Anisotropy of Anomalous Cosmic Rays Observed by Voyager 2 in the Heliosheath

Abstract

Voyager 2 (V2) has been observing anomalous cosmic rays (ACRs) in the heliosheath since August 2007 when V2 crossed the termination shock of the supersonic solar wind. We use the counting rate of ~0.5-35 MeV protons collected during periods when the spacecraft was rolling about the axis pointed to the Earth to infer their direction of flow. The observed flow velocity is the combination of the flow due to motion of heliosheath plasma and a diffusive flow due to a gradient in the ACR intensity. The latitudinal component of the flow (N component) agrees with the convective flow due to the heliospheric plasma flow as determined by the plasma instrument on V2. However, the tangential component of the flow (T component) is smaller than the predicted convective flow, consistent with an intensity gradient in the +T direction and a diffusive flow of ACRs from a source located in the +T direction. This would be consistent with models predicting that the acceleration of higher energy ACRs occurs along the flanks or tail of the heliosphere. A similar result was obtained by analysis of the V1 magrol data during V1's journey through the heliosheath [1].