The heliopause is a boundary that separates the heliosheath (which contains magnetic fields and plasmas that originate in the Sun) from the interstellar medium (which contains magnetic fields and particles of stellar/interstellar origin). Observations of the heliopause were first made by the particles and fields instruments on the Voyager 1 spacecraft, moving radially in the northern hemisphere, which crossed the heliopause on 25 August 2012 at a distance of 121.6 au. We show using observations of the magnetic field and energetic particles that Voyager 2 crossed the heliopause in the southern hemisphere on 5 November 2018 at a distance of ≈119.0 au. Voyager 2 observed a much thinner and simpler heliopause than Voyager 1 as well as stronger interstellar magnetic fields, and it discovered a ‘magnetic barrier’ in the heliosheath adjacent to the heliopause that strongly influences the entry of cosmic rays into the heliosphere. The magnetic field direction observed by Voyager 2 changed smoothly from the time of arrival at the magnetic barrier, through it, and onwards into the interstellar medium, with a small (a few degrees) or no change across the heliopause. These observations, together with the Voyager 1 observations and existing models, show that the magnetic barrier, the heliopause and the neighbouring very local interstellar medium form a complex interconnected dynamical system.
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L.F.B., N.F.N., D.B.B., J.P., L.K.J. and A.S. were supported by the NASA Voyager Project to the NASA/GSFC Magnetometer Team under internal NASA funding. L.K.J. is grateful for support from The International Space Science Institute in Bern in the framework of the team ‘The Physics of the Very Local Interstellar Medium’. L.F.B. thanks N. Pogorelov and H. Washimi for helpful discussions, and N. Pogorelov for preparing Fig. 5. J.D.R. was supported by NASA under grant number 959203 from the Jet Propulsion Laboratory to the Massachusetts Institute.
The authors declare no competing interests.
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Burlaga, L.F., Ness, N.F., Berdichevsky, D.B. et al. Magnetic field and particle measurements made by Voyager 2 at and near the heliopause. Nat Astron 3, 1007–1012 (2019). https://doi.org/10.1038/s41550-019-0920-y
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