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 cos 2 8 > t for the 0 mode and for OJp/Q. > {s(s-I)}-!-and cos 2 8 > (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.

Ordinary mode auroral kilometric radiation, with harmonics, observed by ISIS 1

“…Similarly, the 2nd and 3rd harmonic noise bands extend on both sides of the stimulated resonances at 2f H and 3f H . The low frequency cutoff of the o-mode AKR near f H , rather than near the o-mode propagation cutoff at fN , is consistent with the theoretical results of Hewitt and Melrose [1983] that radiation near f H is by far the most important and the results of Wu and Lee [1979] and Hewitt et al [1982] that it is mainly directed upward. In each case, the fundamental AKR signal is clearly observed to exist only in the frequency region below the x-mode cutoff fx.…”

“…subject of harmonic AKR has been discussed by Lee et al [1980], 0 Hewitt et al [1982] and Hewitt and Melrose [1983]. The result of these j.nvestigations of the cyclotron instability mechanism is that the harmonics can become important at high plasma densities.…”

Auroral kilometric radiation/aurora correlation

“…The importance of the energetic distribution function characteristics has been recently emphasized by Melrose et al [19821, Omidi and Gurnett [1982], Wu et al [1982], Dusenbery and Lyons [1982], and Hewitt and Melrose [1983].…”

Finite Larmor Radius Modifications to the Magnetoionic Wave Modes and Their Effects on Electron Cyclotron Maser Emission

“…We also estimate the parameter regimes in which our approximations break down, and in which a full relativistic treatment is required. In Section 3 we calculate the growth properties of the thermal modes due to maser action driven by a 'hot' loss cone distribution and compare our results with the corresponding results for growth in the magnetoionic modes as given by Hewitt et al (1982), Hewitt and Melrose (1983) and Melrose et al (1984). The results are summarized in Section 4.…”

“…Analytic expressions for Eh may be obtained if purely Calculations for maser emission at the fundamental-in the wave modes of a cold plasma-indicate that growth occurs in parameter regimes satisfying w ::::: fle and sin 2 0 ~ 1 (Lee et al 1980;Omidi and Gurnett 1982;Hewitt and Melrose 1983). In this case A ::::: n 2 ( vi C)2 and for a 'warm' plasma (vi C)2 ~ 1 so A will gerrerally be small.…”