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Ann. Geophys., 24, 1609-1616, 2006
www.ann-geophys.net/24/1609/2006/
© European Geosciences Union 2006
The role of post-sunset vertical drifts at the equator in predicting the onset of VHF scintillations during high and low sunspot activity years
S. Tulasi Ram1, P. V. S. Rama Rao1, K. Niranjan1, D. S. V. V. D. Prasad1, R. Sridharan2, C. V. Devasia2, and S. Ravindran2
1Space Physics Laboratories, Department of Physics, Andhra University, Visakhapatnam, India
2Space Physics Laboratory, VSSC, Trivandrum, India
Abstract. The day-to-day variability in the occurrence of ionospheric scintillations,
which are of serious concern in the trans-ionospheric communications, makes
their prediction still a challenging problem. This paper reports on a
systematic study in quantitatively identifying the precursors responsible,
such as pre-reversal E×B drift velocity, geo-magnetic activity index (Kp)
and the Equatorial Ionization Anomaly (EIA) gradient, for the onset of VHF
scintillations over a low-latitude station, Waltair (20° N dip), during
high (2001) and low (2004) sunspot activity years. The percentage
of occurrences of VHF scintillations over Waltair show a good correlation with
the monthly mean post-sunset vertical drift velocities at the equator,
during both the high and low sunspot activity years. During the days on
which intense (>10 dB) scintillations occur, the ionization anomaly
gradient (dN/dL), measured from ionosonde data of an equatorial (Trivandrum,
0.9° N dip) and an off-equatorial station (Waltair, 20° N dip) shows
an enhancement in the gradient prior to the onset of the scintillations.
However, this enhancement is not seen on days when the scintillations are
weak (<10 dB) or absent. The day-to-day post sunset enhancement in the E×B
drift is found to decrease with increasing Kp-index and this decrease is
more prominent in the equinoxes, less in winter and insignificant in the summer
months. On a day-to-day basis, it is found that the value of the upward drift
velocity at the equator should be ≥30 m/s for the onset of strong
scintillations over Waltair for magnetically quiet days with average
Kp≤2 (6 h prior to the local sunset) during the high sunspot year, 2001.
This threshold value of the upward drift reduces to 20 m/s with the decrease
in the sunspot activity during 2004. Further, these conditions for the onset
of intense scintillations is well defined in equinoxes, less in winter and
least in the summer solstices.
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