Cyclotron instabilities in a plasma stream as a mechanism for wave amplification

Abstract

An analysis is made of the interaction between cold streams of charged particles flowing along a uniform, steady, magnetic field. The special case is considered of plane waves propagating along the field, when the dispersion equation always separates into two transverse modes and a longitudinal mode. Resonance between transverse waves and the gyromotions of the streaming particles results when the particles observe the wave to have a frequency equal to their gyrofrequency. Under such conditions `cyclotron' instabilities may arise. It is shown that wave amplification from convective instabilities is possible if the stream velocity is in the same direction as the group velocity, but greater in magnitude than the phase velocity, of the transverse waves which propagate in the ambient medium. Results are applied to particle streams in the exosphere of the earth, and it is concluded that noise in the very low frequency range will be amplified by ion streams in the presence of ambient electrons, or by electron streams in the presence of ambient protons. For slow streams (V/c < 1/50) the amplified frequencies will have a narrow bandwidth centred close to the gyrofrequency of the ambient particles.