Abstract
Abstract. Relativistic particles entering the Earth's magnetosphere, i.e. cosmic rays and solar energetic particles, are of prime space weather interest because they can affect satellite operations, communications, and the safety of astronauts and airline crews and passengers. In order to mitigate the hazards that originate from such particles one needs to predict the cutoff latitudes of such particles as a function of their energies and the state of the magnetosphere. We present results from a new particle tracing code that is used to determine the cutoff latitudes of 8-15Men-1 alpha particles during the 23/24 April, 1998 geomagnetic storm and the preceding quiet time. The calculations are based on four different geomagnetic field models and compared with SAMPEX observations of alpha particles in the same energy range. The geomagnetic field models under consideration are: (i) the International 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 observations we find that the differences between the observed and the predicted cutoff latitudes are 2.3° ± 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; and 2.5° ± 1.7° (mean) and 7.0° (maximum difference) for the MHD model. All models generally predict cutoff latitudes equatorward of the SAMPEX observations. The MHD model results also show steeper cutoff energy gradients with latitude compared to the empirical models and more structure in the cutoff energy versus latitude function, presumably due to the presence of boundary layers in the MHD model.
Highlights
Space Radiation is a serious hazard for satellite operations, communications, and human space flights
Recent studies have begun to compare observed geomagnetic cutoff latitudes with those calculated using different magnetic field models
The primary goal of our study is to compare the cutoff latitudes as measured by SAMPEX/Mass Spectrometer Telescope (MAST) with those determined from relativistic particle tracing in a magnetic field from a global magnetosphere MHD simulation during a solar energetic particle event on 23 April 1998 at ∼22:40 UT
Summary
Space Radiation is a serious hazard for satellite operations, communications, and human space flights. Recent studies have begun to compare observed geomagnetic cutoff latitudes with those calculated using different magnetic field models. Our study compares the 1995 IGRF, T89c, T96, and MHD geomagnetic field model cutoff energy results with one of the events in the Leske et al (2001) work. The primary goal of our study is to compare the cutoff latitudes as measured by SAMPEX/MAST with those determined from relativistic particle tracing in a magnetic field from a global magnetosphere MHD simulation during a solar energetic particle event on 23 April 1998 at ∼22:40 UT. We expect that during times of high geomagnetic activity, like the ones chosen in this study, the MHD based calculations will be closer to reality compared to those based on empirical models This statement is based on the fact that empirical models like IGRF or Tsyganenko lack principal magnetospheric features, such as a bow shock, a magnetosheath, the cusps, and in the case of the IGRF, even a tail. We use the Leske et al (2001) definition of the cutoff latitude when we compare our model results with the SAMPEX data
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