On the Observation of Electrostatic Harmonics Associated With EMIC Waves
In this study, we report two events of electrostatic harmonics associated with electromagnetic ion cyclotron (EMIC) waves recorded by the Van Allen Probes. Based on the wave and plasma measurements, the wave features are investigated and the possible generation mechanism is discussed. The frequencies of these electrostatic emissions are at the integer and fractional frequencies of the fundamental EMIC waves, which can be across and above the local proton gyrofrequencies. When the frequencies increase, the electric power spectral densities of the electrostatic waves decrease, and their durations become shorter. Considering the bidirectional propagation of the fundamental EMIC waves, we propose that wave-wave resonance probably accounts for the generation of the observed electrostatic emissions. The calculation of cross bicoherence index supports this scenario. This study analyzes a type of electrostatic harmonic wave associated with EMIC waves, essentially different from the electromagnetic harmonics reported before, and provides new insight into the evolution of EMIC waves.
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Geophysical Research Letters
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