Bottom-type scattering layers and equatorial spread &amp;lt;i&amp;gt;F&amp;lt;/i&amp;gt;

D L Hysell,J L Chau,J Chun

doi:10.5194/angeo-22-4061-2004

Abstract

Abstract. Jicamarca radar observations of bottom-type coherent scattering layers in the post-sunset bottomside F-region ionosphere are presented and analyzed. The morphology of the primary waves seen in radar images of the layers supports the hypothesis of kudeki+bhattacharyya-1999 that wind-driven gradient drift instabilities are operating. In one layer event when topside spread F did not occur, irregularities were distributed uniformly in space throughout the layers. In another event when topside spread F did eventually occur, the irregularities within the pre-existing bottom-type layers were horizontally clustered, with clusters separated by about 30km. The same horizontal periodicity was evident in the radar plumes and large-scale irregularities that emerged later in the event. We surmise that horizontal periodicity in bottom-type layer irregularity distribution is indicative of large-scale horizontal waves in the bottomside F-region that may serve as seed waves for large-scale Rayleigh Taylor instabilities. Key words. Ionosphere (equatorial ionosphere; ionospheric irregularties; plasma waves and instabilities)

Highlights

Bottom-type layers are thin scattering layers that occur regularly after sunset during equinox and December solstice in the bottomside F-region over Jicamarca and that serve as precursors of large-scale equatorial spread F (ESF) disturbances
Insights into the layers came from Zargham and Seyler (1989), who showed, with a nonlocal analysis, that the linear growth rate of the ionospheric interchange instability operating in the bottomside equatorial F-region has a broad maximum centered on wavelengths of about 1 km
They suggested that bottom-type layers were composed of kilometric primary waves which are too small to be resolved by coherent scatter radars as anything but a continuous, narrow scattering layer in the bottomside F-region

Summary

Introduction

Bottom-type layers are thin scattering layers that occur regularly after sunset during equinox and December solstice in the bottomside F-region over Jicamarca and that serve as precursors of large-scale equatorial spread F (ESF) disturbances. The evening vortex is a flow feature that arises from shear combined with the evening reversal of the zonal electric field that occurs around twilight It was analyzed theoretically by Haerendel et al (1992) and is routinely observed at Jicamarca using the experimental techniques introduced there by Kudeki and Bhattacharyya (1999), who pointed out that the retrograde motion of the bottomside F-region makes it unstable to horizontal winddriven gradient drift instabilities in the presence of horizontal density gradients, caused in part by the convection associated with the vortex flow. By monitoring the temporal evolution of the radar images, we look for evidence of shear and vortex flow, track the emergence and growth of primary plasma waves and instabilities, and search for a large-scale bottomside structure that could serve as a seed for ESF plumes

Radar observations

Findings

Discussion