Analysis of cyclotron harmonic emissions at the outer planets

Abstract

The flyby missions of Voyagers 1 and 2 at Jupiter, Saturn and Uranus revealed intense waves above the electron gyrofrequency. Observation of waves at the upper hybrid frequency is often accompanied by power at adjacent electron Bernstein harmonics, and the relative power in these modes depends both on the density and temperature ratios of the cold background electron population and the hot magnetospheric electrons which drive the instability. A model of electron distributions which is consistent with observations is used for analysis of the excited waves, their dependence upon plasma parameters, and the time scales of the saturation processes. It is shown that in the presence of two-temperature electron distributions the linear excitation is due to a fluid-like coupling of two eigenmodes for perpendicular propagation and to kinetic destabilization of oblique modes. The dependence of linear growth rates on propagation angle is presented, along with results from particle simulations. A quasilinear diffusion time for relaxing the hot electron loss cone is calculated and compared with simulation results. This time scale is faster than for local saturation by heating the cold population, and also the convective amplification time scale, suggesting that the waves saturate at quasilinear levels, while being convectively localized to the equatorial regions of the outer planetary magnetospheres.