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Published July 2019 | Published
Book Section - Chapter Open

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


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.

Additional Information

© 2019 owned by the author(s) under the term of the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License. Pre-published on: 2019 July 22. This work was supported by NASA under grant 80NSSC18K0223. We thank the ACE/SWEPAM and MAG instrument teams and the ACE Science Center (http://www.srl.caltech.edu/ACE/ASC) for making their solar wind and magnetic field data publicly available. We acknowledge the NMDB database (www.nmdb.eu), founded under the European Union's FP7 program (contract 213007) for hosting and distributing the Newark neutron monitor data from the Bartol Research Institute, supported by the NSF under grants ANT-0739620 and ANT-0838839. We thank Todd Hoeksema and the Wilcox Solar Observatory (http://wso.stanford.edu) for providing the HCS and coronal field map data.

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Published - ICRC2019_1105.pdf


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