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An asymmetric solar wind termination shock

Abstract

Voyager 2 crossed the solar wind termination shock at 83.7 au in the southern hemisphere, 10 au closer to the Sun than found by Voyager 1 in the north1,2,3,4. This asymmetry could indicate an asymmetric pressure from an interstellar magnetic field5,6, from transient-induced shock motion7, or from the solar wind dynamic pressure. Here we report that the intensity of 4–5 MeV protons accelerated by the shock near Voyager 2 was three times that observed concurrently by Voyager 1, indicating differences in the shock at the two locations. (Companion papers report on the plasma8, magnetic field9, plasma-wave10 and lower energy particle11 observations at the shock.) Voyager 2 did not find the source of anomalous cosmic rays at the shock, suggesting that the source is elsewhere on the shock12,13,14 or in the heliosheath15,16,17,18,19. The small intensity gradient of Galactic cosmic ray helium indicates that either the gradient is further out in the heliosheath20 or the local interstellar Galactic cosmic ray intensity is lower than expected21.

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Figure 1: Daily-averaged intensities and streaming of energetic termination shock particles that are accelerated at nearby regions of the shock.
Figure 2: Comparison of the energy spectra of protons and helium nuclei in the heliosheath near the times of the Voyager 1 and Voyager 2 shock crossings.
Figure 3: Temporal changes in the intensities of helium nuclei and electrons.

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Acknowledgements

This work was supported by NASA (NAS7-03001).

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Correspondence to Edward C. Stone.

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Stone, E., Cummings, A., McDonald, F. et al. An asymmetric solar wind termination shock. Nature 454, 71–74 (2008). https://doi.org/10.1038/nature07022

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