| Recently, there has been a renewal of interest in the study of the effects of
solar weather events on the ionization redistribution and irregularity
generation. The observed changes at low and equatorial latitudes are rather
complex and are noted to be a function of location, the time of the storm
onset and its intensity, and various other characteristics of the geomagnetic
storms triggered by solar weather events. At these latitudes, the effects of
geomagnetic storms are basically due to (a) direct penetration of the
magnetospheric electric fields to low latitudes, (b) development of
disturbance dynamo, (c) changes in atmospheric neutral winds at ionospheric
level and (d) changes in neutral composition triggered by the storm time
atmospheric heating.
In the present study an attempt is made to further understand some of the
observed storm time effects in terms of storm time changes in zonal electric
fields and meridional neutral winds. For this purpose, observations made by
the Retarding Potential Analyzer (RPA) payload on board the Indian satellite
SROSS-C2 are examined for four prominent geomagnetic storm events that
occurred during the high solar activity period of 1997-2000. Available
simultaneous observations, from the GPS satellite network, are also used.
The daytime passes of SROSS-C2 have been selected to examine the
redistribution of ionization in the equatorial ionization anomaly (EIA)
region. In general, EIA is observed to be weakened 12-24 h after the
main phase onset (MPO) of the storm. The storm time behaviour inferred by
SROSS-C2 and the GPS satellite network during the geomagnetic storm of 13 November 1998,
for which simultaneous observations are available, is found to be
consistent. Storm time changes in the delay of received GPS signals are
noted to be ~1-3 m, which is a significant component of the total
delay observed on a quiet day.
An attempt is made to identify and delineate the effects of a) meridional
neutral winds, b) the development of the ring currents and c) the disturbance
dynamo electric fields on the low latitude ionization distribution. The
weakening of the EIA is noted to be primarily due to the decrease in the
eastward electric fields driving the equatorial fountain during the daytime.
The meridional neutral winds are also noted to play an important role in
redistribution of ionization in the EIA region. The present results
demonstrate that storm time latitudinal distribution of ionization in this
region can be better understood by taking into account the meridional winds
in addition to E×B drifts. |