Electron Cyclotron Maser Emission near the Cutoff Frequencies

Abstract

An earlier discussion of loss-cone driven cyclotron masers is extended to cover the case where the emission occurs close to the cutoff frequency of the 0 mode or the x mode. In general, wave growth may occur in one or two bands, and when two bands are allowed the lower band is close to the cutoff frequency. With the exception of the x mode at 8 = 1, growth in the lower band is allowed only for OJp/D. > 8 and cos28 > t for the 0 mode and for OJp/Q. > {s(s-I)}-!- and cos28 > (s-I)/8 for the x mode, and growth in the lower band has no particularly favourable features when allowed. For the X mode at 8 = 1 both bands are allowed for all OJp/D. $ 1 and growth in the lower band is possible over a wide range of angles in a very narrow frequency band. The spatial growth rate can be quite large due to the small group speed. However, the large spatial growth rate is offset by the short pathlengths for growth in a slowly spatially varying magnetic field due to the very narrow bandwidth of the growing waves. It is found that growth in the lower band is at best no more effective than growth in the upper band. We discuss the relative merits of growth in the two bands in a suggested application to terrestrial kilometric radiation. We also discuss cyclotron theories for type I solar radio emission, pointing out that our results do not favour such theories, and for solar microwave spike bursts.