Changing anomalous cosmic ray oxygen radial intensity gradients between 1 AU and Voyager with the return to solar minimum

Abstract

Using the Solar Isotope Spectrometer (SIS) on NASA’s ACE spacecraft, we have measured the intensities of anomalous cosmic rays (ACRs) near 1 AU down to energies of 10 MeV/nucleon since August 1997. As solar minimum modulation conditions return, ACR intensities at 1 AU are recovering, but are significantly lower relative to galactic cosmic rays (GCRs) than they were during the intensity decline following the last solar minimum and are still a factor of 4 lower than in 1997. The large-scale radial intensity gradients of ACR oxygen obtained using observations from ACE, Voyagers 1 and 2, and Ulysses are much larger during the present A 0 recovery than they were during the last A 0 solar minimum; in fact beyond 50 AU ACR intensities already exceed those at the last A 0 solar minimum. The 1 AU ACR measurements are now being augmented by data from the Low Energy Telescope (LET) on the twin STEREO spacecraft, which will allow us to extend the energy spectra down to 3 MeV/nucleon to track the behavior of the peak of the ACR oxygen spectrum and study longitudinal intensity variations.