Abstract.The dual frequency signals from the GPS satellites recorded at Rajkot (22.29 • N, 70.74 •

[1] Development or inhibition of ESF during magnetically active periods has been an important space weather topic of interest during the recent past in view of its applications in the satellite based navigational systems. Particularly, the postsunset period exhibits significant variability for storm time development of ESF versus longitude. In this paper, we report the results of a multi-instrumental (ground based and space-borne) and multistation study on the development/inhibition of postsunset ESF during five moderate to intense geomagnetic storms occurred during the low and descending phase of the solar activity period, [2004][2005][2006]. It has been observed that, the prompt penetration of eastward electric fields into low latitudes and subsequent development of ESF occurred in all longitudinal sectors where the local time corresponds to postsunset hours during the entire main phase of the storm. In this paper, we show the development of plasma bubble irregularities over a wide longitudinal extent of 92°owing to the dusk time penetration of eastward electric fields into low latitudes. Either the sudden increase in AE-index and/or a marked decrease in Sym-H index may be used as proxies to determine the occurrence as well as the time of penetration of electric fields into equatorial and low latitudes. However, in such cases where the AE-index does not represent any sudden increase, the dSymH/dt seems to be the better index to determine the time of penetration. In this paper, is also presented an interesting case where the prompt penetration eastward electric fields dominated the existing strong westward electric fields and subsequently caused the onset of spread-F and scintillations at both VHF (244 MHz) as well as L-band (1.5 GHz) frequencies.

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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.