Abstract:
This paper presents some features of the ionospheric response observed in equatorial and mid-latitudes
region to two strong geomagnetic storms, occurring during
Oct. 19–23, 2001 and May 13–17, 2005 and to understand
the phenomena of pre-storm that lead to very intense geomagnetic storms. The result point to the fact that pre-storm
phenomena that leads to intense ionospheric storm are; large
southward turning of interplanetary magnetic field Bz, high
electric field, increase in flow speed stream, increase in proton number density, high pressure ram and high plasma beta.
The magnitude of Bz turning into southward direction from
northward highly depends upon the severity of the storm and
the variation in F2 layer parameter at the time of geomagnetic storm are strongly dependent upon the storm intensity.
A detailed analysis of the responses of the ionosphere shows
that during the storm periods, foF2 values depleted simultaneously both in the equatorial and mid latitude. Observation
also shows that low to moderate variations in ionospheric
F2 at the pre-storm period may signal the upcoming of large
ionospheric disturbances at the main phase. The ionospheric
F2response for low and mid latitude does not show any significant differences during the storm main phase and the prestorm period. The ionospheric response during the pre-storm
period is thought very puzzling. The period is observed to be
depleted throughout with low-moderate effect across all the
stations in the low and mid latitude.