Ion thermal effects on E-region instabilities: 2D kinetic simulations

Abstract

Radars frequently receive strong returns from naturally occurring plasma density irregularities centered around 100 km altitude in the atmosphere. These E-region ionospheric irregularities occur when electrojet polarization electric fields drive plasma instabilities, most commonly the Farley–Buneman (FB) and gradient drift instabilities. This paper presents results from two fully kinetic, 2D simulations of field driven instabilities in the E-region. These simulations show the important impact of ion thermal effects on wave growth and turbulence. The first simulation applies parameters meant to replicate a strongly driven high-latitude FB instability where the polarization electric field exceeds the FB threshold by a factor of 4. In this case, waves grow and saturate with characteristics dominated by ion thermal perturbations resulting from a balance between frictional ion heating and cooling. These characteristics differ substantially from those expected for FB waves. In the second simulation, the driving field exceeds the threshold by a factor of 2, causing FB waves to grow with distinct modifications by ion thermal effects. The thermal effects reproduced by these simulations may account for some of the previously unexplained observational characteristics of E-region irregularities, particularly at high latitude and in the upper E-region. They also explain the results seen in the simulations of Janhunen (J. Geophys. Res. 99 (1994b) 11461).