Shear instability as a source of the daytime quasi-periodic radar echoes observed by the Gadanki VHF radar

Abstract

High-resolution observations of the daytime quasi-periodic (QP) structures of the lower E region irregularities made using the Gadanki VHF radar are presented. The structures are found to display striations with negative altitude rates of ∼10 m s^-1 and periods of 1-3 minutes. More importantly, these structures are found to be associated with a descending irregularity layer having descent rate of ∼0.3 m s^-1. The characteristics of these structures although resemble closely to that of nighttime low altitude structures observed earlier at Gadanki as well as at mid-latitudes, the periodicities observed during daytime are found to be higher than that observed during nighttime. The Doppler velocities are mostly in the range -20-10 m s^-1 and exhibit remarkable shear corresponding to the echoes of larger periodicities. The echoes being mostly confined to the collision dominated lower E region, it is possible to derive meridional neutral wind and the derived wind shows remarkable shear. It is proposed that the Kelvin-Helmholtz instability is the most likely mechanism responsible for the observed structures presented in this paper.