In a solar eclipse, and from our distance, the sun's surface appears to be pretty much exactly that, a surface. However, the reality of it's being an extremely tenuous plasma makes things very much different, even under 28 times the force of Earth's gravity. My question might be: What is the elevation difference between a point where 99.9% percent of the light is coming from below, and another point which is completely "submerged" within the sun, with light flux equal in all directions? What about for other parts of the spectrum? Is that how the thickness of the photosphere is defined? In a related question, is there any topographic relief to be seen between the centers and edges of solar convection granules, even if only a few centimeters?
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$\begingroup$ en.wikipedia.org/wiki/Solar_radius says "The solar radius is usually defined as the radius to the layer in the Sun's photosphere where the optical depth equals 2/3". But their explanation of optical depth isn't clear... $\endgroup$– PM 2RingCommented Dec 14, 2022 at 5:58
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You're asking about the Sun's photosphere, a thin (~250 km thick) layer inside of which the odds of a photon escaping are less than 50% and outside of which the odds are close to 100%. The fuzzy boundary between the photosphere and the chromosphere (the next layer up) is that the chromosphere is where we start seeing hydrogen alpha lines. The fuzzy boundary between the photosphere and the convection zone (the next layer down) is that the transition is an arbitrary 50% probability, which is equivalent to an optical depth of 2/3.
answered Dec 14, 2022 at 22:29