Radiation Exposure Assessments for Solar Proton Ground Level Enhancements

Abstract

Solar proton events (SPEs) represent the single-most important source of acute space radiation exposure to humans and radio-sensitive spacecraft systems for lunar and Mars missions. In this paper, a review of the major sola r proton events for the past 5 solar cycles is presented with a focus on those SPEs that produced ground level enhancements (GLEs) as observed/measured by high latitude neutron monitor (NM) stations. Using NM GLE data one can infer the upper energy portion of the SPE e nergy spectrum, which extends up to several GeV (10 3 MeV). Historically, the scientific community has u sed solar proton data in the 10-100 MeV range from early IMP (Interplanetary Monitoring Platform) and, more recently, GOES (Geostationary Operational Environmental Satellite) spacecraft to make radiation exposure estimates. These SPE data were usually fitted using an exponential in rigidity (momentum) assumption (Malitson and Webber, 1963): φ(>P) = No * Exp(-P/Po) where φ is the integral fluence in protons/cm 2 , No is the normalization constant, P is the proton rigidity in million volts (MV), and Po is th e characteristic rigidity in MV. More recently, Xapsos, et al (2000) have determined that a better fit to the SPE data in the 10-100 MeV range is obtained by using a Weibull function. For GLEs, the proton energies extend to several GeV. Recently, Tylka and Dietrich (2008) ha ve utilized NM data and developed methods (Band [1993] function) to extend the solar particle data over the entire particle energy spectrum. SPE radiation assessments can be grossly underestimated if the entire proton energy spectrum is not taken into considerat ion. In this paper, we compare the various fitting methods to estimate the absorbed do se and dose equivalent for six (6) classic GLEs. Finally, detailed radiation exposure analyses are presented using several different radiation-mitigating shielding materials.