[1] 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.
[1] The Forbush decrease (FD) event, a sudden decrease of galactic cosmic ray intensity measured by neutron monitor (NM), is known as a globally simultaneous phenomenon. However, some of them do not occur simultaneously in universal time. On the basis of the difference of main phase onset time distributions of two kinds of FD events, Oh et al. (2008) suggested that the global simultaneity of FD events depends on the solar wind physical parameters and propagation direction in three-dimensional heliosphere around the Earth. In order to get the hypothesis approved with a higher confidence level, the FD event data set is extended to 218 events with a longer period from 1971 to 2006 using Oulu, Inuvik, and Magadan station data. In addition, the probability of the same distribution of those two different classes of FD events is calculated in each NM station's view. All three NM stations confirm the probability of different distributions of FD main phase onset times of globally simultaneous and nonsimultaneous with a confidence level of 99%, compared with 94% of previous study. The statistics in this study may support the hypothesis that the simultaneous FD events occur when stronger magnetic barriers pass by the Earth through the central part of the magnetic barriers and in contrast that the nonsimultaneous FD events occur only if the less strong magnetic barriers pass the Earth on the dusk side of the magnetosphere.
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