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This is sort of a follow-up to this question, and my answer to it.

The graph I see here details the radio (and other frequencies) emission of the sun. What's most notable and interesting to me is the deviation from the typical blackbody spectrum of a body at around 6000 K. The sun seems much brighter in the S band (and at some larger wavelengths) than a typical blackbody.

My question is: why does this happen? Is there an underlying physical/astrophysical phenomena that we know of that causes this? Or is this something that's undergoing research right now?

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There are other ways of getting emissions than just direct thermal radiation. Most of it happens through plasma interactions in the solar corona and atmosphere than in the chromosphere. This review paper names bremsstrahlung, gyroresonance, cyclotronmaser, and plasma radiation as sources, each with their own brightness temperature way above 6000 K. (See also these lecture slides for a bit more in depth math of the sources).

My own summary would be that there is plenty of energy in the magnetic field and plasma, and this can be released through various wave modes producing radio signals. Further, fast charged particles are spinning around field lines or colliding, producing electromagnetic emissions of various types. None of these are blackbody sources, but they add to the blackbody spectrum to produce the observed curve.

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    $\begingroup$ There is also of course no reason why the summed spectrum from two different regions (the photosphere and the corona) should look like a single blackbody. $\endgroup$
    – ProfRob
    Nov 16 '20 at 22:45

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