We present a new model designed to simulate the process of energetic particle precipitation, a vital coupling mechanism from Earth’s magnetosphere to its atmosphere. The atmospheric response, namely excess ionization in the upper and middle atmosphere, together with bremsstrahlung X-ray production, is calculated with kinetic particle simulations using the GEANT4 Monte Carlo framework. Mono-energy and mono-pitch angle electron beams are simulated and combined using a Green’s function approach to represent realistic electron spectra and pitch angle distributions. Results from this model include more accurate ionization profiles than previous analytical models, deeper photon penetration into the atmosphere than previous Monte Carlo model predictions, and predictions of backscatter fractions of loss cone electrons up to 40%. The model results are verified by comparison with previous precipitation modeling results, and validated using balloon X-ray measurements from the BARREL mission and backscattered electron energy and pitch angle measurements from the ELFIN CubeSat mission. The model results and solution techniques are developed into a Python package for public use.

Energetic electron precipitation (EEP) from the radiation belts into Earth’s atmosphere leads to several profound effects (e.g., enhancement of ionospheric conductivity, possible acceleration of ozone destruction processes). An accurate quantification of the energy input and ionization due to EEP is still lacking due to instrument limitations of low-Earth-orbit satellites capable of detecting EEP. The deployment of the ELFIN (Electron Losses and Fields InvestigatioN) CubeSats marks a new era of observations of EEP with an improved pitch-angle (0°–180°) and energy (50 keV–6 MeV) resolution. Here, we focus on the EEP recorded by ELFIN coincident with electromagnetic ion cyclotron (EMIC) waves, which play a major role in radiation belt electron losses. The EMIC-driven EEP (~200 keV – ~2 MeV) exhibits a pitch-angle distribution (PAD) that flattens with increasing energy, indicating more efficient high-energy precipitation. Leveraging the combination of unique electron measurements from ELFIN and a comprehensive ionization model known as Boulder Electron Radiation to Ionization (BERI), we quantify the energy input of EMIC-driven precipitation (on average, ~3.3x10-2 erg/cm2/s), identify its location (any longitude, 50°–70° latitude), and provide the expected range of ion-electron production rate (on average, 100–200 pairs/cm3/s), peaking in the mesosphere – a region often overlooked. Our findings are crucial for improving our understanding of the magnetosphere-ionosphere-atmosphere system as they accurately specify the contribution of EMIC-driven EEP, which serves as a crucial input to state-of-the-art atmospheric models (e.g., WACCM) to quantify the accurate impact of EMIC waves on both the atmospheric chemistry and dynamics.

AbstractElectromagnetic ion cyclotron waves in the Earth's outer radiation belt drive rapid electron losses through wave-particle interactions. The precipitating electron flux can be high in the 100s keV energy range, well below the typical minimum resonance energy. One of the proposed explanations relies on nonresonant scattering, which causes pitch-angle diffusion away from the fundamental cyclotron resonance. Here we propose the fractional sub-cyclotron resonance, a second-order nonlinear effect that scatters particles at resonance order $n = 1/2$, as an alternate explanation. Using test-particle simulations, we evaluate the precipitation ratios of sub-MeV electrons for wave packets with various shapes, amplitudes, and wave normal angles. We show that the nonlinear sub-cyclotron scattering produces larger ratios than the nonresonant scattering when the wave amplitude reaches sufficiently large values. The ELFIN CubeSats detected several events with precipitation ratio patterns matching our simulation, demonstrating the importance of sub-cyclotron resonances during intense precipitation events.