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Voyager 1 exited the solar wind at a distance of 85 au from the Sun

Abstract

The outer limit of the Solar System is often considered to be at the distance from the Sun where the solar wind changes from supersonic to subsonic flow1. Theory predicts that a termination shock marks this boundary, with locations ranging2 from a few to over 100 au (1 au ≈ 1.5 × 108 km, the distance from Earth to the Sun). ‘Pick-up ions’ that originate3,4 as interstellar neutral atoms should be accelerated to tens of MeV at the termination shock, generating anomalous cosmic rays5,6,7. Here we report a large increase in the intensity of energetic particles in the outer heliosphere, as measured by an instrument on the Voyager 1 spacecraft. We argue that the spacecraft exited the supersonic solar wind and passed into the subsonic region (possibly beyond the termination shock) on about 1 August 2002 at a distance of 85 au (heliolatitude 34° N), then re-entered the supersonic solar wind about 200 days later at 87 au from the Sun. We show that the composition of the ions accelerated at the putative termination shock is that of anomalous cosmic rays and of interstellar pick-up ions.

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Figure 1: Intensity profiles at Voyager 1, Voyager 2 since late 2001.
Figure 2: Data from Voyager 1 LECP for 0.57–1.78 MeV protons.
Figure 3: Anisotropy plots of time-averaged sector rates for periods A, B and C noted in Fig. 1 and solar wind velocity fits thereof.
Figure 4: Composition spectra for the duration of Voyager 1's excursion into the heliosheath from LECP 2002/194 to 2003/044.

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Acknowledgements

This work was supported by a NASA grant to the Johns Hopkins University and by subcontracts at the University of Maryland and Fundamental Technologies.

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Correspondence to S. M. Krimigis.

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Krimigis, S., Decker, R., Hill, M. et al. Voyager 1 exited the solar wind at a distance of 85 au from the Sun. Nature 426, 45–48 (2003). https://doi.org/10.1038/nature02068

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