ULF-Modulation of Whistler-Mode Waves in the Inner Magnetosphere During Solar Wind Compression

The solar wind plays important roles in terrestrial magnetosphere dynamics, including the particle population and plasma wave generation. Here, we report an interesting event that ultralow frequency (ULF) waves are enhanced right after solar wind compression and the compressional mode ULF wave subsequently modulates both the intensity and energy flux direction of whistler-mode waves. Quasi-periodic whistler-mode wave packets are observed around L = 5.6 at noon sector by Van Allen Probes. Growth rate calculation demonstrates that the compressional mode ULF wave can modulate the whistler-mode wave intensity by modulating the energetic electron anisotropy. Moreover, the direction of wave energy flux is observed to alternate between northward and southward at equator, which is probably because the intense ULF waves periodically alter the relative direction of the wave source region with respect to the spacecraft. The current results provide a chain of observational evidences to illustrate how the generation and propagation of whistler-mode waves in the inner magnetosphere are affected by ULF waves during the solar wind dynamic pressure enhancement.