GIGAHERTZ SCINTILLATION OBSERVATIONS AT 22.0-DEGREES-N MAGNETIC LATITUDE IN THE INDIAN ZONE

Postsunset ionospheric scintillation measurements at 4 GHz from the INSAT 1B (74-degrees E) satellite taken during the increasing half of the current solar cycle 22 at Sikandarabad (22.0-degrees N magnetic latitude sub ionospheric) along 149-degrees E geomagnetic meridian in the Indian zone have been analyzed and presented here. Results show that during the low solar activity period, only weak scintillations (peak-to-peak fluctuations < 2 dB) were observed mainly during the summer months. As the solar activity increased, occurrence of summer scintillations more or less remained the same, but the intensity and occurrence probability increased substantially in the equinoxes and to some extent in the December solstice also. During October 1989, severe scintillations with peak-to-peak fluctuations exceeding 20 dB were observed at this latitude, which is normally beyond the daytime crest of the equatorial anomaly. The characteristics of scintillations during equinoctial periods of high sunspot year were such that their onsets were mostly abrupt and they developed fully within a few seconds. Simultaneous measurements taken from an equatorial station Chenglepet (5.2-degrees N magnetic latitude) along the same geomagnetic meridian plane during September-October 1989 show that scintillations at Sikandarabad were conditional to their prior occurrence at Chenglepet. As compared to the equatorial location, the onset of scintillations at 22.0-degrees N magnetic latitude was always delayed and also died out first at the higher latitude. Simultaneous observations showed that the intensity of scintillations at the equatorial location never exceeded 5 dB peak-to-peak, whereas the intensity of scintillations at 22.0-degrees N magnetic latitude frequently exceeded 10 dB peak-to-peak. This is explained on the basis of background ionization and the geometry of the ray paths relevant for these locations. Scintillation activity is in general found to be suppressed during geomagnetic disturbances, but it is observed to be enhanced in the postmidnight hours of the same night for those magnetic storms whose recovery phase starts between the midnight and dawn local time sector. Results are also compared with the observations reported from other regions of the world.