Solar Black Body Emission
I would appreciate
any criticism of my (mis)understanding of the Blackbody Physics involved.
I don't see any major problems. Of course, a
true blackbody would never reflect any incident electromagnetic radiation; a
true blackbody would absorb any & all incident electromagnetic radiation and would emit only
Planck-law thermal ("blackbody") electromagnetic radiation.
But in the real universe there are no
true blackbodies, so no real object will behave that way. However, we can certainly approximate a true blackbody, to the extent that technology & financing allow. Laboratory blackbodies are usually designed to operate where there is little or no incident radiation anyway, so they are designed to look like blackbodies in emission, ignoring how they behave with regard to the incident radiation that is not there anyway (e.g., the reference blackbody used to calibrate the
COBE FIRAS).
Any optically thick plasma will emit electromagnetic radiation in a very nice approximation of a true blackbody, if it is isothermal. The photosphere of the sun does in fact emit as a very nice approximation of a true blackbody, but it is not isothermal. So each temperature layer of the photosphere emits nearly blackbody radiation; we see little from the bottom because there is too much absorbing & scattering material above it, little from the top because it is becoming optically thin, and most from the middle where it is still optically thick but there is not too much stuff above it. So all those blackbody (Planck-law) spectral energy distributions (SEDs) add up to one SED that looks like a blackbody at about 5777 Kelvins. The observed optical spectrum is a sum of that near blackbody continuum plus absorption features from the photospheric gases.
As to whether or not a solid surface would reflect 171 Å incident radiation, quite regardless of its status as a "blackbody", that depends greatly on other considerations I don't see in the conversation. For one thing, if the surface is mostly iron, as Mozina and perhaps brantc seem to think, then I would expect very little reflection, simply because 171 Å is an iron emission line, and anything iron emits it will also absorb. I would expect the vast majority of any 171 Å radiation incident on an iron surface to disappear in the act of ionizing the iron, with only a small fraction being reflected back. And as DD points out, any of that reflected radiation is subject to being absorbed or scattered by the overlying plasma. The result is likely to be an un-observably small amount for us to see. Of course, brantc has relied on his "ghost limb" argument to push the point that we actually do see this reflected light, but as I have already pointed out in the posts above (
The Solar EUV Radius V,
The Solar "Ghost Limb" Is An Optical Artifact,
The Solar "Ghost Limb" Is An Optical Artifact II,
The Solar "Ghost Limb" Is An Optical Artifact III), he is relying on an optical artifact, so his claim is easily falsified.
But the entire discussion is moot in any case. We already know from simple thermodynamical considerations that a solid or rigid or liquid iron surface is physically impossible. So it's asking "if this impossible surface were present, would it reflect an impossible flux of 171 Å radiation?". Impossible is impossible, not just improbable.
)
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at the core.
