Abstract
The International Space Station (ISS) provides the proving ground for future long duration human activities in space. Radiation measurements at low Earth orbit (LEO) in general, and at ISS in particular, form an appropriate tool for the experimental validation of radiation environmental models and nuclear transport code algorithms. 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 time dependent models of the LEO environment. In addition, past studies using computer aided design (CAD) models of ISS have demonstrated that the exposure prediction for a spacecraft at LEO requires the description of an environmental model with accurate directional as well as time dependent behavior. Within the framework of an environment code named GEORAD (GEOmagnetic RADiation), this paper describes the time dependency and directional 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 directional 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 directional cutoff rigidity and transmission coefficient provides a useful tool to validate GCR exposure measurements by solid state particle telescopes which inherently have directional sensitivity. While this paper concentration is on the directional 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, 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 directional profile of GCR ions at different angular distribution with respect to zenith. While the magnitude of the GCR directionality 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 directionality at LEO.
Published Version
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