Observations of the Termination Shock and Heliosheath

Abstract

Voyagers 1 and 2 have both crossed the solar wind termination shock and are now in the heliosheath, the region of shocked solar wind plasma before the heliopause. This paper reviews observations of the termination foreshock, the termination shock (TS), and the heliosheath. The TS foreshock is characterized by anisotropic (streaming along the magnetic field) particles with energies of tens of keV to tens of MeV. The lower energy particles are accelerated at the TS. The particles are observed upstream of the TS when magnetic field lines connect the TS to the Voyager spacecraft. The TS is a weak shock, with a shock strength of about 2. The TS is the first observed particle mediated shock; the energetic particles cause the upstream solar wind to slow many days before the actual TS crossing. Three TS crossings were observed. Two of the TS crossings look like classic quasi-perpendicular supercritical shocks, but the third looked very different, with slowly varying N and V across the shock and two magnetic field ramps. These data have been interpreted as evidence that the TS is reforming upstream. Only about 20% of the flow energy at the TS was transferred to the thermal protons; the rest of this energy probably heats the pickup ions. The heliosheath is a highly variable region. Magnetic field and plasma parameters fluctuate by factors of up to five on scales of tens of minutes and the standard deviations of these parameters are large. The flows are away from the nose of the heliosphere, as expected, but the flow speeds differ at V1 and V2 and the speeds at both spacecraft differ from model predictions. Before the Voyager TS encounters, the TS was thought to be the source of the anomalous cosmic rays (ACRs), but at the Voyager crossing no evidence of the ACR source was observed, so these particles must be accelerated elsewhere on the TS or elsewhere in the heliosphere.