Time Variation of Radial and Latitudinal Gradients of Anomalous Cosmic Ray Oxygen in the Outer Heliosphere

Abstract

We have used data from the Voyager I (VI), Voyager 2 (V2), and Pioneer IO (PIO) spacecraft to monitor the radial a.nd latitudinal gradients of anomalous cosmic ray oxygen in the energy range 7.1 - 17.l MeV /nuc from 1984 to 1989. We find that the latitudinal gradient varies from near zero value in 1984-85 to -5.5%/deg in mid-19871 returning to near zero value by the end of 1988. The magnitude of the latitudinal gradient is inversely correlated with the radial gradient. The variations in the radial and latitudinal gradients are correlated with the tilt of the neutral sheet for tilts ≾30 °, as expected from a sim pie model of particle propagation which includes curvature and gradient drifts in the large-scale heliospheric magnetic field.