Quasi-periodic backscatters from the E region at Gadanki: Evidence for Kelvin-Helmholtz billows in the lower thermosphere?

Abstract

Analyzing the field-aligned coherent radar backscatter observed over Gadanki, India (13.5°N, 79.2°E), with a narrow beam pointing almost vertically, we present convincing experimental evidence for the presence of low-latitudes tilted sporadic ionization layers close to 10 km in vertical extent that move horizontally through the field of view of the radar. Using the data from high temporal (-3 s) resolution experiments, we also show that the line-of-sight Doppler velocities associated with at least some of the quasi-periodic striations have very clear vortex-like structures cutting across a vertical plane inside regions of strong horizontal wind shears. The power as well as the Doppler width peak together, and they often reach their peak values near the center of a vortex, where the magnitude of the Doppler velocity is minimum. The Doppler properties and spatial distribution of the 3 m echoes are explained in terms of a local electrodynamical process that makes ions and electrons move with the vertical neutral wind. Both the wind field and the tilt of the layers are in turn consistent with the presence of Kelvin-Helmholtz billows. We postulate that the billows themselves are triggered by a shear instability in the large ambient zonal wind; strong zonal wind shears clearly have to be present when sporadic E layers are observed. In our case, the breaking of an originally uniform and horizontal sporadic E layer into tilted pieces aligned more or less parallel to one another, and their motion through the radar field of view in the presence of a mean zonal wind, give the echoes their quasi-periodic appearance.