Abstract
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.
Highlights
The equatorial and low-latitude ionosphere is the region where the most significant post-sunset ionospheric dynamics play a great role in the generation of irregularities causing scintillations on trans-ionospheric radio communication signals
This paper reports on the results of a systematic study carried out on the role of the pre-reversal enhancement in the upward E×B drift and the geo-magnetic activity index (Kp) for the development/inhibition of Equatorial Spread-F (ESF) irregularities and quantitatively describes the precursors for the occurrence/absence of scintillations on a night-by-night basis during the high and low sunspot years of 2001 and 2004, respectively
The virtual height of the 4-MHz return signal (h’F) on the ionograms of the equatorial station Trivandrum is scaled at 15-min intervals, and the computed dh’F/dt during the post-sunset hours is considered as the pre-reversal enhancement in an upward E×B drift
Summary
The equatorial and low-latitude ionosphere is the region where the most significant post-sunset ionospheric dynamics play a great role in the generation of irregularities causing scintillations on trans-ionospheric radio communication signals. Fejer et al (1999), Whalen et al (2002), and Lee et al (2005) have shown that the pre-reversal enhancement in the E×B drift velocities are reduced with increasing magnetic activity, as measured by a 6-hour average Kp-index, thereby reducing the occurrence and/or intensity of the scintillations They have shown that the diminution in the vertical drifts with increasing Kp is significant during the equinoxes and winter months, but is insignificant during the summer months. This paper reports on the results of a systematic study carried out on the role of the pre-reversal enhancement in the upward E×B drift and the geo-magnetic activity index (Kp) for the development/inhibition of ESF irregularities and quantitatively describes the precursors for the occurrence/absence of scintillations on a night-by-night basis during the high and low sunspot years of 2001 and 2004, respectively
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