Assessing the Space-Radiation Hazard in Ground-Level Enhanced (GLE) Solar Particle Events (Invited)

The most severe transient radiation hazards for human spaceflight in the historical record are solar particle events associated with so-called Ground-Level Enhancements (GLEs), in which processes at the Sun can accelerate protons to GeV energies in minutes. We report on recent efforts to improve the reliability of assessments of these radiation hazards. These efforts are based on a new analysis of the entire historical data base of GLEs from 1956-2006, using the complete ensemble of measurements from riometers, satellites, and neutron-monitors, from ∼10 MeV to ∼10 GeV. Of particular importance is the functional form of the proton spectrum. We have found that the event-integrated integral proton spectrum can generally be well represented by the so-called Band function (Band et al., ApJ 413, 281-292, 1993; a double power law with a smooth rollover) in rigidity. The residuals of these fits with respect to the data typically range from <10% for satellite measurements to <30% for neutron monitor measurements. We present a new catalogue of Band fit parameters for 58 out of the 66 GLEs that have been observed since 1956. We also examine how the dose-depth profiles calculated from these Band fits differ from earlier calculations, based on assumed functional forms derived from more limited datasets. For some of the larger events, we also show hour-by-hour analysis of the accumulated dose. Supported by ONR, NASA/SRAG (IPR NNJ09HC54I), and NASA LWS/TRT (DPR NNH09AL11I).