Abstract:
Here we examine the structuring of equatorial plasma bubble (EPB) during intense
geomagnetic storm of solar cycle (SC) 24 that occurred on 17 March 2015 using spaced receiver scintillation
observations on a 251 MHz radio signal, recorded by a network of stations in Indian region. As yet, this is
the strongest geomagnetic storm (Dstmin ∼ −223nT) that occurred in present SC. Present study reveals
that the structuring of equatorial spread F (ESF) irregularities was significantly different on 17 March as
compared to quiet days of corresponding month. ESF irregularities of intermediate scale (100 m to few
kilometers) are observed at unusually higher altitudes (≥ 800 km) covering wider longitudinal-latitudinal
belt over Indian region. A presence of large-scale irregularity structures with stronger ΔN at raised F peak
with small-scale irregularities at even higher altitudes is observed. It caused strong focusing effect (S4 >1)
that prevails throughout premidnight hours at dip equatorial station Tirunelveli. Other observational aspect
is that zonal irregularity drifts over low-latitude station Kolhapur exhibited a large deviation of ∼230 m/s
from their average quiet time pattern. During this geomagnetic storm, two southward turnings of significant
strength (BZ ≤−15 nT) occurred at 11.4 IST (Indian standard time) and 17.9 IST. The later southward turning
of interplanetary magnetic field (IMF) BZ resulted in a large eastward prompt penetration electric field (PPEF)
close to sunset hours in Indian longitude. Estimates of PPEF obtained from real-time ionospheric model are
too low to explain the observed large upliftment of F region in the post sunset hours. Possible reason for
observed enhanced PPEF-linked effects is discussed.
