An investigation of equatorial 150-km irregularities

Abstract

We performed experiments using the 50-MHz Jicamarca radar to study the equatorial 3-m scale plasma irregularities which are responsible for the radar backscatter called 15-km echoes observed at upper E-region heights from approximately 140 to 170 km. These studies were motivated by the search for a generation mechanism and the potential for using backscatter from 150-km irregularities to make vertical drift estimates over long time periods for enhancing the databases used in space weather prediction tools. Based on our observations of a counter-electrojet event, we have determined that the zonal component of the electric field is not the main factor in the generation of these plasma irregularities. We observe no significant altitudinal shears in the zonal drifts, but we do observe intensity and Doppler velocity modulations that indicate the presence of gravity waves. Our power-map shift analyses indicate a predominantly meridioanal propagation direction for these waves. The brief interval during which we observe modulations in the vertical drift estimates indicates the presence of a purely zonally propagating wave. The wave parameters extracted from our observations allow us to estimate the zonal wind U for this short time. The differential between the computed U and the measured zonal drift is large enough to consider the associated 2.5-mv / m vertical electric field as a possible driver for the generation of the 150-km irregularities, themselves. Howewer, the absence of observed velocity perturbations at other times on the same day and the other two days means that we have not yet established a generation mechanism for all times. With regard to our aspect width measurements, we conclude that aspect widths of the 150-km echoes are smaller than those of those electrojet and larger than those of F-region irregularities. With regard to zonal interferometry, we conclude that scatterers observed using an east-west baseline appear diffuse and fill the beam.