Magnetospheric Chorus, Exohiss, and Magnetosonic Emissions Simultaneously Modulated by Fundamental Toroidal Standing Alfvén Waves Following Solar Wind Dynamic Pressure Fluctuations.

Magnetospheric quasiperiodic whistler‐mode emissions have long been considered a consequence of the relaxation oscillation or the compressional ultralow‐frequency wave modulation. Here we experimentally demonstrate that the whistler‐mode chorus, exohiss, and magnetosonic emissions can be effectively modulated by the toroidal ultralow‐frequency waves. On 04 August 2017, the solar wind dynamic pressure fluctuations excited the fundamental toroidal standing Alfvén waves in the dayside magnetosphere. These regular toroidal pulsations displayed the approximately same periods as the power variations of the whistler‐mode emissions from 50 Hz to 5 kHz. Along with the decay of the toroidal pulsations, the quasiperiodic feature of these whistler‐mode emissions gradually became indistinct. However, no modulation signatures of background parameters and resonant particles for the whistler‐mode emissions were observable near the equator, and the exact cause for this phenomenon remains to be elucidated. Plain Language Summary: Whistler‐mode emissions contribute significantly to the Van Allen radiation belt electron dynamics, and their power can exhibit quasiperiodic variations on a timescale of tens of seconds to several minutes in the dayside magnetosphere. Since the 1960s, the quasiperiodic whistler‐mode emissions have been considered a consequence of the relaxation oscillation or the compressional ultralow‐frequency wave modulation. Using the Van Allen Probes data, we reveal here new physical mechanism for the quasiperiodic whistler‐mode emissions: The solar wind dynamic pressure fluctuations trigger the magnetospheric fundamental toroidal standing Alfvén waves and then modulate the whistler‐mode chorus, exohiss, and magnetosonic emissions from 50 Hz to 5 kHz. Further investigation of this unexpected phenomenon may deepen our understanding of the growth and propagation of whistler‐mode emissions and facilitate the radiation belt model developments. Key Points: The solar wind dynamic pressure fluctuations excited the fundamental toroidal standing Alfvén waves in the dayside magnetosphereThe regular toroidal pulsations simultaneously modulated the whistler‐mode chorus, exohiss, and magnetosonic emissions from 50 Hz to 5 kHzNo modulation signatures of background parameters and resonant particles for these whistler‐mode emissions were observable near the equator [ABSTRACT FROM AUTHOR]