Geomagnetic Cutoff Rigidity Computer Program

Author: D.F. Smart, M.A. Shea
Center for Space Plasma and Aeronomic Research
The University of Alabama in Huntsville
Huntsville, Alabama 35889

Parameter: Cutoff rigidity for given location

Brief Description:
Prediction of magnetic rigidity (momentum/charge) cutoffs or thresholds for penetration of energetic cosmic ray particles from interplanetary space through Earth's geomagnetic field to the lower atmosphere was pioneered by Carl Stoermer in 1930 as the true nature of these particles was just becoming known. In the 1950's and thereafter the development of increasingly fast computers allowed detailed predictions of charged particle trajectories through complex multipolar models of the geomagnetic field. From the 1970's to the present the standard software for such computations has been that of Margaret (Peggy) A. Shea and Don F. Smart, both originally at the old Air Force Geophysics Laboratory in Massachusetts and now affiliated with the University of Alabama at Huntsville. Updated FORTRAN 77 versions of this software, test files for input and output, and accompanying documentation in Microsoft Word format, have recently been archived at the National Space Science Data Center through funding support from NASA Grant NAG5-8009. This software determines allowed trajectories reaching specified locations near or above the Earth's surface for a given range of magnetic rigidities and incidence directions. The lower limit of the rigidity range for allowed trajectories approximately defines the local cosmic ray cutoff. Cutoff averaging is needed at some locations and directions for penumbral effects of partial shadowing by the body of the Earth. Choices of models for the geomagnetic field include IGRF-95 and other older models in the NASA ALLMAG subroutines.

Availability: The Fortran code is available from the CCMC archive at

Included in software documentation.

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