Aspects of the time-dependent modulation of galactic cosmic rays throughout a three-dimensional heliosphere

Abstract

A dominant feature in the heliosphere is the heliospheric current sheet, separating regions of opposite polarity of the heliospheric magnetic field. This structure is dependent on solar activity through the tilt angle, which increases with increasing solar activity. This alters the waviness of the current sheet, and thus the region swept out by this structure. One of the four major cosmic-ray modulation mechanisms in the heliosphere are drifts due to the curvature of, and gradients in, the heliospheric magnetic field, as well as current sheet drifts. The effect of these particle drifts is known to be reduced by turbulent magnetic field. This study aims to provide some insight into particle drifts along the wavy current sheet, in particular how such effects are modelled in numerical galactic cosmic ray modulation codes, by using an ab initio approach to this problem that models diffusion and turbulence-reduced drift coefficients from first principles, so that they now depend on basic turbulence quantities. This is done using a numerical cosmic-ray modulation model that employs a set of stochastic differential equations to solve the Parker transport equation. Different methods by which current sheet drift effects are usually modelled in cosmic-ray modulation codes are investigated, and are shown to have a significant effect on galactic cosmic-ray modulation. At high levels of solar activity, it is demonstrated that if the drift model takes into account the simultaneous decrease in cosmic-ray Larmor radius with increasing solar activity parameters, it computes intensities in good qualitative agreement with observations. Lastly, a novel approach to modelling drift effects due to the heliospheric current sheet is proposed, and compared with a previous approach to this problem. The novel approach is shown to lead to an improvement in the qualitative agreement of computed cosmic-ray intensities with spacecraft observations relative to previously used approaches to modelling this phenomenon.