Observations of low latitude spread-F using ST mode of the Indian MST radar

Abstract

The ST mode of the Indian MST radar was utilized to conduct observations on the coherent back scatter from the field-aligned F-region irregularities at the low latitude station, Gadanki (13.47°N, 79.18°E). This was made possible since the antenna array is so oriented as to permit the radar beam scan in the magnetic north-south plane. The first observations made on the night of 3-4 July 1992 when strong spread-F was noted on the ionograms recorded at the SHAR Centre about 100 km east of Gadanki, are presented. The basic characteristics of the Doppler spectra and range-time-intensity (RTI) maps are essentially the same as those of equatorial spread-f reported for Jicamarca. The irregularities are confined to a range-extent defined by sharp boundaries on either side. The signal-to-noise ratio is found to vary with range from -10 to +10 dB, which, even on normalizing to the system sensitivity, is somewhat lower than that observed at Jicamarca. The mean radial velocity transverse to the magnetic field and the velocity spread derived from the Doppler spectra range from -20 to 60 ms^-1 and 10 to 60 ms^-1, respectively. The drift velocities are found to be consistent with the progressive time delays observed in the onset of the signal returns with increasing range. The observations presented here suggest that the underlying plasma instability processes are essentially same as that operating at the equator.