Scientists observed that the old idea of the solar wind just being gradually whittled away as we go further into interstellar space is not true. They find with Voyager 2 - and previously with Voyager 1 - that there's a distinct boundary out there, and see how fluids, including plasmas, form boundaries.
Voyager 1 & Voyager 2 were launched within weeks of each other in 1977, carrying different mission goals and trajectories through space, yet they passed into ISM at the same distance from the Sun. The final data from Voyager 1 was obtained when the spacecraft was at 146 astronomical units or more than 21.7 billion km from the Sun.
The plasma wave instrument on Voyager 2 recorded that the density is increasing, in data feeds from a spacecraft that is so now far away that it takes more than 19 hours for the information to be delivered to Earth. The two Voyagers, now in their own orbits around the galaxy, will probably outlast Earth, unless they run into anything, which is considered unlikely.
Image credit: NASA/JPL
Article Source: https://watchers.news/2019/11/05/voyager-2-reaches-interstellar-space-becomes-second-to-leave-the-heliosphere/
Reference: "Voyager 2 enters interstellar space" - Du Toit Strauss, R. et al - Nature Astronomy - DOI: 10.1038/s41550-019-0942-5