Optically thin H Lyman alpha production on outer planets: Low‐energy proton Acceleration in parallel electric fields and neutral H atom precipitation from ring current

Abstract

A Monte Carlo model has been constructed to describe the energization and energy degradation of low‐energy protons in an H2 atmosphere in the presence of parallel electric field. The energy spectrum of Ly α collision events is employed to determine the volume emission rate (VER) of H Ly α emissions. Numerical experiments have been performed to study the effect of initial proton energy, electric field, neutral number density, initial pitch angle, and cutoff limit on H Ly α VER. The present study demonstrates that energization of solar EUV‐generated low‐energy protons by parallel electric fields is incapable of producing optically thin Ly α emissions on Uranus through direct collision with H2. However, nonthermal H atoms, produced through acceleration of protons in parallel electric fields, play an important role in enhancing the Ly α intensity through resonant scattering of solar Ly α flux: resulting emissions are consistent with the broadening of the Ly α line observed on Jupiter by IUE. The Monte Carlo model is also applied to the problem of Doppler‐shifted H Ly α emissions from the auroral atmosphere of Jupiter. Energetic neutral H precipitation from the ring current is discussed as a possible source of optically thin H Ly α emissions on outer planets. Estimates are also given for Hα, Hβ, and H2 band emissions resulting from the impact of fast H (of ring current origin) on H2. The input energies and fluxes of precipitating ring current particles are, however, too uncertain to permit definite conclusions.