Some new aspects of low-latitude E-region QP echoes revealed by Gadanki radar: are they due to Kelvin-Helmholtz instability or gravity waves?

We present and analyze some interesting new aspects of low-latitude quasi-periodic (QP) echoes observed using the Gadanki radar, not reported before, to address the origin of these echoes. We report QP echoes, embedded in a descending echoing region, with periods in the range of 2-12 min decreasing with time. In one event, we show a deeply modulated echoing region, resembling very closely to Kelvin-Helmholtz billow structure. Associated with this event, we find that another echoing region already ongoing at altitudes higher up, which was not quasi-periodic, turned quasi-periodic when the undulated echoing region came in existence. Interestingly, both the echoing regions displayed identical time history in their periodicity. We have also shown QP event lasting for 5 h or even more. We also observe a clear velocity reversal with height having vertical wavelength of ~20 km in association with these echoes. On the basis of the observed features and related analysis we have shown that the QP echoes reported here are difficult to be accounted for by the KH instability. We provided a framework and mechanism considering the gravity wave winds in zonal direction as a potential source in forming quasi-periodic plasma blob structures, which could be unstable to plasma instability, to account for the low-latitude QP echoes