Radial and Latitudinal Gradients of Anomalous Cosmic-Ray Oxygen and Helium and Galactic Cosmic Rays in the Outer Heliosphere

Abstract

We have used measurements from instruments on Voyagers 1 and 2 and Pioneer 10 to derive simultaneous latitudinal and radial gradients of anomalous cosmic-ray helium and oxygen and galactic cosmic rays during the latter part of 1986. We find that the negative latitudinal gradients that first appeared in 1985 when Voyager 1 reached heliolatitudes in excess of the current sheet continue to be observed, with an increased magnitude probably related to the decreasing tilt of the current sheet. The sign of this gradient is opposite to that reported during the last solar cycle when the solar magnetic field polarity was reversed, as predicted by propagation models in which curvature and gradient drifts are important. Although during the 1985-1986 time period radial gradients in the outer heliosphere appeared to decrease, the ratios of the radial and latitudinal gradients remained similar for both anomalous and galactic particles as predicted by drift theory. These observations indicate that the particles move inward preferentially at low latitudes from the heliospheric boundary during the current phase of the solar cycle.