CHANGES IN COSMIC-RAY CUTOFF RIGIDITIES DUE TO SECULAR VARIATIONS OF THE GEOMAGNETIC-FIELD

An analytical expression is derived for the cutoff rigidity of cosmic rays arriving at a point in an arbitrary direction, when the main geom agnetic field is approximated by that of an eccentric dipole. This exp ression is used to determine changes in geomagnetic cutoffs due to sec ular variation of the geomagnetic field since 1835. Effects of westwar d drift of the quadrupole field and decrease in the effective dipole m oment are seen in the isorigidity contours. On account of the immense computer time required to determine the cutoff rigidities more accurat ely using the particle trajectory tracing technique, the present formu lation may be useful in estimating the transmission factor of the geom agnetic field in cosmic ray studies, modulation of cosmogenic isotope production by geomagnetic secular variation, and the contribution of g eomagnetic field variation to long term changes in climate through cos mic ray related modulation of the current flow in the global electric circuit.