Perpendicular ion beam‐driven instability in a multicomponent plasma: Effects of varying ion composition on linear flute mode oscillations

Abstract

Numerous observations of low frequency electrostatic waves associated with the injection of heavy ion beams from sounding rockets have prompted an investigation of the collisionless linear flute mode (k∥ = 0) dispersion relation (Ichimaru, 1973; Seiler et al., 1976; Hamelin and Beghin, 1976) for a multi‐component plasma. Assuming a two‐ion component background plasma whose masses differ by at most a factor of 2 (e.g., O+, O2+; O+, NO+), solutions are obtained as a function of varying ion composition both in the absence of a beam term (i.e., multicomponent ion Bernstein modes) and in the presence of an unmagnetized Ar+ ion beam assuming small thermal spreading of the beam. Numerical solutions for resonantly driven oscillations using small beam densities and perpendicular beam velocities yield increasing growth rates for increasing wave number, k, for the first three modes studied. An analytical approximation to the growth rates, following a procedure similar to one used by Roth et al. (1983), is consistent with this conclusion. The results compare favorably to Langmuir probe electrostatic wave observations made during the injection of an Argon ion beam from the ARCS‐I sounding rocket into the auroral ionosphere. During the injection process the rocket descended through a region in which the ratio of light to heavy ions can vary considerably.