Abstract:
We report preseismic and postseismic distinct features observed in equatorial ionospheric
F region in Indian longitude during two major earthquakes that occurred on 28 March 2005 (Mw = 8.6)
and 26 December 2004 (Mw = 9.1) with epicenters near Sumatra, Indonesia. We used spaced receiver
scintillation observations on a 251-MHz signal transmitted from a geostationary satellite and recorded at
dip equatorial station Tirunelveli, which is ≈2,200 km away from the epicenter of these earthquakes. On 28
March 2005, we noticed significant enhancement (≈88.5 m/s) in F region zonal plasma drift as compared
to their seasonal quiet time drifts, 20 min prior to the start of earthquake. This implies the presence of
additional vertically downward electric field of approximately 3 mV/m, which is linked with the earthquake
mechanism. It is unusual because so far we have not encountered such feature on either magnetically
quiet or disturbed days. Spatial scales associated with equatorial spread F irregularities generated in the
postsunset hours on this earthquake day are considerably smaller. It suggests that the earthquake-linked
electric field influenced the structuring and evolution of equatorial spread F irregularities on this earthquake
day. Another earthquake that occurred on 26 December 2004 triggered a tsunami. We noticed that the
earthquake- and tsunami-linked effects in the F region plasma drifts are distinctly observed even 18.5 hr
after the earthquake.
