On the interpretation of the VHF radar echoes from the equatorial lower E region

Abstract

Reanalysis of the recent radar observations reported by Tsunoda and Ecklund [1999] has been made to explain the VHF radar echoes from the collision dominated lower E region (<95 km). It is shown that the lower E region echoes extending down to 90 km can be explained using the gradient drift instability if the neutral wind is taken into consideration. It is also shown that the inclusion of neutral wind in the growth rate of the gradient drift instability is capable of explaining the day-to-day variability of the occurrence of the irregularities extending to the lower altitudes. Interpretation is given of the low value of Doppler velocities and the unusual velocity reversal observed by the radar at the lower altitudes that is consistent with the existing theory of the equatorial electrojet.