1. Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 37-655, Cambridge, Massachusetts 02139, USA
2. State Key Laboratory of Space Weather, Center for Space Science and Applied Research, Chinese Academy of SciencesPO Box 8701, Beijing 100080, China
3. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA

Correspondence to: John D. Richardson^1,2 Correspondence and requests for materials should be addressed to J.D.R. (Email: jdr@space.mit.edu).

Abstract

The solar wind blows outward from the Sun and forms a bubble of solar material in the interstellar medium. The termination shock occurs where the solar wind changes from being supersonic (with respect to the surrounding interstellar medium) to being subsonic. The shock was crossed by Voyager 1 at a heliocentric radius of 94 au (1 au is the Earth–Sun distance) in December 2004 (refs 1–3). The Voyager 2 plasma experiment observed a decrease in solar wind speed commencing on about 9 June 2007, which culminated in several crossings of the termination shock between 30 August and 1 September 2007 (refs 4–7). Since then, Voyager 2 has remained in the heliosheath, the region of shocked solar wind. Here we report observations of plasma at and near the termination shock and in the heliosheath. The heliosphere is asymmetric, pushed inward in the Voyager 2 direction relative to the Voyager 1 direction. The termination shock is a weak, quasi-perpendicular shock that heats the thermal plasma very little. An unexpected finding is that the flow is still supersonic with respect to the thermal ions downstream of the termination shock. Most of the solar wind energy is transferred to the pickup ions or other energetic particles both upstream of and at the termination shock.

1. Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 37-655, Cambridge, Massachusetts 02139, USA
2. State Key Laboratory of Space Weather, Center for Space Science and Applied Research, Chinese Academy of SciencesPO Box 8701, Beijing 100080, China
3. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA

Correspondence to: John D. Richardson^1,2 Correspondence and requests for materials should be addressed to J.D.R. (Email: jdr@space.mit.edu).

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