Kinetic Modeling of Radiation Belt Electrons With Geant4 to Study Energetic Particle Precipitation in Earth's Atmosphere

• Published in: Earth and space science (Hoboken, N.J.) N.J.), 2023-11, Vol.10 (11), p.n/a
• Main Authors: Berland, G. D., Marshall, R. A., Capannolo, L., McCarthy, M. P., Zheng, L.
• Format: Article
• Language: English
• Subjects: Altitude; Atmosphere; Atoms & subatomic particles; Charged particles; electrons; energetic particle precipitation; Energy; General circulation models; Ionization; Magnetic fields; Plasma; Precipitation; Radiation; radiation belts; Remote sensing; X‐rays
• ISSN: 2333-5084; 2333-5084
• DOI: 10.1029/2023EA002987

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 Balloon Array for RBSP Relativistic Electron Losses mission and backscattered electron energy and pitch angle measurements from the Electron Loss and Fields Investigation with a Spatio‐Temporal Ambiguity‐Resolving CubeSat mission. The model results and solution techniques are developed into a Python package for public use. Plain Language Summary The upper atmosphere and near‐Earth space interact with each other through the transport of charged particle (e.g., electrons) from space into the atmosphere in a process called energetic particle precipitation. This process disturbs the atmosphere and causes X‐rays to be generated, among other direct and indirect effects to the atmosphere, including ozone destruction. This work describes a physics‐based model that simulates this process across realistic input values for energy and electron velocity direction. Results of this work include an estimate of the number of excess ion‐electron pairs generated in the atmosphere from precipitation, how many electrons are lost to the atmosphere versus those that rebound and return to space, and the energy and amount of X‐rays generated by precipitation. The model outputs are checked using balloon‐based measurements of X‐rays in the middle atmosphere and by a low Earth orbiting satellite that spins to measure electrons heading toward and away from Earth. Key Points A Geant4‐based model has been developed to simulate radiation belt energetic particle precipitation (EPP) Mono‐energy and mono‐pitch angle beams are sim