Coherent Gyromagnetic Emission as a Radiation Mechanism

Abstract

The properties of coherent gyromagnetic emission of radiation which can escape from a source of astrophysical interest by a bi-Maxwellian distribution of nonrelativistic electrons (thermal velocities P 1.0 C and PII 0 c, and streaming velocity p, c) are considered. It is found that: (1) Coherent emission occurs only for Pio ;0;. PliO, and only for a more extreme anisotropy for waves close to their cutoff frequencies. (2) Coherent emission in the x-mode at the second harmonic, i.e. at OJ "'" 2Qe > OJp , is the dominant process except for p, > OJ~/Q; when coherent emission in the x-mode at he first harmonic, i.e. at OJ ~ Q. }> OJp , is possible and dominates. It is argued that these results are not strongly dependent on the choice of distribution function. It is pointed out that the use of the quasilinear equations can be justified in any astrophysical context but that the above properties apply even when the quasilinear treatment is not valid. Limitations imposed on the mechanism by the presence of absorption in the thermal plasma and by competing instabilities are shown to be relevant except for emission in the x-mode at the first harmonic.