New features of ion acoustic waves in inhomogeneous and permeating plasmas

Context. 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.
Aims: In this work it will be shown that the IA wave is susceptible to excitation even in hot-ion plasmas when both an electromagnetic transverse wave and a background density gradient are present in the plasma and, in addition, the wave is unstable (i.e., growing) in the case of permeating homogeneous plasmas.
Methods: 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.
Results: In plasmas with density and temperature gradients, the IA wave is effectively coupled with the electromagnetic waves. In comparison to the ordinary IA wave in homogeneous plasma, the Landau damping of the present wave is much smaller; 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 currentless IA wave instability is presented; it has a very low threshold for excitation 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.
Conclusions: The results presented in the work suggest that the role of the IA wave in the heating of the solar atmosphere (chromosphere and corona) should be reexamined.

Appendix A is available in electronic form at http://www.aanda.org