Monte Carlo study of secondary electrons and X-rays produced by different angular distributions of primary precipitating electrons interacting with the atmosphere

Electron precipitation from the outer belt is an important input of energy and electric charge to the atmosphere. Its effect on the electrodynamics of the atmosphere depends on the resulting ionization profile (ionization rate vs. altitude). It is likely that the ionization profile is strongly affected by the angular distribution of precipitating electrons absorbed by the atmosphere. Definitive measurements of precipitating electrons at the top of the atmosphere have not been made; the usual assumption for calculations of this problem is that they have an isotropic distribution over the zenithal hemisphere. However, consideration of the mechanism leading to the precipitation of outer belt electrons suggests a different distribution: a trapped electron in the process of mirroring encounters a region near the top of the atmosphere where its gyro-circumference is equal to its mean-free-path and thus collides with an atmospheric molecule. In this case, precipitating electrons are traveling horizontally when they are absorbed in the atmosphere. In order to investigate differences in the ionization profile that may depend on the angular distribution of precipitating electrons, we have conducted a Monte Carlo study of this problem using the FLUKA code. The two angular distributions described previously were assumed with an energy spectrum typical for outer belt electrons up to 10 MeV; both electrons and X-rays were followed down to energies of 100 keV. The Monte Carlo results are compared to measurements of electrons in the atmosphere below 80 km made from rocket-boosted, parachute-deployed payloads, and to measurements of X-rays made on balloon payloads at altitudes of about 35 km. Also, the flux and energy spectrum of backscattered electrons traveling upward from the atmosphere are determined for the two angular distributions of precipitating electrons, isotropic over the zenithal hemisphere and horizontal absorption.