Variations in cosmic ray cutoff rigidities during the great geomagnetic storm of November 2004

Very strong interplanetary and magnetospheric disturbance observed on 7-13 November 2004 can be regarded as one of the strongest events during the entire period of space observations. In this paper, we report on the studies of cosmic ray cutoff rigidity variations during 7-13 November 2004 showing how storm conditions can affect the direct cosmic ray access to the inner magnetosphere. Effective cutoff rigidities have been calculated for selected points on the ground by tracing trajectories of cosmic ray particles through the magnetospheric magnetic field of the “storm-oriented” Tsyganenko 2003 model [Tsyganenko, 2002a, b; Tsyganenko et al., 2003]. Cutoff rigidity variations have also been determined by the spectrographic global survey method on the basis of experimental data of the neutron monitor network. Correlations between the calculated and experimental cutoff rigidities, as well as between geomagnetic D st index and interplanetary parameters have been investigated. Correlation coefficients between the cutoff rigidities obtained by the trajectory tracing method and the spectrographic global survey method have been found to be in the limits of 0.76-0.89 for all stations except the low-latitude station Tokyo (0.35). The most pronounced correlation has been revealed between the cutoff rigidities that exhibited a very large variation of ~1-1.5 GV during the magnetic storm, and the D st index.