Expert: James Gort - 7/5/2005

Question
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Followup To
Question about solar lightning

Dear Professor Gort,

Thank you for your very lucid explanation of lightning itself and the surface of the sun!  I do have another question though.  

I had thought that the light we see here on earth was from hydrogen-hydrogen hydrogen-helieum helieum-helieum fusion reactions directly.  It sounds like some or all of the light from the sun is comming from the gasses reclaiming their lost electrons in the arches of the photosphere. And that the fusion reactions just power the magnetic fluxuations that cause the gasses to loose their electrons.  So the fusion reactions are the power, but the ionized gasses are the light bulb.  But this doesn't make sense because the sun emits radiation along the entire electromagnetic spectrum and I don't think you can get gamma rays from ionized gas, or maybe you can?  

Sorry about the confusion, but you are reforming some foundational material of my understanding of the universe, and I would really like to get a clear and accurate version of what is going on.

Thank you again, you are a good writer and your explatioins are easy to follow.

Argenteus


Answer
Hello Argenteus,

Actually, we get light only indirectly from the interior fusion reactions.  All the radiated photons from fusion (the entire spectrum - gamma rays, etc.) are continuously being absorbed and re-emitted by atoms and through photon-electron interactions (e.g., electrons losing energy through scattering by other particles, including photons).  The light we see is from the photosphere (upper 500 km), although that also depends on wavelength.

In the photosphere, various mechanisms are going on.  One of the most important is the capture of a free electron by a neutral hydrogen atom (forming a negative H atom).  Since the captured electron could have had any energy, this mechanism is mainly responsible for the "continuum" spectrum.

There's also H (and He, Ca, Na, etc.) emission and absorption going on (somewhat dependent on the height of the atom within the photosphere).  In Hydrogen, emission can result in photons from the UV to IR parts of the spectrum.  But, as you correctly surmise, that doesn't give you much gamma radiation!  In the sun, X-rays and gamma rays are relatively few.  However, the mechanisms that produces them are somewhat different.  The two main mechanisms are synchrotron radiation (electrons spiralling in a magnetic field), and "braking radiation", or electrons slowed (but not captured) when they encounter another particle (say, an H atom).  Which is why solar flares emit fairly good x-rays and gamma rays.  The flares not only consist of ions arching when accelerated by the magnetic field (this is the lightning effect), but high energy protons and electrons, too.  Any accelerated charged particle will emit radiation (e.g., synchrotron and/or braking radiation), and at high energies, these are often in the x-ray or gamma ray region.

It starts to get a little complicated, since there are several different mechanisms at work, but as I said, it's all pretty well understood.  Let me know if that helps answer your question.

Prof. James Gort