Evidence of Nonlinear Interactions Between Magnetospheric Electron Cyclotron Harmonic Waves

Electron cyclotron harmonic (ECH) waves play an important role in the magnetosphere-ionosphere coupling. They are usually considered to be generated by the Bernstein-mode instability with electron loss cone distributions. By analyzing the Van Allen Probes wave data, we present the direct evidence of the nonlinear interactions between ECH waves in the magnetosphere. Substorm-injected electrons excite primary ECH waves in a series of structureless bands between multiples of the electron gyrofrequency. Nonlinear interactions between the primary ECH waves produce secondary waves at sum- and difference-frequencies of the primary waves. Our results suggest that the nonlinear wave-wave interactions can redistribute the primary ECH wave energy over a broader frequency range and hence potentially affect the magnetospheric electrons over a broader range of pitch angles and energies.