Radiation Hazards from Extreme Solar Energetic Particle Events Observed near the Moon

Acute space radiation hazards pose one of the most serious risks to future human and robotic exploration. Large solar energetic particle (SEP) events are dangerous to astronauts and equipment. A fundamental question remains as to howlarge SEP events are formed, how they are related to coronal mass ejections (CMEs) and active region eruptions, and what factors control thedifferential fluxes incident at Earth and other observers during SEPevents. The current evolution of the Sun between solar cycles 23 and 24 andduring cycle 24 remains highly anomalous compared to previous periods of the space age. The Sun has been abnormally quiet over a relatively long solar minimum when galactic cosmic rays (GCRs) achieved the highest flux levels observed in the space age, and thepower, pressure, flux and magnetic flux of the solar wind were at the lowest levels. Despite the continued paucity of solar activity, one of the hardest solar eventsin almost a decade occurred in September 2017 after more than a year ofall-clear periods. The 2017 September event demonstrates the importance of large fluxes of suprathermal seed populations and fast, large CMEs that drive shocks and compressions low in the corona (<5 Rs) where large SEP events are accelerated. The Coronal-Solar Wind Energetic Particle Acceleration (C-SWEPA) modeling effort and the SPE Threat Assessment Tool (STAT) combine the Earth-Moon-Mars Radiation Environment Modules (EMMREM) that describe energetic particles and their effects, with the CORHEL (Corona-Heliosphere) modeling suite developed by the Predictive Science, Inc. (PSI) group. Recent simulations demonstrate how CMEs form powerful compressions and shocks low in the corona that rapidly accelerate high energy particles often up to the GeV energies required for Ground Level Enhancements (GLEs). The most pronounced acceleration of the CME occurs very closeto the Sun (<2 Rs) causing extremely strong compression on the flanks and nose of the CME. We report on observations from the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on the Lunar Reconnaissance Orbiter (LRO) orbiting the Moon revealing the significant radiation hazards associated with large solar energetic particle events, and modeling showing how these events are generated in the low corona.