Magnetic field and particle measurements made by Voyager 2 at and near the heliopause

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. This paper reports measurements of the magnetic fields and energetic particles detected by the Voyager 2 spacecraft as it passed from the heliosphere, through the heliosheath and heliopause, to the interstellar medium. As predicted by models, Voyager 2 encountered a ‘magnetic barrier’ before reaching the heliopause.