Cosmic ray latitudinal effects predicted by a three-dimensional drift model

Abstract

The Ulysses spacecraft measured surprisingly small latitudinal gradients during its first passage through the polar regions of the heliosphere. This is in contrast to predictions made with two-dimensional drift models for A > 0 magnetic polarity epochs during solar minimum modulation conditions. Several modifications to drift models have been investigated to improve the situation, e.g., modifying the heliospheric magnetic field and increasing turbulence in the polar regions in qualitative agreement with observations made by the Ulysses spacecraft. However, these modifications seem to be insufficient to explain the small latitudinal gradients. We now consider the effect of a heliospheric magnetic field that exhibits an asymmetry with respect to the neutral sheet. In addition, we allow enhanced perpendicular diffusion in the latitudinal direction. These particular aspects of modulation are studied with a fully three-dimensional drift model in an attempt to explain the Ulysses measurements.