Effective Rigidity of a Polar Neutron Monitor for Recording Ground-Level Enhancements

The “effective” rigidity of a neutron monitor for a ground-level enhancement (GLE) event is defined so that the event-integrated fluence of solar energetic protons with rigidity above it is directly proportional to the integral intensity of the GLE as recorded by a polar neutron monitor, within a wide range of solar energetic-proton spectra. This provides a direct way to assess the integral fluence of a GLE event based solely on neutron-monitor data. The effective rigidity/energy was found to be 1.13?–?1.42 GV (550?–?800 MeV). A small model-dependent, systematic uncertainty in the value of the effective rigidity is caused by uncertainties in the low-energy range of the neutron-monitor yield function, which requires more detailed computations of the latter.