Solar Energetic Particle Behavior in Near-Mars Space

Solar energetic particles (SEPs), comprised mainly of greater than ~10 MeV protons, have been detected by both MARIE on Mars Odyssey and in the background counter of the MGS Electron Reflectometer (ER) experiment. These particle events, also seen on the earlier Phobos spacecraft, are typically associated with a form of solar activity called Coronal Mass Ejections or CMEs. They are of interest both because of their potential for depositing significant amounts of energy in the Martian atmosphere, and also for their known effects on spacecraft instrumentation and astronauts. SEP event effects in Earth's environment are usually limited to high latitudes, where they have direct access to the polar caps along open magnetospheric field lines, and to the radiation belt regions, where they gain access by dayside entry during the accompanying interplanetary shock passage. In contrast to Earth, weakly magnetized Mars represents a practically unshielded obstacle. To a first approximation, it should absorb the energetic protons arriving along the interplanetary magnetic field direction. This poster describes the use of a model of the solar wind interaction with an unmagnetized body to examine in more detail the behavior of energetic protons around Mars, and the nature of the orbital period dips in the detected fluxes at the spacecraft. We illustrate how an anisotropic influx of energetic protons from the Sun is affected by the presence of Mars, creating SEP shadows on the antisunward extensions of the interplanetary field lines that drape closest to the planet. These shadows should be minimized when the shock is present at Mars and the SEPs are more isotropic, and reverse to the sunward side of the planet after the shock source has passed Mars. We examine to what extent such a picture can explain the observations on MGS and Odyssey.