A comparison of intense electrostatic waves near f_UHR with linear instability theory

Abstract

Intense electrostatic waves beyond the plasmapause have recently been identified at frequencies near the upper hybrid resonance frequency. In addition, the waves occur within a band at an odd, half‐harmonic of the local electron gyrofrequency. These bands of electrostatic turbulence are among the most intense waves detected within the earth’s magnetosphere. Measurements obtained with the ISEE 1 plasma wave receiver show that the intense waves appear to be intensifications of an electrostatic cyclotron harmonic band near the upper hybrid resonance frequency. A straightforward explanation of intense waves at the upper hybrid resonance frequency exists in the electrostatic multi‐cyclotron emission theory. For a broad range of plasma parameters nonconvective instability or large spatial growth rates occur within the cyclotron band encompassing the cold upper hybrid frequency. Comparison of spatial growth rate spectra with measured wave spectra shows that there is excellent qualitative agreement between the linear theory and the observed wave characteristics.