There's something in the comments about this (the supersonic part, anyways). Apparently they are referring to the speed of sound in the solar wind "...which is

There's something in the comments about this (the supersonic part, anyways). Apparently they are referring to the speed of sound in the solar wind "...which is roughly 220,000 mph". Huh. Makes sense, but I wouldn't have thought of that. That's non-obvious enough that it would have been worth a mention in the article.