The equatorial zonal electric field responses to prompt penetration of eastward convection electric fields (PPEF) were compared at closely spaced longitudinal intervals at dusk to premidnight sectors during the intense geomagnetic storm of 17 March 2015. At dusk sector (Indian longitudes), a rapid uplift of equatorial F layer to >550 km and development of intense equatorial plasma bubbles (EPBs) were observed. These EPBs were found to extend up to 27.13°N and 25.98°S magnetic dip latitudes indicating their altitude development to ~1670 km at apex. In contrast, at few degrees east in the premidnight sector (Thailand‐Indonesian longitudes), no significant height rise and/or EPB activity has been observed. The eastward electric field perturbations due to PPEF are greatly dominated at dusk sector despite the existence of background westward ionospheric disturbance dynamo (IDD) fields, whereas they were mostly counter balanced by the IDD fields in the premidnight sector. In situ observations from SWARM‐A and SWARM‐C and Communication/Navigation Outage Forecasting System satellites detected a large plasma density depletion near Indian equatorial region due to large electrodynamic uplift of F layer to higher than satellite altitudes. Further, this large uplift is found to confine to a narrow longitudinal sector centered on sunset terminator. This study brings out the significantly enhanced equatorial zonal electric field in response to PPEF that is uniquely confined to dusk sector. The responsible mechanisms are discussed in terms of unique electrodynamic conditions prevailing at dusk sector in the presence of convection electric fields associated with the onset of a substorm under southward interplanetary magnetic field Bz.

In this paper, we present unique results of equatorial and low‐latitude ionosphere response to one of the major geomagnetic storms of the current solar cycle that occurred during 17–18 March 2015, where Dst reached its minimum of −228 nT. Here we utilized data from magnetometers, chain of ionosondes located at Tirunelveli (8.73°N, 77.70°E; geometry: 0.32°N), Hyderabad (17.36°N, 78.47°E; geometry 8.76°N), and Allahabad (25.45°N, 81.85°E; geometry 16.5°N) along with multistation GPS receivers over Indian sector. The observations showed a remarkable increase of h′F to as high as ~560 km over Tirunelveli (magnetic equator) with vertical drift of ~70 m/s at 13:30 UT due to direct penetration of storm time eastward electric fields which exactly coincided with the local time of pre‐reversal enhancement (PRE) and caused intense equatorial spread F irregularities in ionosondes and scintillations in GPS receivers at wide latitudes. Plasma irregularities are so intense that their signatures are seen in Allahabad/Lucknow. Storm time thermospheric meridional winds as estimated using two ionosondes suggest the equatorward surge of gravity waves with period of ~2 h. Suppression of anomaly crest on the subsequent day of the storm suggests the complex role of disturbance dynamo electric fields and disturbance wind effects. Our results also show an interesting feature of traveling ionospheric disturbances possibly associated with disturbance meridional wind surge during recovery phase. In addition, noteworthy observations are nighttime westward zonal drifts and PRE‐related total electron content enhancements at anomaly crests during main phase and counter electrojet signatures during recovery phase.

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The 2015 St. Patrick's Day geomagnetic storm with SYM‐H value of −233 nT is an extreme space weather event in the current 24th solar cycle. In this work, we investigated the main mechanisms of the profound ionospheric disturbances over equatorial and low latitudes in the Asian‐Australian sector and the American sector during this super storm event. The results reveal that the disturbed electric fields, which comprise penetration electric fields (PEFs) and disturbance dynamo electric fields (DDEFs), play a decisive role in the ionospheric storm effects in low latitude and equatorial regions. PEFs occur on 17 March in both the American sector and the Asian‐Australian sector. The effects of DDEFs are also remarkable in the two longitudinal sectors. Both the DDEFs and PEFs show the notable local time dependence, which causes the sector differences in the characteristics of the disturbed electric fields. This differences would further lead to the sector differences in the low‐latitude ionospheric response during this storm. The negative storm effects caused by the long‐duration DDEFs are intense over the Asian‐Australian sector, while the repeated elevations of hmF2 and the equatorial ionization anomaly intensifications caused by the multiple strong PEFs are more distinctive over the American sector. Especially, the storm time F3 layer features are caught on 17 March in the American equatorial region, proving the effects of the multiple strong eastward PEFs.

[1] This paper describes the quiet time variabilities of the ionospheric total electron content (TEC) derived from the signals from Global Positioning Satellite System (GPS) recorded at several stations in India along with simultaneous observations of equatorial electrojet (EEJ) strength obtained from geomagnetic field variations during January-March 2006 when sudden stratospheric warming (SSW) events occurred. Analysis of the observations presented here confirms that strong correlation exists among the variabilities in EEJ strength and GPS TEC observations. Investigations suggest that there exist large-scale wave like structures with periodicity of quasi 16-day wave in the TEC observations near the equatorial ionization anomaly (EIA) crest quite similar to that of EEJ strength. Our observations also indicate the existence of morning enhancement and evening reduction of TEC and EEJ strength and vice versa during SSW events similar to that reported elsewhere. Using these observations, it is suggested that the quiet time variabilities seen in the GPS TEC over EIA could be caused due to the nonlinear interaction of upward propagating planetary waves (PWs) with atmospheric tides. Presence of similar periods in the EEJ strength and TEC observations near the EIA crest region, supports the view that the large-scale wave like structures seen in TEC near the EIA crest are associated with PWs that are modifying the primary eastward electric field in the equatorial E region and hence the EEJ strength through non linear interactions with atmospheric tides.Citation: Sripathi, S., and A. Bhattacharyya (2012), Quiet time variability of the GPS TEC and EEJ strength over Indian region associated with major sudden stratospheric warming events during 2005/2006,

[1] In this paper, we present response of equatorial and low-latitude ionosphere to an intense solar flare of class X7/2B that peaked at 08:05 UT on 09 August 2011 in the solar cycle 24. Global positioning system total electron content (TEC) observations in the sunlit hemisphere show enhancement of~3 TEC units, while geomagnetic H component observations indicate sudden decrease and increase in their strength at equatorial and low-latitude stations, respectively, at several stations in the sunlit hemisphere. In addition, equatorial electrojet strength over Indian region reveals commencement of counter electrojet. Simultaneous Canadian Advanced Digital Ionosonde observations at Tirunelveli, an equatorial station in India, show the disappearance of ionogram echoes during the flare event indicating absorption of radio signals in the D region. Strong equatorial blanketing type E s layer was observed in the ionogram records at Tirunelveli prior to the occurrence of the solar flare that continued for several hours though it became weak/absent during the flare event. Ionogram records on the control day show regular F layer movement without any blanketing type E s layer. Very low frequency (VLF) observations at Allahabad, an Indian low-latitude station, show enhanced VLF amplitude signal during the same time revealing the sudden enhancement of D region ionization. Using the observations presented here, an attempt has been made to study the impact of the solar flares on the electrodynamics of the equatorial and low-latitude ionosphere.Citation: Sripathi, S., N. Balachandran, B. Veenadhari, R. Singh, and K. Emperumal (2013), Response of the equatorial and low-latitude ionosphere to an intense X-class solar flare (X7/2B) as observed on 09

[1] In this paper, we present the results of a morphological study of Equatorial Spread F (ESF) irregularities over Indian region based mainly on observations of (1) amplitude scintillations on GPS L-band signal and Rate of TEC Index (ROTI) obtained using a network of GPS receivers and (2) amplitude scintillations on a VHF signal using spaced receivers at Tirunelveli, an equatorial station. Occurrence of both amplitude scintillations on the GPS L1 signal and occurrence of significant ROTI recorded at several stations has been investigated. The latitudinal extent of L-band scintillations shows that their strength is weak over the dip equator but stronger over Equatorial Ionization Anomaly (EIA) region, preferentially during vernal equinox. We find an equinoctial asymmetry in both the occurrence of scintillations and ROTI wherein their occurrence is greater in the vernal equinox than in the autumn equinox. Attempts have been made to understand the asymmetry in latitudinal extent using maximum cross-correlation (C I ) of intensity fluctuations obtained from the VHF spaced receivers observations. The observations suggest that occurrence of C I less than 0.5 is more in the vernal equinox than in the autumn equinox suggesting that the maximum height of the Equatorial Plasma Bubbles (EPBs) during vernal equinox may be higher than that during autumn equinox. TIMED/GUVI retrieved peak electron density during the same period also indicates that background electron density is higher and more symmetric during vernal equinox than autumn equinox. Hence, our results suggest that background electron density may be playing a vital role in creating the equinoctial asymmetry.Citation: Sripathi, S., B. Kakad, and A. Bhattacharyya (2011), Study of equinoctial asymmetry in the Equatorial Spread F (ESF) irregularities over Indian region using multi-instrument observations in the descending phase of solar cycle 23,