Geomagnetic cutoff rigidities and geomagnetic coordinates appropriate for the Carrington flare Epoch
Using the trajectory-tracing technique for cosmic rays in the geomagnetic field, vertical cutoff rigidity values for a world grid have been determined for Epoch 1850. These values have been used to derive a world map of iso-rigidity contours that would have been appropriate for the era of the Carrington flare in September 1859. When comparing these iso-rigidity contours with those determined for Epoch 2000, large differences are found, particularly in the Atlantic Ocean region. Geomagnetic cutoff rigidity values and geomagnetic coordinates have been determined for selected mid and low latitude geographic locations for which aurora were sighted during the geomagnetic storms of late August and early September 1859 and compared with the values for those locations calculated for the year 2000. While the geomagnetic latitude differences are relatively small, there are major changes in the vertical cutoff rigidity values for these same locations over this 150-year period. The cutoff differences are attributed to a combination of: (1) the decreasing internal geomagnetic field over the last 150 years and (2) the westward drift of the major features of the geomagnetic field. The relatively small changes in geomagnetic latitude are attributed to the small change in the latitude of the north magnetic pole over this 150-year period. This study emphasizes that while geomagnetic cutoff values are essential for the analysis of high-energy solar proton events, they are not an appropriate parameter for evaluation of the equatorward extent of an auroral display.