Simultaneously Interactions of Ultra‐Low‐Frequency Waves and Whistler Waves, and Whistler Waves and Quasi‐Electrostatic Whistler Harmonics in the Earth's Magnetosheath.

Identifying how different wave modes interaction with each other is an important topic in space physics. Using measurements from the Magnetospheric Multiscale (MMS) mission, we report an event in the Earth's magnetosheath where ultra‐low‐frequency (ULF) waves, whistler waves, and quasi‐electrostatic whistler harmonics were observed simultaneously. Specifically, our analyses show that the parallel components of ULF waves modulate the electron butterfly distributions which provide the free energy to the excitation of the observed whistler waves. Meanwhile, quasi‐electrostatic waves sharing frequencies that are nearly integral multiples of the frequency of the observed whistlers are identified as nonlinear harmonics of whistler waves. The close correlations between the whistler waves and the electric field harmonics, as well as the high bicoherence indexes within the frequency ranges of the whistlers and harmonics, suggest the significant nonlinear couplings between the whistler waves and quasi‐electrostatic whistler harmonics. Our work provides new insights to the interactions between waves at different frequency bands. Key Points: Ultra‐low‐frequency (ULF) waves, whistler waves, and quasi‐electrostatic whistler harmonics are observed simultaneously in the magnetosheath plasmaULF waves modulate the electron butterfly shaped distributions which provide free energy for the excitation of the observed whistler wavesQuasi‐electrostatic waves are identified as the whistler harmonics, and may be generated by the wave‐wave nonlinear coupling [ABSTRACT FROM AUTHOR]