The effect of altitude differences and secular changes on the utilization of cosmic-ray variational coefficients

Abstract

The extended sets of variational coefficients have been examined to ascertain the effects of altitude and also for possible effects due to secular changes in the Earth magnetic field. A comparison of the variational coefficients calculated for two closely positioned cosmic-ray stations (Bagneres de Bigorre and Pic du Midi) indicates that the altitude effects are small. An examination of the variational coefficients calculated for western hemispheric locations, where the geomagnetic field has its most rapid secular change, indicates a maximum phase difference of one-half hour in the first harmonic of the daily variation. Significant changes of the phase of transient anisotropies can be computed only for unreallistic cases of extremely narrow «square wave» modulations. The results of this analysis indicate that the secular variation in the vertical cut-off rigidity (and associated changes in the asymptotic-direction calculations) are such that variational coefficients calculated by using either a 1955 or a 1975 model of the geomagnetic field are adequate for most cosmic-ray analyses.