Generation of UHR Waves as Inferred From Wave and Electron Flux Measurements by Van Allen Probes

Abstract

Observations of upper hybrid resonance (UHR) waves by Van Allen Probes show that the intensity of these waves increases significantly when their frequency is close to an integer multiple of electron gyrofrequency. This article aims at explaining these observations. The explanation assumes that the UHR waves are excited due to cyclotron instability at higher-order cyclotron resonances. Since UHR waves have relatively small group velocity, propagation effect should not play an important role in spectrum evolution of these waves. Thus, the spectrum observed at a given point should be closely related to the wave growth rate at the same point. We calculate the linear growth rate of UHR waves along Van Allen Probe trajectory using the registered plasma parameters and energetic electron density deduced from differential flux measurements. Apart from the hot plasma density, we use other parameters of model distribution compatible with but not inferred from flux measurements. The growth rate along the satellite trajectory calculated in this way fits well the simultaneous spectrum observations.