<p>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 <a href="https://en.wikipedia.org/wiki/Solar_radio_emission" rel="noreferrer">Solar radio emission</a> you can see images of the sun from 17 GHz down to 80 Mhz.</p>
<p><a href="https://i.sstatic.net/VRXCU.png" rel="noreferrer"><img src="https://i.sstatic.net/VRXCU.png" alt="enter image description here" /></a></p>
<p><sub>By Patrick McCauley Mccauley.pi - Own work, CC BY-SA 4.0, <a href="https://commons.wikimedia.org/w/index.php?curid=99231226" rel="noreferrer">https://commons.wikimedia.org/w/index.php?curid=99231226</a></sub></p>
<p>Photos and a detailed theoretical discussion of observations between 150-450 MHz are given in C. Mercier and G. Chambe (2012) <a href="https://www.aanda.org/articles/aa/abs/2012/04/aa18163-11/aa18163-11.html" rel="noreferrer">Morphology of the quiet Sun between 150 and 450 MHz as observed with the Nançay radioheliograph</a></p>

CC BY-SA 4.0
2021-06-22T01:16:10.560Z
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?](https://astronomy.stackexchange.com/q/36366/7982) 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!
uhoh
CC BY-SA 4.0
2021-06-23T21:55:46.720Z
@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 https://www.plasmaphysics.org.uk/research/sun.htm for more
Thomas
CC BY-SA 4.0
2021-06-21T18:33:46.920Z
2021-09-26T02:37:51.290Z
2021-09-26T02:37:51.290Z
Thomas
<p>Radio2space.com's <a href="https://www.radio2space.com/the-radio-sun/" rel="nofollow noreferrer">The Radio Sun</a> says:</p>
<blockquote>
<p>At very low frequencies (below 0.1 GHz) and therefore at very long wavelengths (&gt; 3m) the solar disk appears much bigger and brighter in the center, and its brightness gradually decreases and vanishes after several solar radii.</p>
</blockquote>
<p>and I guess if you sell dishes and feed horns, then &lt; 100 MHz may seem like &quot;very low frequencies&quot;.</p>
<p>This page shows an image of the Sun at 4 GHz and it's relatively dark but lined with bright &quot;spots&quot; in two bands on either side of the equator.</p>
<p>But what does the Sun &quot;look like&quot; 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.</p>
<p>Different but related:</p>
<ul>
<li><a href="https://astronomy.stackexchange.com/q/36366/7982">Do stars have &quot;radio photospheres&quot;? Are they different from their optical photospheres?</a></li>
<li><a href="https://astronomy.stackexchange.com/q/36365/7982">What will it finally take to accurately measure the distance to Betelgeuse?</a> (includes links to radio images of other stars)</li>
</ul>
<hr />
<p><a href="https://i.sstatic.net/LHQau.jpg" rel="nofollow noreferrer"><img src="https://i.sstatic.net/LHQaum.jpg" alt="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)" /></a> 4 GHz. (click for larger)</p>
<blockquote>
<p>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)</p>
</blockquote>

<observational-astronomy><the-sun><radio-astronomy><stellar-astrophysics><radio>
What does the Sun "look like" below 100 MHz?
uhoh
# What does the Sun "look like" below 100 MHz?

(Asked by: username_0 on 2020-08-15)

Radio2space.com's [The Radio Sun](https://www.radio2space.com/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:

* [Do stars have "radio photospheres"? Are they different from their optical photospheres?](https://astronomy.stackexchange.com/q/36366/7982)
* [What will it finally take to accurately measure the distance to Betelgeuse?](https://astronomy.stackexchange.com/q/36365/7982) (includes links to radio images of other stars)

---

[![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)](https://i.sstatic.net/LHQaum.jpg)](https://i.sstatic.net/LHQau.jpg) 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)

**Tags:** observational-astronomy, the-sun, radio-astronomy, stellar-astrophysics, radio

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## 1 answers

### Answer by username_1 on 2021-06-21. Score: 8 | Accepted answer

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](https://en.wikipedia.org/wiki/Solar_radio_emission) you can see images of the sun from 17 GHz down to 80 Mhz.

[![enter image description here](https://i.sstatic.net/VRXCU.png)](https://i.sstatic.net/VRXCU.png)

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](https://www.aanda.org/articles/aa/abs/2012/04/aa18163-11/aa18163-11.html)

> **Comments:**
> **username_0**: 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?](https://astronomy.stackexchange.com/q/36366/7982) 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!  
> **username_1**: @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 https://www.plasmaphysics.org.uk/research/sun.htm for more  

---

