Worldwide trajectory-derived vertical cutoff rigidities and their application to experimental measurements

Since the advent of high-speed digital computers, the trajectory-tracing process has been employed by several researchers to calculate the cutoff rigidity for specific locations of geophysical interest. For the past year we have been using this method to produce a grid of vertical cutoff rigidities for each 5° in latitude and 15° in longitude. This method utilizes the Finch and Leaton sixth-degree simulation of the quiescent geomagnetic field and considers rigidity intervals as small as 0.01 bv to include effects of the penumbra. When cosmic-ray threshold rigidities interpolated from this grid are used to order cosmic-ray intensity data, excellent coherence of the data is obtained even in magnetically anomalous areas. Examination of the data for several latitude surveys shows that for any specific year, the data are grouped in a single general curve, but when the data from different years are compared, time changes due to modulation of the primary cosmic-ray spectrum are apparent. However, slight variations noticeable in very precise experimental measurements are indicative of the fact that we may have reached a limitation in using only vertical cutoff rigidities for ordering experimental data. Vertical cutoff rigidities for a 15 by 15° world grid are listed.