The history of cosmic rays in a dynamical halo - A retrodictive probability approach

Abstract

We examine a particular model of cosmic-ray propagation in the Galaxy that contains convective outflow in the halo. When this convection is important, the solutions for the cosmic-ray density are quite different from those obtained by considering diffusion alone. We derive an equation for propagating, backward in time, the particle population that is the disk at the present time and find that, unlike the bulk of the cosmic rays, these particles have never been very far from the disk. We may therefore define an effective halo thickness D* which is smaller than the actual halo thickness D and conclude that particles in the disk have little to do with those that venture farther into the halo than D*. This fact can, perhaps, reconcile radio evidence for an extensive halo with the evidence from ..gamma..-ray data that the distribution of cosmic rays in the disk is strongly source-dominated.