Gadanki radar observations of F region field-aligned irregularities during June solstice of solar minimum: first results and preliminary analysis

In this paper we present the first results of F region field-aligned irregularities (FAI) made during the summer of low solar condition using the Gadanki mesosphere-stratosphere-troposphere radar. FAI echoes were observed on all 20 nights of radar observations and were mostly confined to the postmidnight hours. Echo morphology is found to be very different from the equinoctial postsunset features reported earlier from Gadanki. Echo SNRs are lower by 25 dB than their equinoctial postsunset counterparts but are quite comparable to those of the equinoctial decaying FAI during the postmidnight hours. The Doppler velocities, which lie in the range of ±100 m s<SUP>-1</SUP>, are predominantly upward-northward during 0000-0300 LT and downward-southward afterward, in contrast to those observed as predominantly downward-southward associated with the decaying equinoctial postmidnight F region FAI. Spectral widths of the summer echoes, which are well below 50 m s<SUP>-1</SUP> and are very similar to those of the decaying equinoctial irregularities, represent the presence of weak plasma turbulence. Simultaneous observations made using a collocated ionosonde show no ionogram trace during 2200-0530 LT except for a few cases of weak spread F events. Concurrent ionosonde observations made from magnetic equatorial location Trivandrum also show very similar results. The observations are discussed in the light of current understanding on the postmidnight occurrence of F region irregularities in the summer of low solar condition