<p>Well, I did some digging and found a helpful chart <a href="https://www.radio2space.com/the-radio-sun/" rel="nofollow noreferrer">here</a>. The image itself is located at <a href="https://www.radio2space.com/wp-content/uploads/2018/12/The-radio-Sun-2.jpg" rel="nofollow noreferrer">this link</a>.</p>
<p>Wikipedia defines the <a href="https://en.wikipedia.org/wiki/S_band" rel="nofollow noreferrer">S band</a> as the section of the electromagnetic spectrum from 2 to 4 GHz. To interpret the respective values the graph displays at these frequencies, we need to convert these two figures to wavelengths:</p>
<p><span class="math-container">$\lambda_1 = \dfrac{c}{\nu} = \dfrac{300\:000\:000\:m/s}{2\:000\:000\:000\:Hz} = 0.15\:m$</span></p>
<p><span class="math-container">$\lambda_2 = \dfrac{c}{\nu} = \dfrac{300\:000\:000\:m/s}{4\:000\:000\:000\:Hz} = 0.075\:m$</span></p>
<p>And, we might as well find the wavelength at 3 GHz, since it's at the middle of the S band and might help us (hint: since the graph is on a log scale, and this calculation comes out to a nice power of 10, it definitely will help us):</p>
<p><span class="math-container">$\lambda_m = \dfrac{c}{\nu} = \dfrac{300\:000\:000\:m/s}{3\:000\:000\:000\:Hz} = 0.1\:m$</span></p>
<p>So, we can take a look at the graph at 0.1 m. The units seem to be in <span class="math-container">$W\: m^{-2} \: (c/s)^{-1}$</span>, and I'm assuming that <span class="math-container">$(c/s)$</span> is just <span class="math-container">$Hz$</span>, which would make these units have the same units as Janskys, though, as Connor Garcia mentions in his comment, Janskys are <span class="math-container">$10^{-26}\: \rm{W\: m^{-2}\: Hz^{-1}}$</span>. <strong>So, the quiet sun in the S band lies around <span class="math-container">$10^{6}\: \rm{Jy}$</span>, and the &quot;disturbed&quot; or &quot;active&quot; sun lies around <span class="math-container">$10^{7}\: \rm{Jy}$</span>.</strong></p>
<p>And as for the second part of your question, the graph also shows some blackbody curves. You asked for 5800 K, but the closest one on the graph is at 6000 K - hopefully that's okay. As expected, the Sun seems to closely match that line for a good portion of the wavelengths on the graph, but seems to deviate in the S band, which is interesting, to say the least. <strong>So, from the blackbody curves in the graph, the sun is brighter than a typical blackbody at ~6000 K in the S band, both in its quiet and disturbed states. This difference is around one order of magnitude in the quiet state, and around two orders of magnitude in the disturbed state.</strong></p>
<p>And lastly, you asked about the difference between the &quot;brightness&quot; of the sun in its active and disturbed states in the S band. <strong>From visual inspection of the graph, it seems like the difference in the S band is about one or two orders of magnitude.</strong></p>

CC BY-SA 4.0
2020-11-16T02:16:04.307Z
Excellent, thank you!! I'll review later today but it looks like you've nailed it.
uhoh
CC BY-SA 4.0
2020-11-16T02:55:33.250Z
Thank you! I've posted a follow-up to the discrepancy between the sun and a typical blackbody in the S band here - hopefully answers to this prove insightful. https://astronomy.stackexchange.com/questions/39866/why-does-the-sun-deviate-from-a-typical-blackbody-spectrum-in-the-s-band
sforsingh
CC BY-SA 4.0
2020-11-16T19:36:39.123Z
@Calc-You-Later +1 vote, outstanding answer, but since a Jansky is 1e-26 W/(m^3Hz), shouldn't your post read 10^6 Jy instead of 10^-20?
Connor Garcia
CC BY-SA 4.0
2020-11-16T19:57:35.190Z
@ConnorGarcia ahh, you're right! Thank you for telling me, I'll edit this straightaway.
sforsingh
CC BY-SA 4.0
2020-11-16T20:01:52.960Z
@ConnorGarcia - also, I'm assuming the "m^3" in your comment is a typo, correct? Last I checked Janskys were W / (Hz * m^2), unless I'm missing something...
sforsingh
CC BY-SA 4.0
2020-11-16T20:02:54.870Z
@Calc-You-Later yep, m^3 is a typo.
Connor Garcia
CC BY-SA 4.0
2020-11-17T01:21:39.197Z
Now that you've got this worked out so nicely, you are welcome to post an answer to [How well can Voyager 1 separate Earth signals from Solar noise these days?](https://space.stackexchange.com/q/14317/12102) There's some information on link budgets that might be somewhat helpful in [this answer](https://space.stackexchange.com/a/24343/12102) and one can either dig into the DESCANSO volume on [Voyager Telecommunications](https://descanso.jpl.nasa.gov/DPSummary/Descanso4--Voyager_new.pdf) or assume a transmit power on Earth of say 20 kW and a 34 or 70 meter dish.
uhoh
CC BY-SA 4.0
2020-11-17T01:23:52.460Z
Oh, there are more goodies in [this answer](https://space.stackexchange.com/a/41809/12102)
uhoh
CC BY-SA 4.0
2020-11-17T01:30:10.717Z
If they are up and running again they will likely ping Voyager regularly. You can watch for uplinks and downlinks to show up [here](https://eyes.nasa.gov/dsn/dsn.html) or download a few days worth of logs and get the actual transmit power and dish name (and intimately size) from [Is there any publicly available logging, or “Wayback Machine” for DSN Now activity?](https://space.stackexchange.com/q/17046/12102) and [Understanding the information contained in the Deep Space Network XML data?](https://space.stackexchange.com/q/32479/12102)
uhoh
CC BY-SA 4.0
2020-11-17T01:42:45.927Z
That question looks really interesting! I'll do some more digging along with looking at the resources you've send and see if I can come up with a substantive answer :).
sforsingh
CC BY-SA 4.0
2020-11-17T02:50:19.810Z
@Calc-You-Later I didn't get a notification of your comment. It's better to use [@replies](https://meta.stackexchange.com/q/43019/303080) to ensure the target recipient of a comment gets a notification.
uhoh
CC BY-SA 4.0
2020-11-18T00:04:04.670Z
@uhoh ahh thank you, I'll make sure to use those.
sforsingh
CC BY-SA 4.0
2020-11-16T02:09:44.843Z
2020-11-16T20:00:27.870Z
2020-11-16T20:00:27.870Z
sforsingh
<p>As far as I can tell when the Voyagers listen to Earth they have the Sun in the same beam. I never did calculate how bright a blackbody Sun would be because I have no idea if that's representative of the Sun's output in S-band</p>
<ul>
<li><a href="https://space.stackexchange.com/q/14317/12102">How well can Voyager 1 separate Earth signals from Solar noise these days?</a> shows that the Voyagers see the Sun and Earth separated by 0.2 to 0.5 degrees each year.</li>
</ul>
<p>I also don't know how wide of a slice in frequency the front end is exposed to, or if the Sun would be strong enough to saturate it.<a href="https://space.stackexchange.com/q/17429/12102">Frequency responses of Voyager's S-band high gain antenna's feed-horn and receiver front end &amp; IF?</a> remains unanswered.</p>
<p>This is related <a href="https://space.stackexchange.com/q/33338/12102">Why is the operating temperature for the Voyagers' receiver noise calculation about 1550K?</a> and answers there are informative.</p>
<p>I assume the designers were optimistic that Voyager might survive this long and designed the system to be able to pick out transmissions from Earth from the Sun's output, but I'm not quite able to put the whole puzzle together yet.</p>
<p>So I'd like to ask:</p>
<p><strong>Question:</strong> How bright is the Sun in S-band? Is there a measured value for power per unit frequency? How close is it to what a blackbody would produce at say 5800 K? At this frequency is there a big difference between when the sun is quiet and when it is active?</p>
<p>See also <a href="https://astronomy.stackexchange.com/q/36366/7982">Do stars have &quot;radio photospheres&quot;? Are they different from their optical photospheres?</a></p>
<p><strong>note:</strong> The linked questions and answers have a lot of Voyager-specific information which is relevant to Voyager's operation in S-band, but I have nothing on the Sun's behavior <a href="https://deepspace.jpl.nasa.gov/dsndocs/810-005/201/201C.pdf" rel="nofollow noreferrer">at 2.1 GHz</a> Actually the first link gives the downlink frequency as</p>
<blockquote>
<p>2113.312500 MHz for Voyager 2</p>
</blockquote>

CC BY-SA 4.0
2020-11-06T18:21:36.453Z
Not an answer but last figure of [these lecture notes](https://web.njit.edu/~gary/728/Lecture10.html) have a flux density figure for the Sun and the major sources of emission. Figure 6 of [Rochat et al 2017](https://www.researchgate.net/publication/321243651_Direct_Observation_of_Asteroid_Interior_and_Regolith_Structure_Science_Measurement_Requirements) has the galactic and atmosphere contributions to the antenna temp (not reversal of x-axis). [DSTSE](https://descanso.jpl.nasa.gov/dstse/DSTSE.pdf) page 442 (pdf page 460) has T(antenna) examples up to 10,000K for Sun in the whole beam
astrosnapper
CC BY-SA 4.0
2020-11-07T01:18:34.977Z
@astrosnapper thanks! DESCANSO rocks! [When was the idea behind the DESCANSO Book Series first conceived, by whom, and what is its “mission”?](https://space.stackexchange.com/q/22968/12102)
uhoh
<the-sun><radio-astronomy><radio>
How bright is the Sun in S-band?
uhoh
# How bright is the Sun in S-band?

(Asked by: username_0 on 2020-11-04)

As far as I can tell when the Voyagers listen to Earth they have the Sun in the same beam. I never did calculate how bright a blackbody Sun would be because I have no idea if that's representative of the Sun's output in S-band

* [How well can Voyager 1 separate Earth signals from Solar noise these days?](https://space.stackexchange.com/q/14317/12102) shows that the Voyagers see the Sun and Earth separated by 0.2 to 0.5 degrees each year.

I also don't know how wide of a slice in frequency the front end is exposed to, or if the Sun would be strong enough to saturate it.[Frequency responses of Voyager's S-band high gain antenna's feed-horn and receiver front end & IF?](https://space.stackexchange.com/q/17429/12102) remains unanswered.

This is related [Why is the operating temperature for the Voyagers' receiver noise calculation about 1550K?](https://space.stackexchange.com/q/33338/12102) and answers there are informative.

I assume the designers were optimistic that Voyager might survive this long and designed the system to be able to pick out transmissions from Earth from the Sun's output, but I'm not quite able to put the whole puzzle together yet.

So I'd like to ask:

**Question:** How bright is the Sun in S-band? Is there a measured value for power per unit frequency? How close is it to what a blackbody would produce at say 5800 K? At this frequency is there a big difference between when the sun is quiet and when it is active?

See also [Do stars have "radio photospheres"? Are they different from their optical photospheres?](https://astronomy.stackexchange.com/q/36366/7982)

**note:** The linked questions and answers have a lot of Voyager-specific information which is relevant to Voyager's operation in S-band, but I have nothing on the Sun's behavior [at 2.1 GHz](https://deepspace.jpl.nasa.gov/dsndocs/810-005/201/201C.pdf) Actually the first link gives the downlink frequency as

> 2113.312500 MHz for Voyager 2

**Tags:** the-sun, radio-astronomy, radio
> **Comments:**
> **username_1**: Not an answer but last figure of [these lecture notes](https://web.njit.edu/~gary/728/Lecture10.html) have a flux density figure for the Sun and the major sources of emission. Figure 6 of [Rochat et al 2017](https://www.researchgate.net/publication/321243651_Direct_Observation_of_Asteroid_Interior_and_Regolith_Structure_Science_Measurement_Requirements) has the galactic and atmosphere contributions to the antenna temp (not reversal of x-axis). [DSTSE](https://descanso.jpl.nasa.gov/dstse/DSTSE.pdf) page 442 (pdf page 460) has T(antenna) examples up to 10,000K for Sun in the whole beam  
> **username_0**: @astrosnapper thanks! DESCANSO rocks! [When was the idea behind the DESCANSO Book Series first conceived, by whom, and what is its “mission”?](https://space.stackexchange.com/q/22968/12102)  

---

## 1 answers

### Answer by username_2 on 2020-11-16. Score: 3 | Accepted answer

Well, I did some digging and found a helpful chart [here](https://www.radio2space.com/the-radio-sun/). The image itself is located at [this link](https://www.radio2space.com/wp-content/uploads/2018/12/The-radio-Sun-2.jpg).

Wikipedia defines the [S band](https://en.wikipedia.org/wiki/S_band) as the section of the electromagnetic spectrum from 2 to 4 GHz. To interpret the respective values the graph displays at these frequencies, we need to convert these two figures to wavelengths:

$\lambda\_1 = \dfrac{c}{\nu} = \dfrac{300\:000\:000\:m/s}{2\:000\:000\:000\:Hz} = 0.15\:m$

$\lambda\_2 = \dfrac{c}{\nu} = \dfrac{300\:000\:000\:m/s}{4\:000\:000\:000\:Hz} = 0.075\:m$

And, we might as well find the wavelength at 3 GHz, since it's at the middle of the S band and might help us (hint: since the graph is on a log scale, and this calculation comes out to a nice power of 10, it definitely will help us):

$\lambda\_m = \dfrac{c}{\nu} = \dfrac{300\:000\:000\:m/s}{3\:000\:000\:000\:Hz} = 0.1\:m$

So, we can take a look at the graph at 0.1 m. The units seem to be in $W\: m^{-2} \: (c/s)^{-1}$, and I'm assuming that $(c/s)$ is just $Hz$, which would make these units have the same units as Janskys, though, as Connor Garcia mentions in his comment, Janskys are $10^{-26}\: \rm{W\: m^{-2}\: Hz^{-1}}$. **So, the quiet sun in the S band lies around $10^{6}\: \rm{Jy}$, and the "disturbed" or "active" sun lies around $10^{7}\: \rm{Jy}$.**

And as for the second part of your question, the graph also shows some blackbody curves. You asked for 5800 K, but the closest one on the graph is at 6000 K - hopefully that's okay. As expected, the Sun seems to closely match that line for a good portion of the wavelengths on the graph, but seems to deviate in the S band, which is interesting, to say the least. **So, from the blackbody curves in the graph, the sun is brighter than a typical blackbody at ~6000 K in the S band, both in its quiet and disturbed states. This difference is around one order of magnitude in the quiet state, and around two orders of magnitude in the disturbed state.**

And lastly, you asked about the difference between the "brightness" of the sun in its active and disturbed states in the S band. **From visual inspection of the graph, it seems like the difference in the S band is about one or two orders of magnitude.**

> **Comments:**
> **username_0**: Excellent, thank you!! I'll review later today but it looks like you've nailed it.  
> **username_2**: Thank you! I've posted a follow-up to the discrepancy between the sun and a typical blackbody in the S band here - hopefully answers to this prove insightful. https://astronomy.stackexchange.com/questions/39866/why-does-the-sun-deviate-from-a-typical-blackbody-spectrum-in-the-s-band  
> **username_3**: @Calc-You-Later +1 vote, outstanding answer, but since a Jansky is 1e-26 W/(m^3Hz), shouldn't your post read 10^6 Jy instead of 10^-20?  
> **username_2**: @ConnorGarcia ahh, you're right! Thank you for telling me, I'll edit this straightaway.  
> **username_2**: @ConnorGarcia - also, I'm assuming the "m^3" in your comment is a typo, correct? Last I checked Janskys were W / (Hz * m^2), unless I'm missing something...  
> **username_3**: @Calc-You-Later yep, m^3 is a typo.  
> **username_0**: Now that you've got this worked out so nicely, you are welcome to post an answer to [How well can Voyager 1 separate Earth signals from Solar noise these days?](https://space.stackexchange.com/q/14317/12102) There's some information on link budgets that might be somewhat helpful in [this answer](https://space.stackexchange.com/a/24343/12102) and one can either dig into the DESCANSO volume on [Voyager Telecommunications](https://descanso.jpl.nasa.gov/DPSummary/Descanso4--Voyager_new.pdf) or assume a transmit power on Earth of say 20 kW and a 34 or 70 meter dish.  
> **username_0**: Oh, there are more goodies in [this answer](https://space.stackexchange.com/a/41809/12102)  
> **username_0**: If they are up and running again they will likely ping Voyager regularly. You can watch for uplinks and downlinks to show up [here](https://eyes.nasa.gov/dsn/dsn.html) or download a few days worth of logs and get the actual transmit power and dish name (and intimately size) from [Is there any publicly available logging, or “Wayback Machine” for DSN Now activity?](https://space.stackexchange.com/q/17046/12102) and [Understanding the information contained in the Deep Space Network XML data?](https://space.stackexchange.com/q/32479/12102)  
> **username_2**: That question looks really interesting! I'll do some more digging along with looking at the resources you've send and see if I can come up with a substantive answer :).  
> **username_0**: @Calc-You-Later I didn't get a notification of your comment. It's better to use [@replies](https://meta.stackexchange.com/q/43019/303080) to ensure the target recipient of a comment gets a notification.  
> **username_2**: @uhoh ahh thank you, I'll make sure to use those.  

---

