Largest GLE in half a century: Neutron monitor observations of the January 20, 2005 event

Abstract

Within a 6-minute span on January 20, 2005, the count rate registered by a neutron monitor at the sea level station of McMurdo, Antarctica increased by a factor of 30, while the rate at the high-altitude (2820 m) site of South Pole increased by a factor of 56. The size of the increase at McMurdo qualifies it as the largest observed at sea level since the famous 1956 event, while the increase at South Pole may have been the largest (in percentage terms) ever registered by a neutron monitor. This paper uses data from the "Spaceship Earth" network of neutron monitors to characterize the time evolution of cosmic rays during the event. We also investigate spectral evolution using multiplicity data from a specially instrumented mobile monitor that was located in McMurdo Sound at the time of the event.