Look at this photo taken in France during the 1999 solar eclipse which I found on Wikipedia. Why does the corona appear white? It has a temperature of about $10^6\,$K so you would think that it would maybe look more like violet?
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3 Answers
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The light that is seen in the photo is white light from the solar photosphere that is Thomson-scattered from free electrons in the corona into the line of sight (see diagram below).
The hot, optically thin corona, hardly emits any visible light itself (just a few isolated emission lines, the brightest of which is in the green at 530 nm, and almost no visible continuum) in comparison to the scattered luminosity from the photosphere.
What you are seeing is essentially light with the same spectrum as the solar photosphere (which appears white to the human eye), since the Thomson scattering cross-section is wavelength independent and does not change the spectrum of the scattered light.
A statement from Rusin et al. 2010 from their paper "Comparing eclipse observations of the 2008 August 1 solar corona with an MHD model prediction":
While the white-light corona is essentially the light of the photosphere scattered on free electrons and dust particles in the vicinity of the Sun and is quite easy to observe, the radiation of the emission corona is associated with specific spectral lines (e.g. Billings 1966). It is the inherent radiation of the corona by highly ionized elements of iron, calcium, nickel, etc. A great majority of the corresponding lines lie in the extreme UV or X-ray part of the spectrum; in the visible region there are only about 28 lines, the brightest of them being Fe XIV at 530.3 nm
NB The pink light is intrinsic light from the solar chromosphere, which is denser than the corona and hotter than the photosphere. The colour arises from various bright ionic emission lines.
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$\begingroup$ "So what you are seeing is essentially light with the same spectrum as the solar photosphere" — this contradicts your comment to my answer, since if this statement is true, then the spectrum is not too far from black body emission, see this comparison of solar spectra with a BB spectrum. $\endgroup$– RuslanCommented Sep 29, 2020 at 20:20
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2$\begingroup$ @Ruslan No it does not. The light from the corona is nothing like a blackbody. The light that is seen is not light from the corona it is merely scattered light. $\endgroup$– ProfRobCommented Sep 29, 2020 at 20:35
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$\begingroup$ That's just the details of the mechanism of how the light comes to the observer. It's like saying that the light from the sky is not blue, because it's merely scattered sunlight (or, the light from the clouds is not white for the same reason, if you want to emphasize wavelength independence here). $\endgroup$– RuslanCommented Sep 29, 2020 at 20:42
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2$\begingroup$ @Ruslan The details are what matters here and explain why the corona appears white. You have clearly not understood where the light is coming from and instead have discussed emission from a blackbody with "an extremely high temperature", thus reinforcing the misconception that the OP had in the first place. $\endgroup$– ProfRobCommented Sep 29, 2020 at 21:02
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1$\begingroup$ @jpaugh I can't see what is ambiguous about my conclusion that the corona is white due to coronal electrons scattering photospheric light and that the intrinsic optical light from the corona is negligible and concentrated in a few bright spectral emission lines? The sky appears blue because of Rayleigh scattering, not Thomson scattering, but otherwise your summary is correct (though it simply rewords my first two paragraphs), so you must have learned something. $\endgroup$– ProfRobCommented Sep 30, 2020 at 17:19
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It's unreasonable to expect that any object with approximately-blackbody emission spectrum will have violet color at any temperature. Actual Planckian locus spans colors from red through orange to yellow, white and finally sky-blue, with the latter color achieved at extremely high temperatures.
Now, in addition to this, generally photos can't be relied on to judge the colors. In particular, a photo may have some unspecified white balance, which may be whatever the camera decided it should be, and additionally it may have been altered by post-processing. The actual color should be determined from the spectral power distrubution of the light source.
But, the above is not really relevant to the solar corona. The color of the corona, visible to an observer on the Earth, is mostly not the thermal emission from the coronal matter. Instead it's the light from the solar photosphere, scattered by the corona. Actual thermal emission from the corona consists of some 28 visible spectral peaks (see the answer by Rob Jeffries for details), and more peaks in the UV. They don't influence the color of the corona as much as the scattered light.
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4$\begingroup$ The spectrum of the corona is nothing like a blackbody and the light in the photo is not the intrinsic spectrum of the corona. $\endgroup$– ProfRobCommented Sep 29, 2020 at 18:54
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$\begingroup$ @RobJeffries yeah I guess so. But I've failed to find the actual SPD curve for visible region. All the data I've been able to find were either lists of specific spectral peaks, or curves for far UV range. But there should be quite a noticeable continuous background in the visible range. I'd appreciate if you (or someone else) could point me to the relevant data. $\endgroup$– RuslanCommented Sep 29, 2020 at 19:09
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$\begingroup$ @RobJeffries the answer is now edited to address your concern. $\endgroup$– RuslanCommented Sep 29, 2020 at 21:14
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The colours that you see in the image are not necessarily what you would see with the naked eye. The image colours will depend on the filter and the characteristics of the film used to capture the image. The information about this image on Wikimedia Commons also suggests that it has been "photoshopped" to enhance contrast.
answered Sep 29, 2020 at 11:59
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$\begingroup$ Thanks. Does that mean it is still white, just a lot fainter than this picture would suggest? $\endgroup$ Commented Sep 29, 2020 at 12:04
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$\begingroup$ Also, contrast is an issue. Whilst a detector outputs numbers, the eyes does not. A full moon up in a clear night sky looks white or yellow in spite of being more similar to a wheatered asphalt. $\endgroup$ Commented Sep 29, 2020 at 12:05
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$\begingroup$ Also note: While I know nothing about the specific image in the OPs question, the colors in many astronomical photos are false colors, possibly derived from images taken in other-than-visible light, that either are chosen to convey some specific information or, are chosen for aesthetic reasons. $\endgroup$ Commented Sep 29, 2020 at 14:50
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6$\begingroup$ Whilst the image may have had its contrast messed about with, the visible light that is seen around the Sun during an eclipse is indeed white. $\endgroup$– ProfRobCommented Sep 29, 2020 at 20:38