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Variations in the cutoff rigidities of cosmic rays (ΔR) 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 ΔR, i.e., the successive values that ΔR 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 ΔR 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. [ABSTRACT FROM AUTHOR]

We have studied the features of the latitudinal behavior of geomagnetic thresholds of cosmic rays R, as well as their sensitivity to the interplanetary medium and magnetospheric parameters during three phases of the magnetic storm on September 7–8, 2017, in the initial, main, and recovery phases. For this purpose, values of R were calculated in two different ways—by the method of spectrographic global survey (Rsgs) and by the method of tracing the trajectories of cosmic-ray (CR) particles in the model magnetic field (Ref). The maximum lowering of thresholds is observed at the storm maximum (Dst = –142 nT), reaching the values of ΔRsgs = –0.52 GV and ΔRef = –0.66 GV. The curve of ΔRsgs variations, depending on the observation station (latitude) cutoff rigidity, assumes a classical form with a maximum dropping the thresholds at midlatitudinal stations. ΔR correlates most strongly with the Dst index, which indicates that the ring current plays a main part in the dependence of variations of CR cutoff rigidities. The significant influence of solar-wind velocity V and interplanetary magnetic field (IMF) parameters on ΔRsgs and ΔRef is also seen. In the main phase, ΔRef depends on B and Bz of the IMF, and ΔRsgs depends on B and By. For ΔRsgs, the correlation with electromagnetic parameters varies, depending on the observation station, in a regular manner. There is no such tendency for ΔRef. [ABSTRACT FROM AUTHOR]

The ability of cosmic rays to penetrate the magnetosphere is characterized by the rigidity of the geomagnetic cutoff R, i.e., the stiffness below which the particle flux is cutoff due to magnetic screening. During a magnetic storm, the topology of the magnetic field changes; this causes variations in the cutoff rigidity ΔR. The dependence of ΔReff (which is calculated from the tracing of the trajectories of cosmic ray particles in a model magnetospheric field) on the parameters of the solar wind, the interplanetary magnetic field, and the magnetosphere during different phases of the strong magnetic storm on November 7–8, 2004 is considered. It is found that the trajectory of ΔReff, i.e., the successive ΔReff values with respect to the studied parameters, does not coincide with the trajectory in the recovery phase during the main phase: hysteresis loops form. Narrow hysteresis loops were obtained to relate ΔReff to the geomagnetic index Dst and the solar wind velocity, while wide loops were obtained to relate it to the electromagnetic parameters. The shape of the hysteresis loops are indicative of a dependence on the latitude of observation: as the latitude increases, the loop area also increases for all studied parameters. [ABSTRACT FROM AUTHOR]

The correlations between variations in the geomagnetic cutoff rigidity of cosmic rays and the Dst and Kp geomagnetic indices and solar-wind and IMF parameters are calculated for the three phases of the magnetic storm of November 20–21, 2003: before the storm and during its main and recovery phases. The correlations are the strongest between variations in the cutoff rigidity and the Dst index during all stages. A significant correlation was recorded with the By component of IMF and the field magnitude B; the correlation with By dominated during the main phase, and the correlation with B was dominant during the recovery phase. There is also a high correlation with the dynamic parameters of solar activity during the main phase, especially with the solar-wind speed. As far as we know, hysteresis phenomena have been discovered for the first time in the relationship between the cosmic-ray cutoff rigidities and the parameters of the helio- and magnetosphere on the scale of the magnetic storm (with Moscow station as an example). Loop-like patterns formed, because the trajectories of variations in the cutoff rigidities versus the studied parameters during storm intensification (development of current systems) did not coincide with the trajectories during the recovery phase (decay of current systems). The correlations of the cutoff rigidities with Dst and Kp indices were characterized by a narrow hysteresis loop, and their correlations with the IMF parameters were characterized by a wide hysteresis loop. The hysteresis loops for the relationship between the cutoff rigidities and solar-wind density and pressure were disordered. [ABSTRACT FROM AUTHOR]

The correlation of the variations in geomagnetic cutoff rigidity of cosmic rays ΔR with the interplanetary medium parameters and geomagnetic activity indices Dst and Kp was calculated for different phases of the superstorm on November 7–8, 2004. The strongest correlation was observed between the cutoff rigidity and Dst at all stages of storm development (correlation coefficients k ≈ 0.70–0.98); some stable influence of the solar wind (SW) density N (k ≈ 0.50–0.80) should also be noted. The dependence of the cutoff-rigidity variation on all of the dynamic parameters of the SW is observed in the main storm phase and is especially strong for the density N and pressure P. The correlation between ΔR and V is negative in all phases, while the correlation of ΔR with N and P is positive during the main phase and negative in the recovery phase. The cutoff-rigidity variations showed no sensitivity to the Bz component of the interplanetary magnetic field (IMF), the azimuthal component By, or the IMF absolute value B before the storm or in its main phase. A significant correlation or anticorrelation of ΔR with B and all the IMF components is observed only in the recovery period. The specific response of the geomagnetic cutoff rigidity to the heliospheric and magnetospheric parameters at different phases of the magnetic storm is apparently determined by the different relative contributions of the magnetospheric global current systems during these periods. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]