Modeling the Albedo Neutron Decay Source of Radiation Belt Electrons and Protons

There are no detailed measurements of the atmospheric albedo neutrons that, bytheir decay in near-Earth space, are a principal source of the radiation belt.Therefore the main tool for evaluating the source strength is theoreticalmodeling of the relevant physical processes. Upward neutron flux at 100 kmaltitude is computed by a Monte Carlo simulation of cosmic ray nuclei incidenton a model atmosphere using the Geant4 particle transport code. Inclusion ofEarth's curvature is required to generate an upward flux at high energy whereneutron production is strongly forward peaked. This leads to energy and zenithangle dependences in the neutron albedo, while the cosmic ray east-west effectintroduces azimuthal dependence. At low energy, ballistic neutron trajectoriesaccount for gravitational deflection and transit time to higher altitude, duringwhich significant decay and loss of neutron kinetic energy can occur, modifyingthe main electron source. Lorentz transformation of the neutron beta-decayspectrum is required for higher energy electrons. Finally, these factors combinein averages over trapped particle drift paths to determine electron and protonsource functions.