The modulation of galactic cosmic-ray electrons in the heliosheath

Abstract

Voyager 1 has observed strong increases in the intensities of 2-160 MeV electrons since crossing the termination shock of the heliosphere in 2004 December. Before this time these intensities were submerged below the detector background, except for occasional transient events. These increases are large compared to the concurrent increases of positive ions such as H, He, and O. A significant part is probably due to temporal effects as the heliosphere was recovering to solar minimum conditions from 2005 to early 2010. The intensity observed by Voyager 2 since its crossing of the shock in 2007 September is 5-10 times lower than that observed by Voyager 1, which is so low that the electron intensity may still be below the background produced by high-energy protons in the detector. This points to a large north-south asymmetry in the properties of the heliosheath. It is shown that the observations suggest that these electrons are not freshly accelerated on the termination shock, but rather that they are of galactic origin—while they may be re-accelerated by that shock. In this paper, these intensities are modeled with numerical solutions of the cosmic-ray transport equation. It is shown that because they are relativistic, the electrons are much more sensitive to the form of the diffusion coefficient at low rigidities than ions, and that this can explain the asymmetry