Changes in cosmic ray cut-off rigidities due to secular variations of the geomagnetic field

A Bhattacharyya,B Mitra

doi:10.1007/s005850050489

Abstract

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

Highlights

The StoÈrmer formula (1955) for cosmic ray cutorigidity, which assumed the geomagnetic ®eld to arise due to a dipole located at the centre of the Earth, has been widely used in many studies
®gure is to be compared with Fig. 3d, which shows contours of vertical cosmic ray cuto rigidity, calculated as described in the ®eld of the best-®t eccentric dipole obtained from IGRF1985
It has long been recognised that secular variation of the geomagnetic ®eld contributes to signi®cant changes in the production rate of radiocarbon and other cosmogenic isotopes (Damon et al, 1978)

Summary

Introduction

The StoÈrmer formula (1955) for cosmic ray cutorigidity, which assumed the geomagnetic ®eld to arise due to a dipole located at the centre of the Earth, has been widely used in many studies. This shortcoming of the eccentric dipole approximation for the geomagnetic ®eld, as far as rigidity calculations are concerned, has been known for a long time, the only justi®cation for employing this approximation in the present study is that in this case an exact analytical expression could be derived for the cuto rigidity for arbitrary direction of incidence of the cosmic ray particles. This formulation can be used to study the changes in isorigidity contours of cosmic ray cuto rigidities due to variation in the location and strength of the eccentric dipole in the recent past.

Results

Conclusion