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Cosmic ray measurements from Voyager 2 as it crossed into interstellar space


The interaction of the interstellar and solar winds is complex, as revealed by differences in intensities and anisotropies of low-energy ions (>0.5 MeV per nucleon) originating inside the heliosphere and those of higher-energy Galactic cosmic rays (>70 MeV per nucleon) originating outside, in the Milky Way. On 5 November 2018, Voyager 2 observed a sharp decrease in the intensity of low-energy ions and a simultaneous increase in the intensity of cosmic rays, indicating that Voyager 2 had crossed the heliopause at 119 au and entered interstellar space about six years after Voyager 1. Unlike Voyager 1, which found that two interstellar flux tubes had invaded the heliosheath and served as precursors to the heliopause, Voyager 2 found no similar precursors. However, just beyond the heliopause Voyager 2 discovered a boundary layer, in which low-energy particles streamed outward along the magnetic field and cosmic ray intensities were only 90% of those further out.

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Fig. 1: Counting rates of different energetic particle species on Voyager 1 and Voyager 2 around the times of their crossings of the heliopause.
Fig. 2: Counting rates and particle pitch angles around the time of the Voyager 2 heliopause crossing.
Fig. 3: Voyager 2 counting rates, streaming index, magnetic field strength and particle pitch angles for the period from day 180 of 2018 to day 20 of 2019.
Fig. 4: Voyager 2 counting rates and particle pitch angles around the time of the particle anisotropy event.
Fig. 5: Rate of ~0.5–35 MeV protons from Voyager 2 averaged over three telescopes (LETs A, C and D) during a spacecraft roll manoeuvre on day 256 of 2018.
Fig. 6: Intensities of O nuclei with 5.4–13.9 MeV per nucleon in the vicinity of the Voyager 1 and Voyager 2 heliopause crossings.
Fig. 7: Energy spectra of electrons (e+ + e), H nuclei and He nuclei observed by the CRS telescopes on Voyager 1 and Voyager 2.

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Data availability

Most of the CRS data can be obtained by clicking on the DATA link at and following other links to obtain rate and flux data. All data that were used in the figures can be provided by the corresponding author on request.


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We thank L. Burlaga for providing magnetic field data before publication. A.C.C. acknowledges support from the International Space Sciences Institute in Bern, Switzerland, to participate in the international team The Physics of the Very Local Interstellar Medium in the autumn of 2018. This work was supported by NASA under grant NNN12AA01C.

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All authors contributed to the production of this manuscript. A.C.C. and E.C.S. wrote the text. A.C.C., B.C.H. and N.L. performed the data analysis and A.C.C. and B.C.H. prepared the figures. All authors participated in reviewing and commenting on the paper and on the editor’s and referees’ comments.

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Correspondence to Edward C. Stone.

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Peer review information Nature Astronomy thanks Horst Fichtner and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

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Stone, E.C., Cummings, A.C., Heikkila, B.C. et al. Cosmic ray measurements from Voyager 2 as it crossed into interstellar space. Nat Astron 3, 1013–1018 (2019).

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