Changes in cosmic ray cut-o rigidities due to secular variations of the geomagnetic ®eld
نویسندگان
چکیده
An analytical expression is derived for the cuto rigidity of cosmic rays arriving at a point in an arbitrary direction, when the main geomagnetic ®eld is approximated by that of an eccentric dipole. This expression is used to determine changes in geomagnetic cutos due to secular variation of the geomagnetic ®eld since 1835. Eects of westward drift of the quadrupole ®eld and decrease in the eective dipole moment are seen in the isorigidity contours. On account of the immense computer time required to determine the cuto rigidities more accurately using the particle trajectory tracing technique, the present formulation may be useful in estimating the transmission factor of the geomagnetic ®eld in cosmic ray studies, modulation of cosmogenic isotope production by geomagnetic secular variation, and the contribution of geomagnetic ®eld variation to long term changes in climate through cosmic ray related modulation of the current ̄ow in the global electric circuit.
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