First Observation of Banded Electron Cyclotron Harmonic Waves at Earth's Magnetopause

This letter presents the first observation of banded Electron Cyclotron Harmonic (ECH) waves by Magnetospheric Multiscale (MMS) satellites near the Earth's subsolar magnetopause. The observation shows the fundamental mode () and its higher harmonics (–6), each with distinct lower and upper bands separated by a spectral gap at , where is the electron cyclotron frequency and is the mode number. Our analysis shows that the observed electron distribution during the wave event exhibits a loss cone and weak beam, modeled using a subtracted bi‐Maxwellian distribution. The linear dispersion analysis shows the growth only for the upper bands (), suggesting banded ECH as‐yet‐unknown processes. These findings offer new insights into electron‐scale plasma dynamics and highlight the need for further work to understand the generation of banded ECH waves in the Earth's magnetosphere.