Large-Scale Radial Gradient of Anomalous Cosmic-Ray Oxygen from 1 to ~30 AU

Abstract

The cosmic~ray instruments on IMP 8 and Voyagers 1 and 2 have been used to measure the energy spectrum and large-scale radial gradient of anomalous cosmic-ray oxygen with -5-30 MeV/nuc for the period 1986/206-310. We find that the flux of anomalous oxygen at 1 AU is continuing to recover, with an average intensity that is a factor of -4 lower than the 1974-78 solar minimum spectrum. The data from the three spacecraft are consistent with a constant radial gradient at -10 Me V /nuc of -15%/ AU from 1 to 28 AU, similar to that for the previous solar minimum period, but a factor of -4 higher than that for the 21- 39 AU region. These results suggest that the radial gradient decreases with radial distance or that there is a longitudinal gradient of -1%/AU. Under either assumption we find negative latitudinal gradients which, at the position of Voyager 1 (-28 AU), are comparable in magnitude to the radial gradients.