Cosmic Rays during Forbush Effects in March 1989 and March 1991: Spectra of Variation, Anisotropy, and Variations of Geomagnetic Cutoff Rigidity

Abstract

Two Forbush effects that occurred during geomagnetic storms in March 1989 and March 1991 are analyzed based on ground-based measurements of cosmic rays at a global network of stations via spectrographic global survey. The rigidity spectra and variation spectra are presented, as well as the pitch-angle anisotropy of cosmic rays at different phases of the development of Forbush effects and variations in the planetary system of geomagnetic cutoff rigidities. It is shown that, when the variation spectra are approximated by a power-law function of particle rigidity in the range of 10–50 GV, the spectrum index in the phase of maximum modulation is softer than in the decline and recovery phases of cosmic-ray intensity. The distance to the subsolar point and the radius of the ring current, as well as the contribution of the ring current to variations in the geomagnetic cutoff rigidity and the Dst index, are determined within an axisymmetric model of the Earth’s limited magnetosphere that takes into account the magnetopause currents and ring current.