Changing anomalous cosmic ray oxygen radial intensity gradients between 1 AU and Voyager with the return to solar minimum
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.
Additional InformationCopyright Universidad Nacional Autónoma de México This work was supported by NASA (grants NAG5-12929, NAS5-03131, and contract NAS7-03001). COSPIN/LET data were obtained courtesy of the Ulysses Data System, and we acknowledge the National Science Foundation grant ATM-0339527, which supports the Climax neutron monitor.
Published - 2008-12.pdf