Rigidity Dependence of Near-Earth Latitudinal Proton Density Gradients

Abstract

The rigidity dependence of the latitudinal gradient of cosmic-ray protons observed by Ulysses poses a challenge to modulation models: it increases as function of rigidity up to about 1.4 GV and then decreases. Although models could reproduce the observed small positive gradient for an A > 0 solar polarity cycle, its maximum was at a rigidity well below 1 GV. After exploring various options, it turns out that changing the rigidity dependence of the perpendicular diffusion coefficient (DC) in the polar direction so that it differs from that of the parallel DC, is the most effective way to obtain good agreement with data. Specifically, we find that this DC must have a flatter rigidity dependence than the parallel DC in order to reproduce the observed rigidity dependence of the latitudinal gradient of protons during an A > 0 solar polarity cycle.