New features of ion acoustic waves in inhomogeneous and permeating plasmas

Abstract

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.

A new low-frequency backward mode in inhomogeneous plasmas

“…The mechanism of coupling between TEM and LES waves propagating in inhomogeneous plasmas, discovered in Ref. 4, is shown here to contain some crucial extra physics. A completely new mode is shown to exist, which was not noticed in Ref.…”

“…In such a case the LES modes (IA, IP, and Langmuir) become coupled with the TEM mode even within linear theory. The coupling is more profound for the IA mode 4 , implying that these electrostatic modes may have some electromagnetic features. The coupling with the IA mode has been studied in detail in Ref.…”

“…The coupling with the IA mode has been studied in detail in Ref. 4 for both cold and hot ions, collisional and collision-less, isothermal and adiabatic. It was shown that in a part of spectrum, for small wave-numbers k, the IA mode may become backward in the sense that ∂ω/∂k < 0, it gets some cut-off caused by the density gradient, and in this same domain it is coupled with TEM wave.…”

“…However, in the previous work 4 it was not realized that the general dispersion equation, which describes coupling between TEM and LES waves in the presence of a density gradient, allows for some additional peculiar low frequency hybrid (LFH) mode in the range below the IA wave frequency kc s and below the ion thermal mode kv T i , where c s , v T i are the sound and ion thermal speeds, respectively. In the large part of the spectrum this LFH mode is backward, ∂ω/∂k < 0, and it appears only in the presence of a TEM wave propagating through an inhomogeneous environment.…”

Alfvén wave coupled with flow-driven fluid instability in interpenetrating plasmas

“…Within kinetic theory, in our recent papers, 9,[24][25][26] we have shown that a new kind of instability may develop in such systems. The instability discussed for longitudinal electrostatic perturbations is purely kinetic by nature and currentless (the latter meaning that the plasma contains no currents in the initial state).…”