fields (DDE) that begin to dominate the low-latitude electrodynamic processes within few hours (~2-3 h) from the onset of a storm and continues for several hours to days7,37-43. Equatorial and low latitude F-region ionosphere modified drastically during geomagnetic storms as compared to quiet days, are mostly due to the combined effects of relatively short-lived PPE7,25,26 and longer lasting ionospheric DDE. PPE and DDE have in general opposite polarity local time dependences. Therefore, the equatorial ionization anomaly (EIA), the electrojet current (EEJ) and the plasma bubble irregularity/equatorial spread F (ESF) processes, can be greatly enhanced or inhibited under the competing influences of these electric fields.

A PPE can cause large dayside enhancement in the TEC as observed by satellite borne and ground-based GPS receivers18,19,44-49. The simultaneous response on the nightside is expected to be a decrease of TEC. An

important characteristic of a TEC storm, especially during a major storm event, is noted as large latitudinal expansion and intensification of the EIA39,47,50-52. On the other hand, a DDE of westward polarity could considerably reduce dayside TEC45 and cause contraction of the EIA, while that of eastward polarity could cause large F-layer uplift during night hours7,39,52-54 resulting in intensification of the EIA36,52. A large westward plasma drift in the evening equatorial ionosphere was observed30 for southern crest of equatorial anomaly station Ascension Island for the severe magnetic storm of 15 July 2000 and attributed it to ionospheric disturbance dynamo. Recently, Dashora et al.

19 studied the TEC response to the geomagnetic storm of 15 May 2005 at Udaipur (24.73˚N, 73.73˚E), a station near the crest of equatorial ionization anomaly and Hyderabad (17.33˚N, 78.47˚E) a low latitude station using GPS TEC. They found that the large enhancement (or decrement) in vertical TEC is attributed to prompt penetration of electric fields and travelling atmospheric disturbances (TADs). Bagia et al.

18 studied the 24 August 2005 geomagnetic storm for Rajkot (22.29˚N, 70.74˚E) station using GPS TEC and found that the enhancement to vertical TEC is attributed to prompt penetration of electric fields during the storm day and thermospheric composition changes and disturbance dynamo electric fields on the subsequent recovery day. In the present paper, the effect of eastward and westward prompt penetration electric fields (PPE), travelling atmospheric disturbances (TAD) and disturbance dynamo electric fields (DDE) at Bhopal (23.2°N, 77.4°E, MLAT 14.2°N), a station near the anomaly crest, for the two severe geomagnetic storms occurred on 15 May 2005 (SSC: 0239 hrs UT, SymH: -305 nT) and 24 August 2005 (SSC: 0615 hrs UT, SymH: -179 nT) have been discussed. 2 Data and method of analysis

The GPS Ionospheric Scintillation and TEC Monitor (GISTM) GSV4004A is used for TEC data recorded at Bhopal (23.2°N, 77.4°E, geomagnetic 14.2°N). GISTM system consists of NovAtel OEM4 dual frequency GPS receiver and a low phase noise oven-controlled crystal oscillator (OCXO) that is required for monitoring phase scintillation. The GPS receiver system can track up to 11 GPS signals at the L1 frequency (1575.42 MHz) and the L2 frequency (1227.6 MHz). It measures phase and amplitude (at

JAIN et al. : TEC RESPONSE DURING SEVERE GEOMAGNETIC STORMS

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50-Hz rate), code/carrier divergence (at 1 Hz rate) for each satellite being tracked on L1. It computes TEC from combined L1 and L2 pseudo range and carrier phase measurements. TEC and TEC rates are logged every 15 seconds. TEC is measured in TECU (1 TECU = 1x1016 el m-2). Each GPS satellite has its own unique sequence of number and is called a pseudo random number generally abbreviated as PRN.

The line of sight (slant) TEC (or STEC) data obtained from GPS receiver includes the receiver and satellite bias errors. These errors have to be eliminated during the post processing. These errors are known to vary over periods of weeks to months. Therefore, these errors do not pose any serious threat to the shorter period studies of about one week or so. Here, the storm time TEC has been compared with quiet time TEC values of nearby period of few days; these errors have not been removed completely. STEC for 13-17 May and 22-26 August 2005 were obtained from GISTM receiver and their response to two geomagnetic storms is presented in this paper. STEC is converted to VTEC by assuming the penetration point of the satellite ray path with the ionospheric thin shell at a height of 350 km from the Earth surface using following formula:

VTEC = χcos . (STEC)

where, χ , is the angle of incidence at 350 km altitude

of GPS ray path from satellite to ground receiver; χcos , an oblique factor, is defined as55:

2

100E

E

HR

)cos(.R1cos

+−=

eχ

where, ER , is the radius of the Earth; e, the elevation

angle of the satellite; and ionoH , the height of

ionospheric penetration point (IPP) usually assumed to be 350-400 km. Rama Rao et al.

56 demonstrated that the IPP height of 350 km for Indian region can be suitably used for vertical TEC, provided the elevation angle cut-off is taken to be greater than 50˚. Therefore, average VTEC values were obtained by averaging every 15 min values of VTEC having elevation angle > 50˚ to calculate the percentage deviation of VTEC with respect to average of international five quiet days of respective months for all visible PRN. In order to see the large spatial distribution of VTEC for individual PRN, the

elevation angle greater than >30˚ is considered which covers ~16˚-28˚N (geomagnetic ~9˚-18˚N) and ~72˚-82˚E from Bhopal station as can be seen from the footprints of GPS ray path (Fig. 1) at 350 km IPP for a the typical quiet day (27 May 2005). EEJ data

The proxy to the prompt penetration of electric field can be the equatorial electrojet strength (EEJ), which is the difference of horizontal component (∆H)e

at equator and (∆H)off-e at the low latitude station. In the Indian region, Tirunelveli (8.42°N, 77.48°E; geomagnetic latitude 0.57°S) (∆H)Tir and Alibagh (18.63°N, 72.87°E; geomagnetic latitude 10.03°N) (∆H)Ali are used to calculate the EEJ. 3 Results and discussion

In the present paper, the effect of two severe geomagnetic storms on 15 May 2005 (SSC: 0239 hrs UT, SymH: -305 nT) and 24 August 2005 (SSC: 0615 hrs UT, SymH: -179 nT) during the solar minimum period from the observations of GPS-TEC at Bhopal, a station situated near the crest of anomaly, has been discussed.

3.1 Storm on 15 May 2005

An eruption of solar active region AR 10759 produced a fast halo CME and was associated with a long-duration M8 class X-ray flare with peak emission at 1657 hrs UT on 13 May 2005. The disturbance reached the earth on 15 May 2005. The interplanetary event corresponding to the geomagnetic storm on 15 May 2005 is shown in Figs 2 (a-f) for the

Fig. 1 — Coverage of IPP latitude and longitude by GPS satellites from Bhopal station on the typical day 27 May 2005


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period 13-17 May 2005. The solar wind (proton) speed (Vp), the proton density (Np) and IMF Bz are shown in Figs 2 (a-c) with a resolution of 64, 64 and 16 s, respectively. These parameters were obtained as level 2 data from the website http://www.srl. caltech.edu/ACE/ASC/. The variation of symmetric geomagnetic disturbance index (Sym-H), rate of Sym-H (dSymH/dt per 10 min) and percentage deviation of VTEC are given in Figs 2 (d-f), respectively. Sym-H index was obtained from the WDC-Kyoto website.

In preparing Fig. 2, solar wind parameters have been shifted in time in accordance with sudden storm commencement (SSC) as observed in Sym-H index for better comparison of the parameters obtained from the satellite and ground based instruments. This also minimizes the errors in computation of time shifts of the shock arrival time from ACE satellite to the magnetopause. The interplanetary shock is not visible at the time of SSC due to non-availability of solar wind speed and proton density for the period

0210-0348 hrs UT. From Fig. 2, the shock can be identified by the abrupt increase in solar wind speed from 400 to 1000 km s-1 and density increase from 4 to 30 cm-3.

After the SSC, IMF Bz fluctuate between southward and northward and turned strongly and steeply southward at 0530 hrs UT. It reached to the lowest value of ~ 48 nT at 0553 hrs UT and remained for 15 min. It then turned northward and touched its normal value at 0825 hrs UT. Such a sudden decrease in IMF Bz caused steep decrease in Sym-H and commenced the main phase of the geomagnetic storm. The value of Sym-H reached to minimum -305 nT at 0821 hrs UT. For the steep decrease (increase) in IMF Bz and subsequent occurrence of under-shielding (over-shielding) condition, the prompt penetration electric field directed eastward (westward) during the day and westward (eastward) during night, which produces the upward (downward) and downward (upward) vertical plasma drifts, respectively in the

Fig. 2 — Variation of: (a) Solar wind (proton) speed Vp; (b) proton density Np; (c) IMF Bz; (d) Sym-H; (e) dsymH/dt; and (f) percentage deviation of VTEC with respect to average of five quiet days during 13 - 17 May 2005. Solar wind parameters (solar wind speed, proton density and IMF Bz) are obtained from ACE website (http://www.srl.caltech.edu/ACE/ASC/). Sym-H index are obtained from World Data Centre for Geomagnetism, Kyoto web site (http://wdc.kugi.kyoto-u.ac.jp/aeasy/index.html)


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low latitudes and equatorial ionosphere6,8,25-27,29-31,33,34. Since the measurement of zonal winds and ionosphere electric fields of E-region are practically difficult57, therefore, for the evidence of prompt penetration of electric fields to low latitude, sudden increase (decrease) in AE index and/or a marked decrease (increase) in Sym-H index may be used as proxies to determine the occurrences as well as the time of penetration of electric field into the equatorial and low latitudes27,28,58,59. However, in such cases where the AE index does not represent any sudden increase (decrease), then dSymH/dt is a better index to determine the time of penetration59. The rate of Sym-H showed a rapid decrease (-12.7 nT per 10 min) at 0625 hrs UT. This suggests that sharp decrease in IMF Bz created the under-shielding condition and the prompt penetration of electric fields occurs at low latitudes around 0625 hrs UT. Such penetration effects were reported by Basu et al.

58 for 22 September 1999 and 22 October 1999 storms; Haung et al.

28 for three storms (i.e. 6-7 April 2000, super storm 29 October 2003 and 9 November 2004). The other proxy to the prompt penetration of electric field can be the equatorial electrojet strength (EEJ). Rastogi & Klobuchar60 showed that during magnetically disturbed periods, the presence or absence of the anomaly was found to depend only upon the strength of the electrojet current and the varying EEJ current, that is, the equatorial electric field is very effective in moving the ionization to anomaly latitudes of 15-20˚. Hence, EEJ have been used to infer the direction of prompt penetration electric field, and hence, the vertical movement of F-region plasma over equator. In Fig. 3, hourly EEJ strength has been plotted for the entire duration. The rapid increase in EEJ from ~40 to ~170 nT at ~0600 hrs UT on 15 May corresponds to the penetration of high latitude convection electric field over the dip equator and the rapid decrease in EEJ to -45 nT was observed at ~0800 hrs UT. This rapid increase (decrease) was due to the enhanced eastward (westward) electric field over dip equator and enhanced EEJ (counter EEJ or CEEJ) current. This prompt penetration of fields are directed eastward (westward) during the day which uplifts the plasma from equatorial dip due to E×B drift and diffuses along the magnetic field lines and modified equatorial ionization anomaly (EIA) drastically. It is clearly seen from Fig. 2(f) that for 15 May, percentage deviation in VTEC is +68%, which shows positive response to

geomagnetic storm. This deviation is observed at 0937 hrs UT ~ 3 h after the onset of the main phase of the storm (~ 0625 hrs UT). This delay is expected due to slow process of equatorial ionization anomaly (EIA) development and any perturbation in equatorial vertical drift observed in the EIA zone density variation60,61. Also, in the Indian sector, the time of maximization of the daytime anomaly is around 0800-0900 hrs UT (1330–1430 hrs IST) for normal EEJ days60,61. So the time 0625 hrs UT corresponds to the initial phase of EIA built up in the present longitude observations. Thus, the anticipated observation time of modified EIA after around 3 h (~ 0925 hrs UT) is well matched with the observation time.

At Bhopal, the normal anomaly peak (~60 TECU) is found to lay ~13-15˚N magnetic latitude (MLAT) around ~0830-1030 hrs UT for a quiet day (27 May 2005) as inferred from Figs 4 (a and c). On 15 May, this anomaly peak shifted spatially (toward pole) up to ~16.5˚N (MLAT) between 0930 and 1015 hrs UT [Fig. 4(b)]. Two distinct lesser peaks are also discernible around 0730 hrs UT at ~16˚N (MLAT), ~15˚N (MLAT) and one around 0800 hrs UT at ~13˚N (MLAT). These peaks are clearly visible in the VTEC variation plot of different satellites from Fig. 4(d). The satellite trajectory can give sufficient information about ionospheric conditions in space and time precisely. Therefore, to see precisely the exact time of maximum enhancement and magnetic latitude of individual satellites observation, the storm time percentage deviation of VTEC is plotted for

Fig. 3 — Variation of equatorial electrojet strength (EEJ) for 13-15 May 2005


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6 satellites (#PRN 1, 14, 15, 18, 21 and 22) in left panel of Figs 5 (a – c). Percentage deviation of VTEC (blue line) is computed with respect to average quiet days for the respective PRN. These figures show two to three enhanced peaks in different time scales. The first peak is pronounced around 0700-0800 hrs UT and others after 0830 hrs UT. Similar peaks are reported by Dashora et al.

19 for Udaipur (24.73˚N, 73.73˚E) and Hyderabad (17.33˚N, 78.47˚E). The maximum percentage deviation of VTEC (~+120%) is observed for PRN #14 at ~0930 hrs UT. The satellite trajectory is also plotted in the same graph (sky line, left panel) for easy reference as a function of time (hrs UT) and space (geomagnetic latitude in right ordinate). The rate of TEC index (ROTI, in units of TECU per 5 min) is a measure of the size of the irregularity62 are also plotted (brown line) in the right panel of Figs 5 (a-c) and compared with the nearby control day 13 May 2005 (dotted line). Large variation (>0.15 TECU per 5 min) in ROTI for the

mentioned PRNs during the storm period suggests the disturbed ionospheric conditions in EIA region during 0700 – 1130 hrs UT as the ROTI alone is used as an indicator for the occurrence of irregularity and its strength in the ionosphere. The large fluctuations in ROTI are matched with deviation of VTEC (left panel) for the storm of 15 May 2005.

The temporal and latitudinal deviation in VTEC is discernible from Figs 5 (a-c). Clear +50 to +98% deviation in VTEC (increase of 27 to 40 TECU) around 0730 – 0800 hrs UT for PRN# 14, 15, 18, 21 and 22 has been seen. The peaks almost cover the EIA region in the Indian sector (MLAT 13˚-17˚N and 75˚-79˚E). Its effect would be observed in EIA zone around 0930 hrs UT. This justifies the observed peaks in VTEC around 0940 hrs UT in PRN # 1, 14 and 22. Same peaks were also reported by Dashora et al.

19 for Udaipur (24.73˚N, 73.73˚E) and Hyderabad (17.33˚N, 78.47˚E) in GPS TEC and discussed the formation of these peaks. The formation of peaks in VTEC around

Fig. 4 —VTEC map in hrs UT and geomagnetic latitude for: (a) quiet day 27 May 2005, and (b) storm day 15 May 2005; VTEC plot for (c) the quiet day, and (d) the storm day. Legend on right side in (c) and (d) show the PRN’s by different colours


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Fig. 5 — Variation of VTEC and ROTI on 15 May 2005 for: (a) PRN #1 and 14; (b) PRN # 15 and 18; and (c) PRN # 21 and 22

Left panel: Percentage deviation of VTEC (blue line) from quiet days VTEC average of the respective PRN as a function of time. Trajectory of the satellite (sky line) is also plotted as a function of time with geomagnetic latitude.

Right panel: Rate of TEC index (ROTI) (brown line) compared with ROTI of the near by control day 13 May 2005 (dashed line).

0750 hrs UT in PRN #14, 15, 18, 21 and 22 attributed to local low latitude ionospheric response to the dawn-to-dusk (eastward) and dusk-to-dawn (westward) PPE fields (ref 19 and references therein) which are enhanced during the rapid change of southward and northward turning, respectively34. These PPE field modify the zonal electric field of local low latitude ionosphere and uplift (suppress) the plasma drift. The enhancement in VTEC and formation of wave like different peaks after 1000 hrs UT was attributed to EIA development and equator-ward travelling ionospheric disturbances (TID’s) (ref 19 and references therein).

3.2 Storm on 24 August 2005

An eruption of solar active region AR 798 produced a halo CME and was associated with a long duration M5.6 class X-ray flare with peak emission at 1727 hrs UT on 22 August 2005. The disturbance reached the earth on 24 August 2005. The interplanetary event corresponding to the geomagnetic storm on 24 August 2005 is shown in Fig. 6 for the period 22 - 26 August 2005. From Figs 6 (a and b), the shock can be identified by the abrupt increase in the solar wind speed (proton) from about 448 to 539 km s-1 and the density increase from 7.7 to19.7 cm-3 denoted by vertical dashed line (SSC 0615 hrs UT).

After the SSC, IMF Bz fluctuate with small amplitude in the southward and northward direction and turned strongly southward at 0853 hrs UT. It reached the lowest value of ~ -53 nT at 0931 hrs UT and then turned northward again and touched to normal value at ~1042 hrs UT. It turned southward again and fluctuated with large amplitudes between (-34) and (-2) nT till 1223 hrs UT and almost remained around zero afterwards [Fig. 6(c)]. Such a sharp decrease in IMF Bz caused steep decrease in Sym-H index value and commenced the main phase of the geomagnetic storm. The sym-H index value reached a minimum of -179 nT at 1151 hrs UT followed by the recovery phase [Fig. 6(d)]. This sharp decrease in IMF Bz and subsequent occurrence of under-shielding condition, the convection electric fields from high latitudes penetrated into the low latitudes. This is confirmed by the marked decrease of Sym-H index and sharp decrease of dSymH/dt (-6.3 nT per 10 min) at ~0915 hrs UT [Fig 6(e)] as used by Haung et al.

27 for Jicamarca and Millstone Hill radars and Ram et al.

59 with ground based and space-borne instruments for Pacific regions. This is


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supported by the rapid increase in EEJ around 0900 hrs UT (Fig. 7). The rapid decrease in Sym-H and its minimum value time suggest that the effect of prompt penetration electric fields was in the early evening sector of the Indian region. Low latitude ionosphere showed positive response to this geomagnetic storm and more than +70% deviation in average VTEC was reported on 24 August 2005 as shown in Fig. 6(f).

As observed at Bhopal, the normal anomaly peaks lie in the range ~ 12.5-13.5˚N (MLAT) around 0800 hrs UT with peak value ~35 TECU for the quiet day 20 August 2005 as shown in Figs 8(a and b). On the storm day, two enhanced anomaly peaks (~ 70 TECU and ~ 60 TECU) were developed, one around 1130 hrs UT and other around 1300 hrs UT as shown in Figs 8 (c and d). The enhanced peaks were distributed in a large area from 12˚ to 16.5˚N (MLAT) around 1100 hrs UT and from 13˚ to 14.5˚N (MLAT)

Fig. 6 — Variation of: (a) Solar wind (proton) speed Vp; (b) proton density Np; (c) IMF Bz; (d) Sym-H; (e) dsymH/dt; and (f) percentage deviation of VTEC with respect to average of five quiet days during 22-26 August 2005. Solar wind parameters (solar wind speed, proton density and IMF Bz) are obtained from ACE website (http://www.srl.caltech.edu/ACE/ASC/). Sym-H index are obtained from World Data Centre for Geomagnetism, Kyoto web site (http://wdc.kugi.kyoto-u.ac.jp/aeasy/index.html)

Fig. 7 — Variation of equatorial electrojet strength (EEJ) during 22-24 August 2005


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around 1300 hrs UT which shows temporal as well as the latitudinal shift (towards pole) of anomaly peak.

The percentage VTEC deviation and ROTI for different PRN have been plotted in Figs 9(a-c) [similar to Fig. 5(a)]. Out of the two peaks, the first peak around 1115 hrs UT and other around 1330 hrs UT can be clearly seen in the left panel of Figs 9(a-c). For the peak around 1115 hrs UT, +54 to +133% deviation in VTEC (increase of ~20 to 30 TECU) occurred and distributed in a large area (MLAT 15-18˚N, 73-80˚E). For the peak around 1330 hrs UT, +37 to +106% deviation of VTEC occurred (15-23˚N and MLAT 12-17˚N) covering geographic longitude 77-82˚E. It covered almost the Indian EIA region and the extension of EIA region toward pole was higher than normal values. Large variation of ROTI (0.1 to 0.35 TECU per 5 min.) during the storm period suggested the disturbed ionospheric conditions in EIA region during 1100 – 1330 hrs UT (right panel of Figs 9(a-c) and were matched with positive deviation of VTEC (left panel).

The high latitude convection electric field penetrated to low latitude when IMF Bz turned southward during the under-shield condition ~ 0915 hrs UT on 24 August. Around this time, EEJ strength increased suddenly from 13 to 69 nT and then decreased to -27 nT. This sudden increase and decrease showed the prompt penetration of electric field in under-shield and over-shield conditions. Since the prompt penetration of electric fields was during the local post-noon hours, its eastward polarity uplifts more plasma from dip equator by enhanced E×B drift. This uplifted plasma then diffused along the magnetic lines to higher latitudes and the enhanced EIA was observed. The EIA development is a slow process and any perturbation in equatorial vertical drift is observed in the EIA zone density variation after a delay of 3-4 h (refs 60,61). Therefore, any increment over and above the day-to-day variability in the VTEC in the EIA zone due to modified fountain effect should be present after around 3 h. Thus, the anticipated time to reach such effects in the EIA zone

Fig. 8 — VTEC map in hrs UT and geomagnetic latitude for: (a) quiet day 20 August 2005, and (b) storm day 24 August 2005; VTEC plot for (c) the quiet day, and (d) the storm day. Legend on right side in (c) and (d) show the PRN’s by different colours


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would be after 1230 hrs UT. This time matched well with the observed enhancement around 1315 hrs UT followed by the sharp decrease in VTEC for all PRN’s. In this sector, the sharp decrease in VTEC was due to the higher recombination rates after sunset which may be the cause for formation of peak around 1315 hrs UT.

The peak around 1115 hrs UT were attributed to the local low latitude ionospheric response to the eastward PPE fields due to the under-shield condition at high latitude during the sudden turning of IMF Bz to southward. These PPE fields modified the zonal electric fields which uplift the low latitude ionospheric plasma to the higher altitudes where the recombination rate is less, thereby, increase in the electron density and enhanced VTEC were observed. Just after, the IMF Bz turns northwards sharply which penetrated the prompt electric fields into low and equatorial latitudes which is westward during the day. Therefore, a downward drift of the plasma decreased the electron density sharply due to higher recombination rate at a lower attitude leading to the decrease in VTEC and thus the peaks around 1115 hrs UT were formed.

On the subsequent day (25 August 2005), the suppressed VTEC was observed for the whole day after 0400 hrs UT [Fig. 10(a)]. The EEJ strength was very low (~15 nT) and fluctuated virtually for whole day and so there was no net direction persisted to develop the EIA, due to this, the diffused pattern in VTEC Map [Fig. 10(b)] was observed. The causative mechanism for this suppression is somewhat complicated. This effect may be due to late disturbance dynamo electric fields or due to thermospheric composition changes or may be the combined effect of the both. Recently, the same observations were reported by Bagia et al.

18 for the 24 and 25 August for Rajkot (22.29˚N, 70.74˚E) using GPS TEC. They also attributed the enhanced VTEC on 24 August to the prompt penetration of electric fields. They also observed the enhancement in O/N2 ratio on 24 August and depletion on 25 August using TIMED/GUVI images. Since the electron loss at F2 peak depends upon the recombination/attachment with N2, as N2 density decreases (increases) the electron density will increase (decrease) giving rise to positive (negative) storm effect. So on 25 August, the depletion in VTEC was observed mainly due to the thermospheric composition changes18, however, disturbance dynamo electric field also played important role.

Fig. 9 — Variation of VTEC and ROTI on 24 August 2005 for: (a) PRN #19 and 27; (b) PRN # 24 and 28; and (c) PRN # 8 and 11 Left panel: Percentage deviation of VTEC (blue line) from quiet days VTEC average of the respective PRN as a function of time. Trajectory of the satellite (sky line) is also plotted as a function of time with geomagnetic latitude. Right panel: Rate of TEC index (ROTI) (brown line) compared with ROTI of the near by control day (dashed line)


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3.3 GPS position accuracy during storms

It is well known that position accuracy from GPS navigation satellites is largely dependent on the ionospheric conditions through which its signals received at ground receiver besides the geometric positions of satellites in space for position solutions. Since, GPS positioning errors may increase during the storm periods and to investigate it, latitudinal and longitudinal errors in meters on storm days (15 May and 24 August) have been compared with the quiet days (14 May and 25 August) for the period (0600-1200 hrs UT and 0400-1400 hrs UT) as shown in Figs 11(a and b). It is discernible from both the figures that during the storm period latitudinal and longitudinal errors both increased. Also, the range error is directly proportional to TEC along the ray path and since 6.15 TEC units correspond to the range error of 1 m on L1 frequency; hence the change in VTEC of ~60 TECU and ~30 TECU correspond to 9.76 m and 3.252 m, respectively for the 15 May and 24 August storms. These range error may have devastating effects on navigation system20,22.

4 Conclusions

In the present paper, VTEC response of the two severe geomagnetic storms on 15 May and 24 August 2005 during the solar minimum period has been shown from the observations of GPS-TEC at Bhopal, a station situated near the crest of anomaly region. The SSC and main phase occurred in local day time for both the storms. Prompt penetration electric fields originate due to under-shielding and over-shielding conditions and peaks like structure in EIA

region were observed for both the storms. This is similar to one reported by Dashora et al.

19 for 15 May 2005 storm at Udaipur (24.73˚N, 73.73˚E) and Hyderabad (17.33˚N, 78.47˚E), but almost similar results have been reported for 24 August 2005 storm at Bhopal (23.2°N, 77.4°E, MLAT 14.2°N). The travelling ionospheric disturbances (TID’s) were responsible for the formation of wave like nature in VTEC in the Indian longitude sector covering range of latitudes and longitudes for 15 May, whereas it was not observed for 24 August. The suppression in VTEC was observed on 25 August 2005, which was mainly due to thermospheric composition changes and also possibly due to the disturbance dynamo electric fields.

Since the range error is directly proportional to TEC along the ray path (1 m for 6.15 TEC units on L1 frequency), the range error for 15 May and 24 August storms are 9.76 and 3.252 m, respectively. The range errors are more during the high solar activity and this can have devastating effects on navigation system20,22. Therefore, this study highlights the intense threat to GPS based navigation system from space weather events.

In this study, it was found that although both the storms occur during the local day hours but they have their own characteristics and have different effects on the low latitude ionosphere depending on their strength, phase and local time of occurrence. Such studies are useful for developing the regional TEC model for better understanding and prediction at the crest of equatorial anomaly region in which storm parameters are used as input.

Fig. 10 — (a) Variation of VTEC, and (b) VTEC map for 25 August 2005


22

Acknowledgments

One of the authors, Amit Jain, acknowledges financial support from UGC, New Delhi through Teacher Research Fellowship. The solar wind speed, the proton density and IMF Bz data were obtained from ACE website (http://www.srl.caltech.edu/ ACE/ASC/). The Sym-H index values were obtained from World Data Centre for Geomagnetism, Kyoto website. The authors thank Indian Institute of Geomagnetism (IIG), Mumbai for providing the EEJ data. Authors thank reviewers for their suggestions in improving the content of the paper.

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