Don Gurnett

Think of a cold front that forms when a very cold air mass comes down to the U.S. from Canada, here we find a very hot plasma mass coming outward from the sun that encounters the cold plasma in the interstellar medium. It does not surprise me that a sharp boundary forms.

In a historical sense, the old idea that the solar wind will just be gradually whittled away as you go further into interstellar space is simply not true, we show with Voyager 2 -- and previously with Voyager 1 -- that there's a distinct boundary out there. It's just astonishing how fluids, including plasmas, form boundaries.