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@MichaelPorter wrote: "It sounds like the word "surface" is being used in almost a mathematical sense here, as opposed to a physical surface."

Indeed! But you'd notice this surface because waves in the plasma can go *out* of this surface but never *in*, because this is the surface where the outflowing solar wind first becomes supersonic. It's very much like the reverse of a black hole. The event horizon of a black hole is a surface where light can go *in* but never *out*.

You can also sometimes see this sort of surface in a river near the top of a waterfall: there's a line where the water starts moving faster than waves in the water!