Anisotropic Trapped Proton Effects on the International Space Station

Most trapped proton effects studies assume a particle omni-directionality, while in reality the trapped particle environment is highly directional. This effect, called the “East-West effect,” has been observed and measured on several Space Shuttle missions. Normally one assumes that the Shuttle flies at different attitudes during the course of the mission and the directionality effects get “smeared out.” The International Space Station (ISS), however, will fly at a fixed attitude. Using the SPENVIS on-line capability and the anisotropic proton models of Badhwar-Konradi (B-K) and Watts (Vector Flux model: VF1), trapped proton differential spectra were generated for selected altitudes (51.6 deg inclination) for both solar minimum and maximum. Incorporating a particle transport code and a shielding model of the ISS, transmitted proton spectra can be computed as a function of shield thickness and polar and azimuthal angles (“look direction”) for ISS habitable modules. These data allow one to identify thin regions where additional shielding may be located to minimize crew exposures. The directional results are compared with omnidirectional calculations.