Geomagnetic cutoff rigidities are essential for the analysis of Space weather events and in particular Solar Particles Events (Ground Level Enhancements), Forbush Decrease and cosmic rays anisotropies observations by ground based cosmic rays detectors (e.g. neutron monitors, muon telescopes, etc.). The main parameters affecting the observed quantities are the energy spectra of the sources (primary galactic and solar cosmic rays) and the magnitude of the magnetic fields along the propagation paths. Magnetic storms significantly influence the latter and alter the discrimination threshold of the stationary ground based observatories.

In this work we are presenting the analysis of the dependence of the geomagnetic cuttoff rigidity as a function of the magnetospheric magnetic field variations at a given epoch of the geomagnetic field expressed in terms of disturbance storm index (Dst), planetary and local K indices. The analysis is then applied to a particular geomagnetic storm and the analytic results are bench-marked to the measured neutron monitor count rates. This is done in the absence of solar particles in order to isolate the effect of the storm on the galactic cosmic rays interactions with the magnetospheric magnetic field. The so obtained results can be used to obtain the ground based CR intensities change due to the primary component and not to magnetic fields effects.

This work is part of the real-time galactic cosmic rays energy spectrum analysis of the Royal Meteorological Institute of Belgium space weather ground level alert service.