A new method of computing cosmic-ray cutoff rigidity for several geomagnetic field models

An approximate method of computing cosmic-ray cutoff rigidities is presented. The geomagnetic field is represented by the combination of a sixth order magnetic field whose origin lies entirely within the earth, and that due to a filamentary equatorial ring current of various radii and magnetic moments. The sixth order spherical harmonic expansion of the scalar magnetic potential is that obtained by Finch and Leaton by numerical analysis of surface magnetic field measurements for epoch 1955. Vertical cutoff rigidities have been computed for all IGY stations of geomagnetic latitude greater than ±45° listed in the Annals of the International Geophysical Year, vol. 8. In addition, the vertical cutoff rigidities have been calculated for geographic latitude steps of 5° in geographic longitude steps of 10° for the sixth order field alone in regions of geomagnetic latitude greater than ±45°. The results are compared with the values given by Quenby and Webber, and by Quenby and Wenk, and with those derived from the magnetic shell parameter L. Comparisons of the values of vertical cutoff rigidity obtained for Port aux Francais, Northern Italy, and Minneapolis with those obtained by McCracken, Lynch, and Kellogg, respectively, by calculation of particle trajectories, are consistent with the 5 per cent precision claimed for the present method.