Proceedings of the National Academy of Sciences: https://www.pnas.org/content/118/17/e2106371118

"The Sun helped confirm Voyager’s feat. Solar storms had erupted earlier in 2012, and the next year they shocked the plasma that Voyager 1 was speeding through, causing electrons there to oscillate and give off radio waves that the spacecraft detected. The frequency of those radio waves indicated that Voyager had indeed entered a much denser domain (2).
Voyager 1 thus became the first spacecraft ever to reach the interstellar medium. Contrary to some media reports, the craft had not left the solar system. Roughly a trillion icy bodies revolve around the Sun far beyond the orbits of Neptune and Pluto; every now and then one of them plunges toward the Sun and we see a new comet in the sky. The farthest of these distant icy objects are probably 1 to 2 light-years, or 63,000 to 126,000 astronomical units, away. Someone in the center of the continental United States who walks three miles west has gotten closer to the Pacific Ocean, relatively speaking, than Voyager has to the solar system’s edge.
On November 5, 2018, Voyager 2 also crossed the heliopause."