Radar observations of F region equatorial irregularities

Abstract

Experimental results obtained with the Jicamarca radar and a new digital processing system during spread F conditions are presented. The data consist of two‐dimensional maps showing backscatter power and samples of frequency spectra of the backscatter signals as a function of altitude and time. Almost simultaneous spread F backscatter power and incoherent scatter observations of electron density and vertical drifts are presented for one occasion. It is shown that spread F can occur at the bottomside, at the topside and the steep bottom of the F region, and in the valley between the F and E regions when the electric field is either positive, negative, or null. The existence of plumelike structures extending hundreds of kilometers in altitude and physically connecting the spread F on the topside with the bottomside is one of the highlights of the experimental results. They are interpreted as evidence of a Rayleigh‐Taylor instability. A mechanism involving ‘bubbles’ or low‐density plasma is proposed to extend the instability to the stable regions on the top. Other unstable processes are proposed for spread F at other altitude ranges. The frequency spectra show a large variety of shapes. Simple or multiple peak spectra from a few hertz to a few hundred hertz wide are found. An interpretation of the spectral shapes is presented in terms of turbulent motions and the angular extent of k vector angles of the fluctuation waves with respect to perpendicularity. A puzzling phenomenon, referred to as explosive spread F, which consists of the simultaneous onset of short time enhancements in the backscatter power and involves selected heights in an altitude range of the order of 100 km, is presented.