Modelling heliospheric current sheet drift in stochastic cosmic ray transport models

Abstract

Drifts are one of the major cosmic ray modulation mechanisms in the heliosphere. Three types of drifts occur in the background heliospheric magnetic field, namely curvature, gradient and current sheet drifts. The last compo- nent occurs because of the switch in magnetic field polarity across the heliospheric current sheet and is the main topic of study. We discuss and implement a new approach to model drifts in a numerical modulation model. The model employs stochastic differential equations to solve the relevant trans- port equation in five (three spatial, energy and time) dimen- sions. What is of interest is the fact that the model can han- dle current sheet tilt angles up to the theoretical maximum of α = 90° and still remain numerically stable. We use the additional insights gained from the numerical model to in- vestigate the effectiveness of drifts along the current sheet by examining the relationship between the current sheet path length and the cosmic ray propagation time. It is found that diffusion can disrupt the drift process very effectively, lead- ing to diffusive short circuiting of the current sheet by the cosmic rays.