Thermal effects on parallel-propagating electron cyclotron waves

Thermal effects on the dispersion of right-handed (RH) electron cyclotron waves propagating parallel to a uniform, ambient magnetic field are investigated in the strictly non-relativistic (‘classical’) and weakly relativistic approximations for real frequency and complex wave vector. In each approximation, the two branches of the RH mode reconnect near the cyclotron frequency as the plasma temperature is increased or the density is lowered. This reconnection occurs in a manner different from that previously assumed at parallel propagation and from that at perpendicular propagation, giving rise to a new mode near the cold plasma cut-off frequency ω_xC. For both parallel and perpendicular propagation, it is noted that reconnection occurs approximately when the cyclotron line-width equals the width of the stop-band in the cold plasma dispersion relation. Inclusion of weakly relativistic effects is found to be necessary for quantitative calculations and for an accurate treatment of the new mode near ω_xC. Weakly relativistic effects also modify the analytic properties of the dispersion relation so as to introduce a new family of weakly damped and undamped solutions.