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Originally Posted by The Bad Astronomer
This is simply wrong. You stipulate you need a resonance cavity to get sound waves to pass through the Sun, and that is simply wrong. Sound waves will happily travel through a gas, even a relatively thin one (take a look at any supernova remnant filament for proof of that).
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I agree with you that sound will travel through about anything other than empty space. I wasn't trying to imply the sun is solid, simply that the iron layer provides the resonance cavity that makes helioseismology work in the first place. Here's what I mean:
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http://soi.stanford.edu/results/heliowhat.html
Why does the Sun act as a resonant cavity? Acoustic waves become trapped in a region bounded on top by a large density drop near the surface, and bounded on the bottom by an increase in sound speed that refracts a downward propagating wave back toward the surface. A standing wave is created.
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That "large density drop" they are refering to is the layer of iron. That's why it rings like a bell being struck by sand. Its an iron bell experiencing electrical activity at the surface.
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Your basis for this argument is incorrect. Helioseismology starts with the Sun being a gas, and shows that this can be used to "see" the far side of the Sun. You don't need a solid for sound waves to travel. You don't even need resonance of any kind. Just a medium that can conduct sound waves.
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The wave do resonate and bounce off the "large density drop" they refer to. That is a layer of iron. That's all I'm suggesting. I agree with you that sound can travel though about anything other than empty space, but the resonance cavity does exist and has been documented as has the density change between the stuff that is OUTSIDE and the stuff that is INSIDE. We have to ask WHY there is a large density increase and what causes this abrupt change in density.