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Ann. Geophys., 24, 97-105, 2006
www.ann-geophys.net/24/97/2006/
© European Geosciences Union 2006
Space weather impact on the equatorial and low latitude F-region ionosphere over India
R. S. Dabas1, R. M. Das1, V. K. Vohra1, and C. V. Devasia2
1Radio and Atmospheric Sciences Division, National Physical Laboratory, New Delhi-110012, India
2Space Physics Laboratory, Vikram Sarabhai Space Center, Trivandrum – 695 022, India
Abstract. For a detailed study of the space weather impact on the
equatorial and low latitude F-region, the ionospheric response features are
analysed during the periods of three recent and most severe magnetic storm
events of the present solar cycle which occurred in October and November 2003, and
November 2004. The F-layer base height (h'F), peak height (hmF2) and
critical frequency (foF2) data, from Trivandrum, an equatorial station and
Delhi, a low latitude location, are examined during the three magnetic storm
periods. The results of the analysis clearly shows that the height of
the F-region (both h'F and hmF2), at the equator and low latitude, simultaneously
increases by 200 to 300 km, in association with maximum negative excursion
of Dst values around the midnight hours with a large depletion of ionization over
the equator, which is followed by an ionization enhancement at low latitude
during the recovery phase of the storm. At Delhi, fast variations up to 200 m/s
are also observed in the F-layer vertical upward/downward velocity,
calculated using Doppler shifts, associated with the maximum negative
excursion of Dst. This shows that during magnetic disturbances, the equatorial
ionization anomaly (EIA) expands to a much wider latitude than the normal
fountain driven by the E/F-layer dynamo electric fields. It is also observed
that during the main phase of the storm, at low latitude there is generally
an enhancement of F-region ionization with an increase in h'F/hmF2 but
in the equatorial region, the ionization collapses with a decrease in h'F/hmF2, especially
after sunset hours. In addition, at the equator the normal pre-sunset hours'
enhancement in h'F is considerably suppressed during storm periods. This
might be due to changes in magnitude and direction of the zonal electric
field affecting the upward E×B drift and hence the plasma distribution in
the form of a decrease in electron density in the equatorial region and an
increase in the low latitude region. In association with disturbance
electric fields, the enhanced storm-induced equatorward meridional winds in
the thermosphere can also further amplify the F-layer height rise at low
latitudes during the post-midnight hours, as observed in two of the storm
periods.
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