Abstract:
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.
