Statistical Distribution of the Peak Frequency of ECH Waves in the Outer Magnetosphere From Magnetospheric Multiscale Satellite Observations

Electron cyclotron harmonic (ECH) waves are electrostatic emissions with frequencies between the harmonics of the electron gyrofrequencies. Their frequency properties provide clues for understanding their generation and are keys to evaluating their scattering efficiency. Based on Magnetospheric Multiscale satellite observations, we explored the statistical frequency properties of first-harmonic band ECH waves in the outer magnetosphere. The frequencies at the peak power of ECH waves are found to be day-night and dawn-dusk asymmetries, with higher values in the regions from dawn to post-noon, and these asymmetries are more evident during weaker geomagnetic activity. Furthermore, the frequencies at the peak power of ECH waves decrease gradually with increasing |MLAT| and are positively correlated with their amplitudes at each magnetic local time or |MLAT|. Information on the frequency properties of ECH waves presented in this study can be crucial for future modeling of their contributions to magnetospheric electron dynamics.