Time evolution and dynamics of equatorial backscatter plumes 1. Growth phase

Abstract

Much of our understanding of equatorial spread‐F phenomena has resulted from VHF radar studies of backscatter ‘plumes’ i.e., patterns of backscatter that appear to extend from the bottomside into the topside of the F layer. By using ALTAIR, a fully steerable backscatter radar, to map selected plumes repeatedly, it was found that the dynamic behavior of plumes can be characterized by a growth and a decay phase. The growth‐phase characteristics described in this paper are found to be generally consistent with equatorial irregularity generation as predicted by the Rayleigh‐Taylor and gradient‐drift instabilities. In particular, (1) plumes are generated primarily in the local time around F layer sunset, (2) plumes develop from local perturbations in plasma density in the bottomside of the F layer, (3) plumes develop upward with growth velocities ranging from 125 to 350 m/s, and (4) backscatter strength increases during upward plume development and decreases when upward plume growth slows or ceases. However, certain characteristics, namely, an east‐west asymmetry in backscatter strength found in the bottomside F layer, have been found that suggest key roles played by the eastward neutral wind and by altitude modulation of the bottomside F layer in establishing the initial conditions for plume growth.