Electron cyclotron waves: dispersion and accessibility conditions in isotropic and anisotropic plasmas

Dispersion and accessibility conditions for electron cyclotron waves are investigated for arbitrary weakly relativistic plasmas and for specific isotropic and loss-cone distributions. The transition between the cold plasma and vacuum dispersion relations is investigated as a function of temperature and density. The behaviour of mode structure (including mode coupling), cut-offs and resonances are also examined. Generalizations are obtained of earlier results which indicate that access by extraordinary waves to regions nearthe cyclotron layer from the low-field side is easier in weakly relativistic plasmas than predicted by cold plasma theory because of a reduction in the cut-off frequency of the fast extraordinary mode. This effect is found to be more pronounced in loss-cone distributions than in isotropic distributions, permitting access at temperatures considerably lower than those predicted in the isotropic case. Extra loss-cone modes are found to appear near the cyclotron frequency in loss-cone plasmas which also exhibit instabilities near the cyclotron harmonics.