On the Phase of the 27 Day Modulation of Anomalous and Galactic Cosmic Rays at 1 AU during Solar Minimum

Abstract

We report on the 27 day oscillations in the intensities of He and O ions of the anomalous cosmic rays (ACRs) observed near Earth by the Wind spacecraft during the 1995-1997 solar minimum period. Oscillations persist throughout the period with amplitudes as large as 50%. Galactic cosmic ray (GCR) oscillations, observed by neutron monitors, are in phase with the ACRs, but with much smaller amplitude (2%-3%). For a sustained period of almost a year, peak intensities occur near north-to-south crossings of the heliospheric current sheet and valleys near south-to-north crossings. In GCR observations in the 1974-1976 solar minimum, 27 day oscillations with a similar phase are seen. Understanding these observations poses a severe challenge for models of solar modulation as well as for models of the solar and heliospheric magnetic fields.