Proton intensity gradients and radiation dose equivalents in the inner heliosphere: Modeling and Prediction

Solar Energetic Particles (SEPs) provide a significant radiation hazard for manned and unmanned interplanetary (IP) space missions. In order to estimate these hazards, it is essential to quantify the gradients of SEP intensities in the IP medium. The Earth-Moon-Mars Radiation Exposure Module (EMREM) is a newly developed project aimed at characterizing the time-dependent radiation exposure in IP space environments. EMMREM consists of two major workhorses, a 3D energetic particle transport code (EPREM: Energetic Particle Radiation Environment Module) and a space radiation transport code (a modified version of BRYNTRN : Baryon Transport Model). EPREM ingests SEP intensities measured at 1 AU (by instruments on board ACE, SOHO, GOES, and IMP spacecraft) and projects them into the three-dimensional heliosphere. The model treats particles along each magnetic field line for transport, adiabatic focusing, adiabatic cooling, convection, pitch-angle scattering, and stochastic acceleration in the framework of a modified formalism of the diffusion-focused transport equation [Kota et al., ICRC, 1, 125, 2005]. Using proton measurements from SOHO/ERNE at energies between 2 MeV and 100 MeV during the May 27-31 2003 SEP events, we utilize EMMREM to study the radial and longitudinal dependence of proton peak intensities, event fluences, and radiation dose equivalents (for various aluminum and water shield thicknesses) at 8 different locations between 1 and 5 AU. We compare projected intensities at Mars and 5 AU with observations from Odyssey and Ulysses spacecraft and comment on the capabilities of EMMREM in predicting particle intensities in the heliosphere. We also discuss the effects of EPREM transport parameters and initial conditions on the propagated particle intensities.