Galactic Cosmic-ray Transport in the Global Heliosphere: A Four-Dimensional Stochastic Model

We study galactic cosmic-ray transport in the outer heliosphere and heliosheath using a newly developed transport model based on stochastic integration of the phase-space trajectories of Parker's equation. The model employ's backward integration of the diffusion-convection transport equation rising Ito calculus and is four-dimensional in space+momentum. We apply the model to the problem of galactic proton transport in the heliosphere during a negative solar minimum. Model results are compared with the Voyager measurements of galactic proton radial gradients and spectra in the heliosheath. We show that the heliosheath is not; as efficient in diverting cosmic rays during solar minima as predicted by earlier two-dimensional models.