Daytime low-altitude quasi-periodic echoes at Gadanki: understanding of their generation mechanism in the light of their Doppler characteristics

Abstract

In this paper, we investigate the genesis of low-altitude quasi-periodic (LQP) E region echoes observed during daytime by the Gadanki radar. We show that such echoes reflect much wider Doppler spectra compared to that of typical daytime low-altitude E-region echoes. Further, the Doppler spread resembles that of meteor induced spread echoes. Considering that the meteor spread echoes are generated by the gradient drift instability on the sharp density gradient associated with the plasma trail, we invoke plasma density gradient and a similar physical process to account for the LQP spread echoes. Taking into account all the pertinent issues related to the formation of plasma density structures in the collision dominated lower E region, we surmise that the required plasma density structures are generated by a weakly driven Kelvin-Helmholtz instability. These structures get polarized by the background electric field or neutral wind and become gradient drift unstable manifesting LQP echoes.