Wikipedia's Coronagraph; Invention says:

High Altitude Observatory's Mark IV Coronagraph on top of Mauna Loa, uses polarization to distinguish sky brightness from the image of the corona: both coronal light and sky brightness are scattered sunlight and have similar spectral properties, but the coronal light is Thomson-scattered at nearly a right angle and therefore undergoes scattering polarization, while the superimposed light from the sky near the Sun is scattered at only a glancing angle and hence remains nearly unpolarized.

How does this work?

For example, I looked at the daytime sky through a polarizing filter and I didn't notice substantial sky darkening? How could a polarizer have darkened the sky sufficiently to make the solar corona more visible?

  • $\begingroup$ This is a great question! Just fyi, never, ever look at the Sun, even if you use a polarizing filter. If you want to look at the sky, make sure you look in a different direction than the Sun! $\endgroup$ – uhoh Jul 8 '19 at 1:06
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    $\begingroup$ thank you for editing, now i know more about how that coronagraph worked, later i will try to do an experiment, substracting a polarized image of the surroundings of the sun at some angle to another image polarized at another angle $\endgroup$ – Sartem Cacartem Jul 8 '19 at 3:22
  • $\begingroup$ I strongly urge you not to do anything like that. Looking at the Sun, even through a polarizer, is extremely dangerous, you can damage your vision permanently even if you think the light is dim for many reasons. One of them is that the polarizers may not lower the intensity of wavelengths that are invisible, but can still damage your eyes! Don't do it! $\endgroup$ – uhoh Jul 8 '19 at 3:27
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    $\begingroup$ i will not look at the sun, i will take images with a camera $\endgroup$ – Sartem Cacartem Jul 8 '19 at 3:31
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    $\begingroup$ whew! okay but you may damage your camera as well. Good luck and don't look through the camera either! Don't be like this silly idiot ;-) bbc.com/news/av/world-us-canada-41003929/… $\endgroup$ – uhoh Jul 8 '19 at 3:34

When you look at the Sun in white (visible) light, what you see is actually sunlight that has scattered from free electrons in the solar corona. This is known as "Thomson scattering".

If we view the corona around the Sun (from our perspective), then the scattered light that reaches us has scattered through a large angle (roughly 90 degrees). The sunlight incident upon the electrons in the corona is unpolarised (that is, the electric field vector of the light is randomly oriented but at right angles to the direction of travel of the light). Such light accelerates the free electrons which re-emit light (this is what scattering is). But the scattered light is polarised when it is scattered through a large angle, because we effectively can only "see" the electrons accelerating along a line that forms a tangent to the limb of the Sun. The component of the acceleration along the line of sight does not cause any radiation to be emitted towards us.

The light we see from the daylight sky is also produced by scattering. In this case it is "Rayleigh scattering" from electrons that are bound into molecules and atoms in the Earth's atmosphere. The polarisation effects of this scattering are almost identical in nature to that caused by Thomson scattering. However, when we look at the sky right next to the Sun, then this will have been scattered through a very small angle. In these circumstances the scattered light will be unpolarised.

Thus we have a means of seeing a contrast between (polarised) light coming from the corona and (unpolarised) light coming from the sky in the same line of sight. By viewing the coronal area using a linear polariser, and then presumably rotating the plane of polarisation and taking a series of pictures, and comparing this with images taken without a polarising filter, we can isolate the small fraction of light from around the Sun that is due to Thomson scattering in its corona.

You made the comment that you don't see the sky darken when looking through a polarising filter. Well, either your filter isn't working or you were looking at the sky roughly 90 degrees away from the Sun and had the polarising filter in just the right orientation, or your sky is very polluted (the scattering caused by particulates does not cause the same degree of polarisation). Daytime sky is most definitely polarised depending on angular separation from the Sun. This map shows how the degree of linear polarisation changes with position as the Sun moves across the sky. The Sun is where the black circle is (black indicating unpolarised light). See also this answer.

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  • $\begingroup$ @SartemCacartem for more on the appearance of a clear blue sky through a polarizer, see this and this and this for example. $\endgroup$ – uhoh Jul 8 '19 at 8:22
  • $\begingroup$ i did see the sky darken in some parts of course but it wasnt enough to see the solar corona, now i understand how this technique works by taking advantage of the corona's poalrization $\endgroup$ – Sartem Cacartem Jul 8 '19 at 13:07
  • $\begingroup$ @SartemCacartem Yes, the sky around the Sun will be slightly brighter when your filter's polarisation is aligned with the polarisation vector from the corona then when at 90 degrees to that vector. But it is only slightly. $\endgroup$ – Rob Jeffries Jul 8 '19 at 16:48

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