Record neutron monitor counting rates from galactic cosmic rays

Abstract

Neutron monitors have recorded the flux of high-energy Galactic cosmic rays for more than half a century. During the recent, prolonged, deep minimum in solar activity, many sources indicate that modulated Galactic cosmic rays have attained new Space Age highs. However, reported neutron monitor rates are ambiguous; some record new highs while others do not. This work examines the record of 15 long-running neutron monitors to evaluate cosmic ray fluxes during the recent extraordinary solar minimum in a long-term context. We show that ground-level neutron rates did reach a historic high during the recent solar minimum, and we present a new analysis of the cosmic ray energy spectrum in the year 2009 versus year 1987. To do this, we define a reference as the average of eight high-latitude neutron monitors, four in the Northern Hemisphere (Apatity, Inuvik, Oulu, Thule) and four in the Southern Hemisphere (Kerguelen, McMurdo, Sanae, Terre Adelie). Most stations display changes in sensitivity, which we characterize by a simple linear trend. After correcting for the change in sensitivity, a consistent picture emerges. With our correction, all stations considered display new highs at the recent solar minimum, approximately 3% above the previous record high. These increases are shown to be consistent with spacecraft observations.