Low-frequectcy plasma oscillations for quasi-transverse propagation

Abstract

The spectra and damping of some new branches of hot-plasma oscillations propagating perpendicular to or almost perpendicular to the external magnetic field H0 are discussed. Investigation has been performed of (a) an oscillation branch adjoining longitudinal ion-cyclotron oscillations, (b) extraordinary ion-cyclotron and magneto-acoustic waves and (c) ordinary ion-cyclotron and hybrid waves. Phase velocity of these waves along the magnetic field V = ω/k‖ is considerably larger than ion thermal velocity (k‖ is the wave-vector projection in the magnetic-field direction).Waves adjoining longitudinal cyclotron oscillations have a wavelength of the order of the Larmor radius of an ion with thermal velocity and frequencies of the order of ion cyclotron frequency; the phase velocity of these waves along the magnetic field is considerably smaller than thermal electron velocity υe. These waves can be separated from the general dispersion queation for the case of a low-pressure plasma when x ≪ 1, where (n0 is the density; Ti, the plasma ion temperature). For x ≪ cos2 θ ≪1 (θ is the angle between H0 and the propagation direction) this branch changes to longitudinal ion cyclotron oscillations.The dispersion equation for an ordinary wave for V≪ve has two solutions, corresponding to the hybrid wave with frequency ω = ω0 = k‖kVsVA/ωi), where Vs = (Te/mi)1/2, ωi = eH0/mi C and VA = H0/(4πn0)1/2 and the ion-cyclotron wave with ω≈sωi (s = 1, 2,…).For the oscillations pointed out the damping rates which are caused by Cerenkov absorption by plasma electrons and ions and cyclotron absorption by plasma ions have been found. Oscillation excitation by fluxes of low-density charged particles passing through plasma along a magnetic field has also been examined. Cerenkov and cyclotron wave excitation by both a hot and a cold beam, have been investigated.