A five degree by fifteen degree world grid of trajectory-determined vertical cutoff rigidities

Vertical cutoff rigidities have been calculated for a world grid 5 degrees in latitude and 15 degrees in longitude using the Finch and Leaton sixth-degree simulation of the quiescent geomagnetic field. These calculations utilized the trajectory-tracing procedure at intervals of 0.01 BV throughout the penumbral region to determine an "effective" vertical cutoff rigidity defined by Shea et al. (1965). When experimental cosmic-ray intensity data are plotted using threshold rigidities derived from this grid, greater coherence of these data is found than is obtained using previously published threshold values. However, small deviations from an intuitively pleasing smooth curve indicate we may have reached a limitation in the usefulness of vertical cutoff rigidities, and perhaps second-order effects should be included when using cutoff rigidity as a parameter in the analysis of very precise cosmic-radiation intensity data.