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Radio2space.com's The Radio Sun says:

At very low frequencies (below 0.1 GHz) and therefore at very long wavelengths (> 3m) the solar disk appears much bigger and brighter in the center, and its brightness gradually decreases and vanishes after several solar radii.

and I guess if you sell dishes and feed horns, then < 100 MHz may seem like "very low frequencies".

This page shows an image of the Sun at 4 GHz and it's relatively dark but lined with bright "spots" in two bands on either side of the equator.

But what does the Sun "look like" below 100 MHz? Is it just a fuzzy, unresolved blob that fades to zero at several solar radii, or can one resolve the Sun's edge.

Different but related:

The radio Sun: radio image of the Sun recorded by VLA. The brightest regions are part of corona nearby but beyond sunspots. Courtesy (NRAO/AUI) 4 GHz. (click for larger)

The radio Sun: radio image of the Sun recorded by VLA. The brightest regions are part of corona nearby but beyond sunspots. Courtesy (NRAO/AUI)

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Below about 300 MHz you can only see the sun's corona as the frequency is too low to penetrate the coronal plasma from further below. In Wikipedia's Solar radio emission you can see images of the sun from 17 GHz down to 80 Mhz.

enter image description here

By Patrick McCauley Mccauley.pi - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=99231226

Photos and a detailed theoretical discussion of observations between 150-450 MHz are given in C. Mercier and G. Chambe (2012) Morphology of the quiet Sun between 150 and 450 MHz as observed with the Nançay radioheliograph

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  • $\begingroup$ Great answer, thank you! By the way you might consider posting an additional answer to Do stars have “radio photospheres”? Are they different from their optical photospheres? There's one there about red giants but something about our own Sun where we can directly compare optical and radio diameters would be super! $\endgroup$
    – uhoh
    Jun 22, 2021 at 1:16
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    $\begingroup$ @uhoh Actually, I don't think that red giants have a photosphere like the sun at all. Given their much smaller mass/radius ratio, the kinetic energy/temperature of the plasma (as calculated per the virial theorem) is too small for ions/protons to excite any atomic transitions i,e, to lead to a cooling of the plasma through inelastic collisions (which requires keV energies). It is this collisional cooling that is responsible for the sun's photosphere. See my web page plasmaphysics.org.uk/research/sun.htm for more $\endgroup$
    – Thomas
    Jun 23, 2021 at 21:55

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