Quite a nice read on sun spots. I think you can make some basic arguments about the reduced temperature being from more efficient conversion of energy into heating the Corona rather than the photosphere. Again a little bit of physics would not have hurt here with an explanation that spectral lines split with magnetic field allowing us to measure the strength of magnetic field with a prism or diffraction grating. I am a little disappointed at getting fatigued while podcasting. MP4 recording     MP3 recording -- -- Your browser does not support the audio element.