Modulation of Whistler Mode Waves by Ultra-Low Frequency Wave in a Macroscale Magnetic Hole: MMS Observations

Abstract

We report the quasi-periodic whistler-mode waves corresponding to electron temperature anisotropy T⊥/T// < 1 at the center of a macroscale magnetic hole (MH) near the Earth's magnetopause observed by the Magnetospheric Multiscale mission. A significant ultra-low frequency (ULF) wave of background magnetic field was observed in this MH. The ULF wave dramatically exacerbates the evolution of MH and the formation of the donut-shaped electron pitch angle distribution (PAD) in the macroscale MH. When the whistler-mode waves were generated, they are consistent with troughs of the ULF wave and density increases, which is also related to the butterfly type PAD at those moments. The analysis of dispersion relation and cyclotron resonance condition indicate that whistler-mode waves were mainly generated by the butterfly type PAD of electrons. Our results provides new insights into one possible excitation mechanism of whistler-mode wave.