Hey, who knew. MIT has an instrument on the Voyager 2 spacecraft! Evidently, the Plasma Science instrument is still going strong. The one on Voyager 1 died before it reached the termination shockwave, but Voyager 2 just crossed it recently, and the instrument is working pretty well.
But with Voyager 2, the Plasma Science instrument not only detected the boundary, making detailed measurements of the solar wind's temperature, speed and density as the spacecraft crossed through it, but it actually encountered the shockwave repeatedly. Because the outflow of the solar wind varies with changes in the sun's activity level, building up during large solar flares and quieting during lulls in sunspot activity, the boundary itself pulsates in and out. These pulsations can wash across the craft multiple times, just as a boat landing onshore may cross the ocean's edge multiple times as waves crash in and then recede.
While Voyager 1 apparently made a single crossing, Voyager 2 apparently crossed the boundary five times, producing a wealth of new data. It's even possible that if there are large variations in that solar outflow, the shock layer "could push past Voyager again," says Richardson.