Analysis of the Radiation Hazard Observed by RAD on the Surface of Mars During the September 2017 Solar Particle Event

We report dosimetric quantities measured by the MSL-RAD detector on the surface of Mars during the 10-12 September 2017 solar particle event (SPE). Despite 23 g cm-2 of CO2 shielding provided by the atmosphere above RAD, dose rates rose above background galactic cosmic ray (GCR) levels by factors of two to three over the course of several hours and leveled off at sustained peak rates for about 12 hours before declining over the following 36 hours. As the SPE flux was gradually declining, a shock front reached Mars and caused a sudden drop of about 15% in instantaneous dose rates. No solar particles followed the shock arrival, and the magnetic shielding of GCRs by the shock reduced their intensity to levels below those seen before the start of the event. This event is the largest seen to date by RAD on Mars.