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@johncarlosbaez It sounds like the word "surface" is being used in almost a mathematical sense here, as opposed to a physical surface. Aside from the boundary you've described, is there some way that a probe (or an astronaut with a very good A/C system in their suit 😄) would notice this surface?

I've always found it interesting how gases and fluids seem to form abrupt boundaries. I'm thinking of the photosphere and chromosphere, here. In terms of solar wind and the outer reaches of the solar system, the description of the heliosphere always reminded me of the hydraulic jump that you see in the kitchen sink.

https://www.sciencealert.com/role-of-surface-tension-not-gravity-hydraulic-jump