Relationship between geomagnetic thresholds of galactic cosmic rays and heliospheric parameters during the May 10–12, 2024 storm

Galactic cosmic rays (CRs) are charged energetic particle fluxes that penetrate interplanetary space. Under the influence of the geomagnetic field, charged cosmic ray particles change their trajectories. Some of these particles, with rigidity below the threshold value characteristic of the measurement site (called the geomagnetic cutoff rigidity, or geomagnetic threshold, R), do not reach the Earth surface. In this study, geomagnetic thresholds R as well as their changes relative to quiet conditions (ΔR) for six stations at different latitudes have been calculated using two methods: 1) a method in which cosmic ray particle trajectories are calculated numerically in a model magnetic field of the magnetosphere (Reff); 2) a spectrographic global survey based on observational data from cosmic ray recordings by a global network of stations (Rsgs) during a very strong magnetic storm on May 10–12, 2024. The obtained results were compared. It was found that the changes in cosmic ray geomagnetic thresholds for this storm, calculated by both methods, correlate well with each other. The most geoeffective components are the vertical component of the interplanetary magnetic field Bz and its full scalar value B, whereas the By component of the field has virtually no effect on changes in threshold rigidities for all stations. ΔR also correlates well with the geomagnetic indices Dst and Kp, with the electric field parameter Ey, and with the solar wind's dynamic parameters (density and pressure), while no significant correlation is observed with solar wind speed. Analysis has shown that the response of ΔR to the controlling electromagnetic parameters and geomagnetic activity indices remains virtually unchanged with the latitude of the observation station for the given storm.