The Unusual Behavior of Anomalous and Galactic Cosmic Ray Intensities at 1 AU During the Present Solar Minimum

Abstract

Since the early 1970's, anomalous cosmic ray (ACR) intensities at 1 AU at solar minimum have generally tracked the galactic cosmic ray (GCR) intensities as measured by neutron monitors. Throughout the current A<0 cycle, however, the ACR intensities are a factor of 3-4 lower than expected from scaling neutron monitor rates; a similar discrepancy seems to have been present during the last A<0 period in the mid-1980's. Also, although there have been no major solar particle events for over 2 years, and sunspot numbers have been at minimum levels for at least a year, the ACR intensities are at present a factor of ~2 lower than their maximum values during each of the last 3 solar minima, suggesting that heliospheric conditions are not yet at minimum modulation levels. This is probably associated with the fact that ACRs drift inward along the heliospheric current sheet (HCS) during A<0 cycles, and the tilt of the current sheet is still relatively high (~23�) for solar minimum. However, while ACR intensities are low, GCR intensities are at a record high, and compared with the last A<0 cycle, we find that both ACR and GCR intensities are actually much higher now for a given HCS tilt angle than they were in the mid-1980's.