Influence of the Solar Wind and Geomagnetic Activity Parameters on Variations in the Cosmic Ray Cutoff Rigidity during Strong Magnetic Storms

Abstract

The correlation between the variations of geomagnetic cutoff rigidity ΔR and interplanetary parameters and the Dst index of geomagnetic activity during one moderate and six strong storms of solar cycles 23 and 24 has been calculated. The ΔR values have been obtained with two methods: (1) the spectrographic global survey method (SGS), in which the determination of the cutoff rigidity RSGS is based on observational data from the neutron monitor network, and (2) a method in which the particle trajectories are calculated numerically in a model magnetic field of the magnetosphere to determine the cutoff rigidity Reff. In general, the results obtained by the two methods are in close agreement. The Dst index of geomagnetic activity has the greatest effect on ΔR, and the correlation increases with storm intensity. The sensitivity of ΔR to interplanetary parameters vary greatly for different storms. The most geoeffective interplanetary parameter is the solar-wind speed V. A significant anticorrelation of ΔR and V can be traced for almost all storms. The correlation of ΔRSGS with the Bz component of the interplanetary magnetic field is observed only for two storms, on November 7–14, 2003, and November 7–8, 2004, for which the absolute Bz value was very high (≈−50 nT). At the same time, there is a rather high correlation of ΔReff with Bz for most storms. The azimuthal component of the interplanetary field By and the solar-wind dynamic pressure P show almost no connection with ΔR.