On the stability of stationary nongyrotropic distribution functions: Coupling and purely growing waves

Abstract

Unperturbed particle distributions exhibiting gyrophase bunching can linearly couple the eigenmodes of parallel propagation and destabilize otherwise stable magnetoplasmas. Previous research on the parallel stability of nongyrotropic populations has either considered homogeneous but time‐varying distributions (TNG) associated with closed phase spaces (solutions of the homogeneous Vlasov equation) or homogeneous, stationary distributions (SNG) arising from open phase spaces (solutions of the Vlasov, or transport equation with source and sink terms). Destabilization of otherwise stable media in the TNG case was only reported when the unperturbed gyrophase bunching generated a finite perpendicular current that coupled the electrostatic and electromagnetic parallel eigenmodes at frequencies shifted by multiples of the cyclotron frequency of the TNG species. Here we demonstrate that introduction of SNG in a particle population of an otherwise stable magnetoplasma can (1) bring about unstable linear mode coupling among the parallel eigenmodes for finite perpendicular currents (as was the case for TNG distributions) albeit now without the frequency shifts, and (2) stimulate nonoscillatory purely growing waves (zero real frequency within a finite wavenumber band) in nongyrotropic environments that only couple parallel electromagnetic waves (zero unperturbed perpendicular current), behavior not encountered in TNG media.