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

摘要

Relativistic particles entering the Earth's magnetosphere, i.e.cosmic rays and solar energeticparticles, are of prime space weather interest because theycan affect satellite operations, communications, and the safety of astronautsand airline crews and passengers.In order to mitigate the hazards that originate from such particlesone needs to predict the cutoff latitudes of such particles as afunction of their energies and the state of the magnetosphere.We present results from a new particle tracing codethat is used to determinethe cutoff latitudes of 8-15Men-1 alpha particles during the23/24 April, 1998 geomagnetic storm and the preceding quiet time.The calculations arebased on four different geomagnetic field models and comparedwith SAMPEX observations of alpha particles in the same energy range.The geomagnetic field models under consideration are: (i) theInternational Geomagnetic Reference Field (IGRF) model,(ii) the Tsyganenko "89" model (T89c), (iii) the Tsyganenko "96" model (T96),and (iv) a global magnetohydrodynamic (MHD) model of Earth's magnetosphere.Examining 11 SAMPEX cutoff latitude observationswe find that the differences between the observed and thepredicted cutoff latitudes are2.3° ± 2.0° (mean) and 7.9° (maximum difference) for the IGRF model;3.9° ± 2.4° (mean) and 6.9° (maximum difference) for the T89c model;4.0° ± 1.4° (mean) and 5.5° (maximum difference) for the T96 model;and2.5° ± 1.7° (mean) and 7.0° (maximum difference) for the MHD model.All models generally predict cutoff latitudes equatorward ofthe SAMPEX observations.The MHD model results alsoshow steeper cutoff energy gradients with latitude compared tothe empirical models and more structurein the cutoff energy versus latitude function, presumably due to thepresence of boundary layers in the MHD model.