Hysteresis Phenomena in the Relationship between the Cosmic Ray Cutoff Rigidity and Parameters of the Magnetosphere during a Storm on May 15, 2005

Variations in the cutoff rigidities of cosmic rays (Δ R ) depending on the parameters of the solar wind, the interplanetary magnetic field (IMF), and geomagnetic activity were studied at different phases of the strong magnetic storm on May 15, 2005. We found that the trajectories Δ R , i.e., the successive values that Δ R takes depending on the parameters under study during the main phase of the storm did not coincide with the trajectories in the recovery phase, which led to the formation of hysteresis loops. The clearest hysteresis loops are formed for the relationship between Δ R and the Bz component of the IMF, the density and pressure of the solar wind, and the Dst index of geomagnetic activity. The area of the hysteresis loops increases with the latitude of the cosmic ray observation station.