Latitudinal and Radial Gradients of Anomalous 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 radial and latitudinal gradients of anomalous cosmic-ray oxygen during the latter part of 1985. We find that in the energy interval from 7.1-30.6 MeV/nucleon the latitudinal gradient is −3 to −4%/deg. The sign of the latitudinal gradient is opposite to that reported for the last solar cycle when the solar magnetic field polarity was reversed, as predicted by propagation models in which curvature and gradient drifts are significant. The ratios of the radial and latitudinal gradients of anomalous oxygen, anomalous helium, and galactic cosmic rays are similar, also in agreement with drift theory predictions. Based on drift theory, these observed ratios, which are near unity, lead to estimates of κ/β in the range of 3-8 × 10^(22) cm²/sec at 24.9 AU for particles with rigidities of ∼2-3 GV.