CNO Redux
Ok, we're working here on "cause". Something has to generate a lot of heat in the corona. So how exactly does the plasma reach those temps sitting above a 6000K photosphere?
How,
exactly, nobody knows. However, the problem is not that there are no explanations available, but rather how to choose between the likely candidates.
Cranmer, 2008 gives a nice review of the general choice between wave dissipation and magnetic loop reconnection, and has a lot of useful references in it. We know that there is a clear correlation between small scale magnetic structure and coronal structure at much higher altitudes (see, for instance, chapter 9 in
Solar Astrophysics by Peter V. Foukal; Wiley-VCH, 2004).
Schrijver, et al., 1997 was a big deal when it came out (and has 185 citations so far, a respectable number). They were able to make models based on the high resolution images of the sun's "magnetic carpet" (
Title & Schrijver, 1998) and show that the loss of energy from the shearing magnetic structures can drive coronal heating, at least in principle. One also sees the keyword "nanoflares" associated with this idea (i.e.,
Hudson, 1991;
Kopp & Poletto, 1993 & citations thereto & etc.).
There is no fundamental problem having a multi-million degree corona sitting on a 6000 degree photosphere, although it may look that way if your approach is too naive. The 2nd law of thermodynamics does not stop refrigerators from working, because they pump heat "uphill" by doing work. likewise, magnetic processes can pump energy "uphill" and heat the corona. The only real problem to watch out for is to make sure the photospheric energy reservoir is up to the task. Since we know that it is, then there is no problem.
Neutron capture signatures in particular are a bit unusual aren't they? What known forces of nature might generate them in the atmosphere of a body in our solar system?
Why should neutron capture signals be unusual? After all, it's not as if neutrons are unusual, and that's really about all you need; a few neutrons, a few nuclei and - voila, neutron capture. In Earth's atmosphere, and I suppose any planetary atmosphere (why not?) neutron capture gamma rays are observed in the polar regions, maybe connected with auroral displays, when energetic solar wind protons impact the upper atmosphere. We see narrow line emission from neutron capture (neutrons are knocked loose by protons and then are captured by other nuclei), and we see narrow line emission from nuclear de-excitation (collisionally excited nuclei relax to the ground state by emitting gamma rays); see for instance
Letaw, et al., 1989, who identify neutron capture on
14N and
16O, and Compton scattering of annihilation gamma photons;
Share & Murphy, 2001, who don't identify line sources in their abstract;
Share & Murphy, 2002, who identify
14N de-excitation and
12C spallation;
Harris, Share & Leising, 2003, who show that gamma ray line emission from Earth's atmosphere is modulated on the period of the solar cycle, consistent with solar wind excitation as the ultimate source.
I'm not sure the idea of "any other way" actually applies here. What we are looking for is the "logical reason" we might observe gamma rays in an atmosphere of a body in our solar system and also neutron capture signatures. There may be many 'possible' ways to do it, but how many ways does nature do it given these specific conditions? In other words we need to be looking from the "most likely" scenario.
http://www.sciencemag.org/cgi/content/summary/307/5712/1054?ck=nck
http://www.innovations-report.de/html/berichte/geowissenschaften/bericht-43938.html
It seems like the "most likely" way we might explain gamma rays is due to an electrical discharge. It certainly happens in our atmosphere and in the atmospheres of many planets. Why not the solar atmosphere as well?
Well, if it's "most likely" you want, then we can easily rule out electrical discharges. The links you provide refer to gamma rays associated with sprites & lightning,
Terrestrial Gamma-Ray Flashes (
TGFs). But
TGF gamma rays, and lightning gamma emission are broadband, exactly as one would expect from an electrical discharge. However, I am talking about narrow line emission, which you will never see from an electrical discharge. And furthermore, the narrow line emission is readily identifiable with specific known sources; i.e., nuclear de-excitation, neutron capture, positron annihilation & etc. Fusion reactions, CNO processes included, produce narrow line gamma ray emissions which will be easily identifiable with the parent process. That's why I made the point in critiquing your paper that you rely on the other narrow lines for a weak indirect argument, and totally ignore the
direct narrow line emission from the CNO reactions. Find those narrow lines in coronal loops and you will have something to say about CNO fusion in the solar atmosphere.
While sprites & lightning cannot themselves generate narrow line gamma ray emission, it has been suggested that they can excite nuclei, which will then decay to the ground state, with associated narrow line emission. Whether or not this is in fact the case remains unclear (
Boggs, et al., 2005). So you can get high energy from electrical discharges, at least in principle, but then that was never a point in dispute.
You are left bereft of
direct evidence. You have no
direct evidence of CNO fusion in the solar atmosphere, and no
direct evidence of electrical discharge in the solar atmosphere. In both cases you have to rely on pure faith, exactly what I am supposed to be doing
