Currently the 2nd comment on the page of article addresses that.

Actually, supersonic simply refers to anything moving faster than the speed of sound in a particular medium. The Earth's atmosphere has a particular speed of sound but so does the solar atmosphere and wind. Apparently, the speed of sound in the solar wind is roughly 220,000 mph. Supersonic and subsonic are the terms that scientists use to describe such things.

and then links to this...

http://iopscience.iop.org/0004-637X/623/1/511/fulltext/61043...