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. In this study, we report 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 several events exhibiting such properties, and show that electron resonant interactions with whistler-mode waves may enhance relativistic electron precipitation by EMIC waves. During a prototypical event which benefits from conjugate THEMIS measurements, we demonstrate that below the minimum resonance energy (Emin) of EMIC waves, the whistler-mode wave may both scatter electrons into the loss-cone and also accelerate them to higher energy (1-3 MeV). These accelerated electrons 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.

Energetic electron losses by pitch-angle scattering and precipitation to the atmosphere from the radiation belts are controlled, to a great extent, by resonant wave particle interactions with whistler-mode waves. The efficacy of such precipitation is primarily controlled by wave intensity, although its relative importance, compared to other wave and plasma parameters, remains unclear. Precipitation spectra from the low-altitude, polar-orbiting ELFIN mission have previously been demonstrated to be consistent with energetic precipitation modeling derived from empirical models of field-aligned wave power across a wide-swath of local-time sectors. However, such modeling could not explain the intense, relativistic electron precipitation observed on the nightside. Therefore, this study aims to additionally consider the contributions of three modifications – wave obliquity, frequency spectrum, and local plasma density – to explain this discrepancy on the nightside. By incorporating these effects into both test particle simulations and quasi-linear diffusion modeling, we find that realistic implementations of each individual modification result in only slight changes to the electron precipitation spectrum. However, these modifications, when combined, enable more accurate modeling of ELFIN-observed spectra. In particular, a significant reduction in plasma density enables lower frequency waves, oblique, or even quasi-field aligned waves to resonate with near $\sim1$ MeV electrons closer to the equator. We demonstrate that the levels of modification required to accurately reproduce the nightside spectra of whistler-mode wave-driven relativistic electron precipitation match empirical expectations, and should therefore be included in future radiation belt modeling.