Dispersion and wave excitation in nongyrotropic plasmas

Tn a nongyrotropic plasma no symmetry of the distribution function with respect to the velocity space coordinates exists, and, in particular, axial symmetry with respect to the background magnetic field is broken. The propagation of small-amplitude waves and their stability under such conditions are studied here in the framework of linearized Maxwell-Vlasov theory. Two types of nongyrotropic distributions are investigated: stationary and rotating distributions. Stationary nongyrotropy may be maintained by sources and sinks in velocity space. Gyrophase organization couples modes that are decoupled in a gyrotropic plasma, and drives unstable waves in a parallel as well as a perpendicular propagation direction with respect to the background magnetic field. In a rotating nongyrotropic plasma there is an additional coupling of waves at different frequencies that is analogous to a three-wave scattering process. The frequency difference of the coupling modes is just the gyrofrequency of the rotating distribution or its harmonics. This frequency-shifted coupling distributes the energy in a, cascade fashion over an extended frequency range. It tends to stabilize the plasma and inhibits wave excitation.