New features of ion acoustic waves in inhomogeneous and permeating plasmas

TL;DR

This paper reveals new conditions under which ion acoustic waves in plasmas can be excited and grow, especially in inhomogeneous and permeating plasmas, with implications for solar atmospheric heating.

Contribution

It introduces a new understanding of ion acoustic wave excitation and instability mechanisms in inhomogeneous and permeating plasmas, using fluid and kinetic theories.

Findings

Ion acoustic waves can be excited in plasmas with hot ions due to electromagnetic and density gradients.

The Landau damping of these waves is significantly reduced compared to ordinary IA waves.

Permeating plasmas exhibit a low-threshold kinetic instability for IA waves.

Abstract

It is generally accepted that the ion acoustic (IA) wave in plasmas containing ions and electrons with the same temperature is of minor importance due to strong damping of the wave by hot resonant ions. In this work it will be shown that the IA wave is susceptible to excitation even in plasmas with hot ions when both an electromagnetic transverse wave and a background density gradient are present in the plasma, and in addition the wave is in fact unstable (i.e., growing) in the case of permeating homogeneous plasmas. The multi-component fluid theory is used to describe the IA wave susceptibility for excitation in inhomogeneous plasmas and its coupling with electromagnetic waves. The growing IA wave in permeating homogeneous plasmas is described by the kinetic theory. In plasmas with density and temperature gradients the IA wave is effectively coupled with the electromagnetic waves. In comparison to ordinary IA wave in homogeneous plasma, the Landau damping of the present wave is much smaller, and to demonstrate this effect a simple but accurate fluid model is presented for the Landau damping. In the case of permeating plasmas, a kinetic mechanism for the current-less IA wave instability is presented, with a very low threshold for excitation as compared with ordinary electron-current-driven kinetic instability. Such growing IA waves can effectively heat plasma in the upper solar atmosphere by a stochastic heating mechanism presented in the work. The results of this work suggest that the IA wave role in the heating of the solar atmosphere (chromosphere and corona) should be reexamined.