Radial and Latitudinal Gradients of Anomalous Cosmic Rays in the Outer Heliosphere

Abstract

We use recent data from the Voyager 1 and 2 and Pioneer 10 spacecraft to determine the radial and latitudinal gradients of anomalous cosmic rays (ACRs) in the outer heliosphere. The latitudinal gradient is small and positive, as opposed to the large and negative values observed during the last solar minimum in 1987. The radial gradient is small and steady. We compare our results with current theories of the acceleration and propagation of ACRs. We also suggest that the large increase in the intensity of ACR helium that we observe during 1992 is due primarily to a change in · the ACR source flux as interplanetary conditions approach quasi-equilibrium. The continuing increases at lower energies in 1993 and 1994 are consistent with a decreasing level of modulation during a period of quasi-equilibrium.