Mars Ground Level Enhancements in the Context of the Solar Energetic Particle Clock

In this work we discuss the growing ensemble of solar energetic particle events registered on the Martian surface, including their temporal appearance and solar sources. Solar energetic particle events have been observed from the surface of Mars since soon after the August 2012 landing of the Radiation Assessment Detector onboard Curiosity. The Martian atmosphere prevents protons and heavy ions below 140 – 190 MeV/n kinetic energy from directly reaching the Martian surface. This cut-off is high enough to limit the number of solar energetic particle events measured on the surface to only 19 in ∼ 12 1/2 years. Yet we find in this analysis that proton acceleration that gives rise to Mars ground level enhancements takes place predominantly at or near the solar eruption site, while transport to Mars incurs a delay in onset, and, as we show here, peak intensity. These delays are a function of the longitudinal magnetic connection distance, defined here as the angular distance between the foot point of solar wind magnetic field lines that intersect the Mars environment and the source longitude of the solar magnetic eruption. A distinct clustering of solar source locations at or near the Mars foot points at the Sun’s western limb relative to Mars is apparent, indicating lower flux thresholds from such preferred locations. We have developed a simple peak flux prediction tool for the Martian surface that could be used to reduce radiation exposure during future human exploration missions.