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Cool heliosheath plasma and deceleration of the upstream solar wind at the termination shock

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.

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Figure 1: An overview of data near the termination shock.
Figure 2: High-resolution (192 s) solar wind speed ( V R ) near the termination shock crossings.
Figure 3: High-resolution data near the termination shock crossings (shaded regions).
Figure 4: Most of the solar wind flow energy does not go into the solar wind plasma.
Figure 5: The termination shock is very different from other shocks observed in the heliosphere.

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Acknowledgements

The work at MIT is supported by NASA. C.W. is grateful for support from NNSFC. Magnetic field data are shown courtesy of the Voyager magnetometer team (principle investigator N. Ness). We thank G. Gordon, Jr and L. Finck for development of and assistance with the plasma analysis.

Author Contributions J.D.R. analysed the plasma data and wrote the paper. J.C.K. performed the calculations for and write-up of the shock parameters. C.W. calculated the termination shock motion. J.W.B. and A.J.L. assisted with design of the instrument and manuscript preparation.

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Correspondence to John D. Richardson.

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Data from the Voyager 2 plasma experiment are available at http://web.mit.edu/space/www/voyager.html.

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Richardson, J., Kasper, J., Wang, C. et al. Cool heliosheath plasma and deceleration of the upstream solar wind at the termination shock. Nature 454, 63–66 (2008). https://doi.org/10.1038/nature07024

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