27-Day Modulation of Cosmic Ray Intensities During the Last Two Solar Minima

Abstract

Variations in both anomalous cosmic ray (ACR) and galactic cosmic ray (GCR) intensities at the solar rotation period (∼27 days) are often observed, and appear throughout much of 2016 into 2019 during the present A>0 polarity solar minimum. During the previous A<0 solar minimum, such variations were evident at energies from a few MeV to many GeV from mid-2007 through late 2008, and were much larger for ACRs than for GCRs at comparable energy. The particle intensities had an inverse power-law correlation with the solar wind speed, intensity peaks occurred only near alternate crossings of the heliospheric current sheet, and the variations in amplitudes were larger than typical for an A<0 epoch. The heliospheric environment was unusual compared with earlier cycles, with lower solar wind dynamic pressure, weaker interplanetary magnetic field strength, and record-setting GCR intensities. The present high-energy GCR intensities are higher than ever before reported in an A>0 cycle (although lower than in several A<0 cycles), indicating that unusual modulation conditions still prevail. Using high statistical precision measurements from the ACE spacecraft along with neutron monitor data, we present observations of the 27-day intensity variations in both ACRs and GCRs during the last two solar minima and describe how they depend on particle energy, rigidity, and spectrum, and on interplanetary conditions such as solar wind speed, magnetic field strength, andmcurrent sheet tilt. We compare the variations observed in this cycle with those in the previous opposite polarity cycle and discuss implications for particle transport in the heliosphere.