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Relativistic particles entering the Earth's magnetosphere, i.e.\udcosmic rays and  solar energetic\udparticles, are of prime space weather interest because they\udcan affect satellite operations, communications, and the safety of astronauts\udand airline crews and passengers.\udIn order to mitigate the hazards that originate from such particles\udone needs to predict the cutoff latitudes of such particles as a\udfunction of their energies and the state of the magnetosphere.\udWe present results from a new particle tracing code\udthat is used to determine\udthe cutoff latitudes of 8-15Men<sup>-1</sup> alpha particles during the\ud23/24 April, 1998 geomagnetic storm and the preceding quiet time.\udThe calculations are\udbased on four different geomagnetic field models and compared\udwith SAMPEX observations of alpha particles in the same energy range.\udThe geomagnetic field models under consideration are: (i) the\udInternational Geomagnetic Reference Field (IGRF) model,\ud(ii) the Tsyganenko "89" model (T89c), (iii) the Tsyganenko "96" model (T96),\udand (iv) a global magnetohydrodynamic (MHD) model of Earth's magnetosphere.\udExamining 11 SAMPEX cutoff latitude observations\udwe find that the differences between the observed and the\udpredicted cutoff latitudes are\ud2.3° ± 2.0° (mean) and 7.9° (maximum difference) for the IGRF model;\ud3.9° ± 2.4° (mean) and 6.9° (maximum difference) for the T89c model;\ud4.0° ± 1.4° (mean) and 5.5° (maximum difference) for the T96  model;\udand\ud2.5° ± 1.7° (mean) and 7.0° (maximum difference) for the MHD model.\udAll models generally predict cutoff latitudes equatorward of\udthe SAMPEX observations.\udThe MHD model results also\udshow steeper cutoff energy gradients with latitude compared to\udthe empirical models and more structure\udin the cutoff energy versus latitude function, presumably due to the\udpresence of boundary layers in the MHD model

Cosmic ray cutoff prediction using magnetic field from global magnetosphere MHD simulations