Efficient Scattering Loss of Energetic Electrons by Enhanced Higher-Band ECH Waves Observed by Van Allen Probes

Electron cyclotron harmonic (ECH) waves, which are responsible for the diffuse aurora, are usually observed with the strongest intensity in the first band. Here, we present observations of enhanced higher-band ECH with wave intensities above 10(-3) mV(2)m(-2) Hz(-1) by Van Allen Probes. Fully thermal simulation results indicate that these waves can be locally excited by the observed electron distributions, and higher plasma density and lower magnetic strength are favorable for generating the enhanced higher-band ECH. The pitch angle diffusion coefficient < D-alpha alpha > of the higher-band ECH can exceed 10(-5) s(-1) inside the loss cone for energetic electrons (300 eV-similar to 15 keV), greater than the momentum diffusion coefficient < D-pp > by similar to 100 times. It is suggested that the enhanced higher-band ECH can also efficiently scatter these electrons into the loss cone and contribute to the diffuse aurora. Plain Language Summary Electron cyclotron harmonic (ECH) waves are electrostatic emissions usually observed with the strongest intensity in the first band. Numerous works have demonstrated that the first-band ECH waves can be responsible for diffuse auroral electron precipitation. However, so far the role of higher-band ECH waves has not been confirmed. Here, we report an enhanced higher-band ECH event with wave intensities above 10(-3) mV(2)m(-2) Hz(-1) from Van Allen Probes. Using the realistic parameters, we conduct fully thermal simulations to investigate the generation of such ECH. The results indicate that growth rates can well reproduce the observed wave spectral characteristics in all four harmonic bands, and the higher density and lower magnetic field strength are favorable for generating enhanced higher-band ECH. We calculate the diffusion coefficients to estimate the effect of the higher-band ECH on electrons. The pitch angle diffusion coefficients induced by higher-band ECH are similar to 2 orders of magnitude greater than the momentum diffusion coefficients over a broad pitch angle range and exceed 10(-5) s(-1) inside the loss cone for energetic electrons. This indicates that the enhanced higher-band ECH can also efficiently scatter these energetic electrons into the loss cone and potentially contribute to the diffuse aurora, similar to the first-band ECH.