A Dynamic Model for the Validation of Cosmic Rays Anisotropy at Low Earth Orbit

The International Space Station (ISS) provides the proving ground for future long duration human activities in space. Ionizing radiation measurements at low Earth orbit (LEO) in general, and at ISS in particular, form the ideal tool for the experimental validation of radiation environmental models, nuclear transport code algorithms and nuclear reaction cross sections. Indeed, prior measurements onboard the space transportation system (STS; shuttle) have provided vital information impacting both the environmental models and the nuclear transport code development by requiring dynamic (time dependent) models of the LEO environment. In addition, past studies using computer aided design (CAD) models of ISS have demonstrated that the ion specific based flux or cumulative based dose exposure prediction for a spacecraft at LEO requires the description of an environmental model with accurate anisotropic (directional) as well as dynamic behavior. Within the framework of an environment code named GEORAD (GEOmagnetic RADiation), this paper describes the dynamic and anisotropic capabilities of GEORAD as applied to the interaction of galactic cosmic rays (GCR) with the geomagnetic field at LEO. The described model is a component of GEORAD which computes anisotropic cutoff rigidity and the corresponding transmission coefficient, both of which are used as input into a deterministic particle transport algorithm for exposure estimation within ISS. The GEORAD capability to compute anisotropic cutoff rigidity and transmission coefficient provides a useful tool to validate GCR exposure measurements by solid state particle telescopes which inherently have strong directional sensitivity. While this paper concentration is on the anisotropic characteristics of GCR ions at LEO, GEORAD suite is applicable to radiation environment prediction at LEO, medium Earth orbit (MEO) and geosynchronous Earth orbit (GEO) at quiet solar periods. GEORAD interest is in the study of the geomagnetic environment from a long term point of view (i.e., a meaningful portion of solar cycle), and therefore it does not account for any short term distortion of the geomagnetic field due to solar activity. With the concentration of the paper on the study of GCR ions at LEO, for the formation flying ISS, the paper presents the anisotropic profile of GCR ions at different angular distribution with respect to zenith. While the magnitude of the GCR anisotropy at LEO depends on a multitude of factors such as the ions rigidity, transmission, attitude and orientation of the spacecraft along the velocity vector, the paper draws quantitative conclusions on the effect of GCR anisotropy at LEO.