A Comparison of Heliospheric Magnetic Fields in the Ab Initio Approach to Cosmic-ray Modulation

In the ab initio approach to cosmic-ray modulation much attention has been given to examination of models for turbulence and for diffusion. In this paper we shift the emphasis somewhat to consider the effect of a heliospheric magnetic field (HMF) other than the standard Parker field. The Parker field scales as one over heliocentric radial distance in the equatorial regions of the heliosphere, but as one over heliocentric radial distance squared in the polar regions, thus making the magnitude of the field much smaller in polar regions than in equatorial regions. Modification to the Parker field, as well as more recent three-dimensional models of the HMF, typically has an azimuthal component that also scales as one over heliocentric radial distance in the polar regions of the heliosphere, thus increasing the magnitude of the field compared to the Parker model. An increase in the magnitude of the magnetic field increases the QLT parallel mean free path at low energies, but leaves it unchanged at high energies, while the perpendicular mean free path predicted by non-linear guiding center theory increases at all energies. The drift coefficient decreases with increasing magnetic field magnitude. We discuss the effect of this disparate behavior of the transport coefficients on cosmic ray modulation and illustrate it with our ab initio modulation model.