Authors: R. A. Leske(1), A. C. Cummings(1), R. A. Mewaldt(1), E. C. Stone(1), C. M. S. Cohen(1), M. E. Wiedenbeck(2) (1)California Institute of Technology, Pasadena, CA USA (2)Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA USA

Anomalous cosmic rays (ACRs) originate as pickup ions that are accelerated to MeV/nucleon energies at or near the heliospheric termination shock. In their journey to 1 AU, both ACRs and galactic cosmic rays (GCRs) probe energetic particle transport conditions throughout the heliosphere. Modulated ACR and GCR intensities at 1 AU track each other through time, but with some differences between opposite polarity solar cycles due to drift effects. Comparing intensities in the same polarity minima, measurements from the Solar Isotope Spectrometer (SIS) on the Advanced Composition Explorer (ACE) show that peak ACR oxygen intensities above ~8 MeV/nucleon in the recent A>0 solar minimum were slightly below the levels seen during the last A>0 minimum in 1997, while GCR iron intensities at ~300 MeV/nucleon from the Cosmic Ray Isotope Spectrometer (CRIS) exceeded those in 1997 by ~30%. Similarly, in the 2009 A<0 minimum, peak ACR intensities were similar to those in the 1987 A<0 cycle, but GCR intensities reached record-setting levels. The GCR observations indicate that there must have been a decrease in heliospheric modulation during recent solar minima, but apparently ACR intensities have been reduced relative to GCRs.

ACR intensities depend not only on solar modulation, but also on their source strength, which might vary with changes in the heliosphere. Contributing factors may include a reduction in the acceleration efficiency of ACRs due to a decrease in the strength or turbulence levels of the interplanetary magnetic field, or a drop in the production of pickup ions (ACR seed particles) due to less ionization in this epoch of weaker solar activity.

We present more than 24 years of ACR and GCR intensities measured by ACE throughout two complete solar cycles and discuss possible reasons for the differences in the behavior of ACRs relative to GCRs in the recent solar minima.