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