Solar energetic particles in near-Mars space

The space radiation environment near Mars has taken on new interest due to the resurrection of plans to send humans to explore the red planet. In addition, solar energetic particles represent a possibly significant input of energy to the atmosphere of Mars during major events, with consequences for atmospheric ionization, chemistry, and possibly escape. Measurements of solar events by the MARIE and GRS experiments on Mars Odyssey illustrate how Mars affects the low-Mars-orbit fluxes of these particles, apparently blocking some particles' access to the spacecraft. The extent to which the presence of Mars reduces the fluxes in Mars orbit from their interplanetary values, and the circumstances and geometry of those reductions, is examined using a simple model and some observationally inspired assumptions about the nature of solar energetic particle events. The results suggest how Mars orbiter SEP results can be interpreted, and also how near-Mars fluxes for a particular interplanetary event can be predicted.