Investigating Whistler‐Mode Wave Intensity Along Field Lines Using Electron Precipitation Measurements.

Electron fluxes in Earth's radiation belts are significantly affected by their resonant interaction with whistler‐mode waves. This wave‐particle interaction often occurs via first cyclotron resonance and, when intense and nonlinear, can accelerate subrelativistic electrons to relativistic energies while also scattering them into the atmospheric loss cone. Here, we model Electron Losses and Fields INvestgation's (ELFIN) low‐altitude satellite measurements of precipitating electron spectra with a wave‐electron interaction model to infer the profiles of whistler‐mode intensity along magnetic latitude assuming realistic waveforms and statistical models of plasma density. We then compare these profiles with a wave power spatial distribution along field lines from an empirical model. We find that this empirical model is consistent with observations of subrelativistic (<200 keV) electron precipitation events, but deviates significantly for relativistic (>200 keV) electron precipitation events at all MLTs, especially on the nightside. This may be due to the sparse coverage of wave measurements at mid‐to‐high latitudes which causes statistically averaged wave power to be likely underestimated in current empirical wave models. As a result, this discrepancy suggests that intense waves likely do propagate to higher latitudes, although further investigation is required to quantify how well this high‐latitude population can account for the observed relativistic electron precipitation. Plain Language Summary: Whistler‐mode waves, the most prevalent type of plasma wave in Earth's magnetosphere, often interact with electrons by resonating with them, causing them to be accelerated and lost into Earth's atmosphere (in other words, precipitated). These waves are generated at the equator and typically stay constrained to within 20° in latitude; however, they can sometimes propagate to greater than 30° where they can accelerate electrons to relativistic energies. It is difficult to quantify how large of a contribution these mid‐to‐high‐latitude waves have on radiation belt electrons due to a lack of off‐equatorial spacecraft wave measurements. However, previous studies have shown that the energy spectra of precipitating electron fluxes may be used to infer the latitudinal extent of whistler‐mode waves. We therefore compare measurements of relativistic precipitation from NASA's Electron Losses and Fields INvestgation (ELFIN) mission (a pair of CubeSats built and operated by UCLA) with large ensemble test‐particle simulations informed by current empirical models of waves. Discrepancies in this comparison suggest that this elusive population of mid‐to‐high‐latitude whistler‐mode waves is most apparent at Earth's nightside and may even help explain some of the more intense precipitation events observed by the ELFIN CubeSats. Key Points: We evaluate the role of whistler‐mode waves on relativistic electron precipitation by modeling ELFIN case‐studies and statisticsObserved precipitation >200 keV exceeds results obtained with empirical models of latitudinal wave power distributions, notably at nightThe discrepancy may be explained by mid‐to‐high‐latitude intense whistler‐mode waves which are missing from current empirical models [ABSTRACT FROM AUTHOR]