Heliospheric modulation of cosmic ray protons: Role of enhanced perpendicular diffusion during periods of minimum solar modulation

Abstract

The effect of enhanced perpendicular diffusion in the polar regions of a simulated heliosphere seems required in drift models for the modulation of galactic cosmic rays in order to find a latitude dependence of cosmic ray protons in the inner heliosphere that is compatible with Ulysses observations. This effect was studied in detail to establish how important it is and to understand the role it plays in global cosmic ray modulation, particularly during minimum modulation periods. After searching through a large parameter space, it was found that enhanced perpendicular diffusion was indeed necessary to obtain compatibility with observations in the inner heliosphere, especially during A > O magnetic field polarity epochs (e.g., from 1990 to ∼2000). This approach, however, also increased the radial dependence during this epoch. Switching to A < O epochs (e.g., from ∼1981 to 1989), the same set of parameters did not fit the observed 1987 radial dependence for 200 MeV cosmic ray protons, so a completely different set of diffusion coefficients had to be used, including less enhanced perpendicular diffusion. Apart from a polarity difference, minimum modulation conditions and, by implication, the diffusion tensor seem to be different during A > O and A < O solar minimum modulation periods.