Abstract:
Based on TEC observations by India’s GPS Aided GEO Augmented Navigation (GAGAN) GPS network, we report the dayside low
latitude ionospheric variations over the Indian region during the moderate main phase step-I of the 17 March 2015 geomagnetic storm.
In addition, we assess the efficacy of GPS inferred TEC maps by International GNSS service (IGS) in capturing large scale diurnal features
of equatorial ionization anomaly (EIA) over the Indian region during this period. Following the prompt penetration electric field
(PPE) at 0605 UT, equatorial electrojet (EEJ) enhances by 55 nT over 75 ± 3
oE longitudes where main phase step-I is coincided with
local noon. Initial moderate EIA gradually strengthens with the storm commencement. Although GAGAN TEC exhibits more intense
EIA evolution compare to IGS TEC maps, latitudinal extent of EIA are comparable in both. The enhanced EEJ reverses by 0918 UT
under the effect of overshielding electric field, the later is accompanied by northward turning of interplanetary magnetic field (IMF) Bz.
The weakening of well evolved EIA reflects in IGS TEC maps after 45 min of the overshielding occurrence. In contrary, GAGAN TEC
shows the corresponding feature after 0115 h. Resurgence of EIA, following the PPE 1115 UT, shows up in GAGAN TEC but IGS
TEC maps fails in capturing this feature. The observed low latitude TEC variations and EIA modulations are explained in terms of the
varying storm time disturbance electric fields. The anomalies between the GAGAN TEC and IGS TEC maps are discussed in terms of
the possible limitations of the IGS TEC maps in capturing storm time EIA variability over the Indian region.
