Observations of Relativistic Electron Precipitation Due To Combined Scattering of Whistler‐Mode and EMIC Waves.

The two most important wave modes responsible for energetic electron scattering to the Earth's ionosphere are electromagnetic ion cyclotron (EMIC) waves and whistler‐mode waves. These wave modes operate in different energy ranges: whistler‐mode waves are mostly effective in scattering sub‐relativistic electrons, whereas EMIC waves predominately scatter relativistic electrons. In this study, we report the direct observations of energetic electron (from 50 keV to 2.5 MeV) scattering driven by the combined effect of whistler‐mode and EMIC waves using ELFIN measurements. We analyze five events showing EMIC‐driven relativistic electron precipitation accompanied by bursts of whistler‐driven precipitation over a wide energy range. These events reveal an enhancement of relativistic electron precipitation by EMIC waves during intervals of whistler‐mode precipitation compared to intervals of EMIC‐only precipitation. We discuss a possible mechanism responsible for such precipitation. We suggest that below the minimum resonance energy (Emin) of EMIC waves, the whistler‐mode wave may both scatter electrons into the loss‐cone and accelerate them to higher energy (1–3 MeV). Electrons accelerated above Emin resonate with EMIC waves that, in turn, quickly scatter those electrons into the loss‐cone. This enhances relativistic electron precipitation beyond what EMIC waves alone could achieve. We present theoretical support for this mechanism, along with observational evidence from the ELFIN mission. We discuss methodologies for further observational investigations of this combined whistler‐mode and EMIC precipitation. Plain Language Summary: Energetic electron precipitation into the upper atmosphere is an important loss process of outer radiation belt fluxes. Whistler‐mode and electromagnetic ion cyclotron (EMIC) waves are two of the most important wave modes responsible for energetic electron scattering to the Earth's ionosphere through wave‐particle interaction. These wave modes typically drive losses of electrons in different energy ranges (above 1 MeV for EMIC waves and tens to hundreds of keV for whistler‐mode waves), occurring in different spatial regions. We report the first observations of energetic electron scattering driven by the combined effect of whistler‐mode and EMIC waves. Our results from equatorial and low‐altitude observations, and a data‐driven test particle simulation explain the wide energy range of electron precipitation from tens of keVs to a few MeVs due to the combined whistler‐mode and EMIC waves effect and explain the unusually high intensity of relativistic electron precipitation at such times. Key Points: We report observations of energetic electron precipitation likely driven by concurrent whistle‐mode and electromagnetic ion cyclotron (EMIC) wavesThe combined scattering of whistler‐mode and EMIC waves leads to electron precipitation over a wide energy range of 50 keVs to a few MeVsThis study highlights the potential nonlinear effects for explaining the observed energetic electron fluxes in the inner magnetosphere [ABSTRACT FROM AUTHOR]