Ion thermal effects on E-region instabilities: linear theory

Linear instabilities and turbulent processes in equatorial and high-latitude electrojets lead to formation of plasma density irregularities which create much of the common E-region radar clutter. Studies of these irregularities have focused on the role of the Farley-Buneman (FB) and gradient drift instabilities with the assumption that the plasma behaves isothermally or adiabatically. In the last decade, this restriction has been relaxed, resulting in the prediction of D and E-region thermally driven instabilities, which now have some supporting observational evidence. Even more recently, fully kinetic particle-in-cell (PIC) simulations show that ion thermal effects strongly modify the nonlinear behavior of the FB instability, especially at the top of the electrojet. This paper describes the linear theory of the combined FB and thermal instabilities. The theory predicts that instabilities will develop over a wider range of altitudes than predicted for the adiabatic or isothermal FB instability. It also predicts that preferred directions of thermally modified FB waves may differ significantly from that of standard Farley-Buneman waves. These theoretical developments have important observational consequences.