C.' A Study of Vertically Incident Cosmic-Ray Trajectories Using Sixth-Degree Simulations of the Geomagnetic

A study has been- ie of vertically incident coamic.ray trajr.ctories at more than 400 positions on the earth's surface using sixtth-degree simulations of the quiescent geomagnetic field. Twenty-six of these positions are near the South African magnetic anomaly, and six other points &re in a region of the North Atlantic where Pomerantz and Agarwal have reported anomalous-results. ' The trajectories have been calculated for rigidities separated by intervals as small as 0.01 bv to allow for the effects of the penumbra. When these trajectories are used to determine cosmic-ray cutoff rigidities, differences gre.ater than 15 percent frna the Quenby and Wenk threshold values are four.d in the vicinity of South Africa, the South Atlantic, and the Canary Islands. Various latitude surveys are shown to exhibit consistency when plotted against the cutoff rigidities determined t1a. this computational method. It is concluded that although currently a krailable simulations of the geomagnetic field are not completely adequate over so,-- portions of the L Kr th to d.ic.c:be the cosmic-ray effects in their entirety, there is PMsmentially no difftret ee in the cutoff rigidities determined using two currently accepted field models, eve