Nonconvective and convective electron cyclotron harmonic instabilities

Abstract

We present a systematic parameter study of the spatial growth rates of electrostatic waves in an electron plasma consisting of a cold component and a hot component with a weak ‘loss cone’ perpendicular velocity distribution. The cold electron density controls which harmonic band can be excited. When the upper hybrid frequency based upon the cold electron density alone is between 1 and 2 times the electron cyclotron frequency, only the first harmonic band is unstable; when the upper hybrid frequency is between 2 and 3 times the cyclotron frequency, the first and second harmonic bands are unstable; and so on. Sufficiently large cold density eventually stabilizes the low harmonic bands. The cold electron temperature Tc controls the spatial amplification; when 0 < Tc/TH < a few times 10−2, where TH is a characteristic energy of the hot electrons, the instability is nonconvective. The first and second harmonic bands can be simultaneously nonconvective. The nonconvective property has been found for a wide range of hot and cold electron densities. It persists even when the hot electron loss cone is almost completely filled. Sufficiently large Tc/TH changes the instability from being nonconvective to being convective. Since cold electrons which come from the ionosphere would be confined to the loss cone if they were not turbulently scattered, the ionospheric source velocity distributions induce a nonconvective instability. However, cold electrons can be heated by electrostatic wave turbulence. Until Tc/TH = 5 × 10−2, nonresonant, quasilinear diffusion heats the cold electrons more rapidly than resonant diffusion heats or scatters the hot electrons into the loss cone. We propose that cold electron heating removes the nonconvective property, so that the spatial amplification of the instabilities can be reduced to a magnitude consistent with their hot and cold electron sources in steady state.