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The day-to-day variability in the occurrence of ionospheric scintillations,which are of serious concern in the trans-ionospheric communications, makestheir prediction still a challenging problem. This paper reports on asystematic 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 VHFscintillations over a low-latitude station, Waltair (20° N dip), duringhigh (2001) and low (2004) sunspot activity years. The percentageof occurrences of VHF scintillations over Waltair show a good correlation withthe monthly mean post-sunset vertical drift velocities at the equator,during both the high and low sunspot activity years. During the days onwhich intense (>10 dB) scintillations occur, the ionization anomalygradient (dN/dL), measured from ionosonde data of an equatorial (Trivandrum,0.9° N dip) and an off-equatorial station (Waltair, 20° N dip) showsan enhancement in the gradient prior to the onset of the scintillations.However, this enhancement is not seen on days when the scintillations areweak (<10 dB) or absent. The day-to-day post sunset enhancement in the E×Bdrift is found to decrease with increasing Kp-index and this decrease ismore prominent in the equinoxes, less in winter and insignificant in the summermonths. On a day-to-day basis, it is found that the value of the upward driftvelocity at the equator should be ≥30 m/s for the onset of strongscintillations over Waltair for magnetically quiet days with averageKp≤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 decreasein the sunspot activity during 2004. Further, these conditions for the onsetof intense scintillations is well defined in equinoxes, less in winter andleast in the summer solstices

The role of post-sunset vertical drifts at the equator in predicting the onset of VHF scintillations during high and low sunspot activity years