<p>@Astrosnapper's <a href="https://astronomy.stackexchange.com/a/29500/7982">answer</a> to <em>Helioseismology of the Sun, what is actually measured?</em> begins with:</p>

<blockquote>
  <p>The two main methods to detect the solar <em>p</em>-mode oscillations, which have a period of about 5 minutes (so frequency of ~3 milliHertz) are line of sight Doppler shift and change in irradiance.</p>
</blockquote>

<p>3 mHz is the peak of the power spectrum, but a 2D histogram of shows significant power at frequencies beyond 8 mHz, corresponding to a period of only 128 seconds.</p>

<p>What does this mean? A dopplergram is collected over an extended period of time, maps of line-of-sight velocity determined via doppler shift of a given spectral line, and fit to a model based on spherical harmonics. Below is an example of a dopplergram and a histogram of power as a function of frequency and spherical harmonic. The shading left-to-right is the doppler shift associated with the rotation of the Sun, I'm not sure if the orbital motion of the Earth and other kinematic effects have been subtracted.</p>

<p>I went with 8 mHz and calculated the time and phase delay due to the different distances of parts of the 695,700 km radius Sun at a given instant of time. (script):</p>

<p><a href="https://i.sstatic.net/gwpl4.png" rel="nofollow noreferrer"><img src="https://i.sstatic.net/gwpl4.png" alt="enter image description here"></a></p>

<p>I found some older data and a nice description <a href="https://soho.nascom.nasa.gov/publications/CDROM1/papers/rhodes/index.html" rel="nofollow noreferrer">Measurements of Frequencies of Solar Oscillations from the MDI medium-l Program</a> by E.J. Rhodes, Jr., A.G. Kosovichev, P.H. Scherrer, J. Schou &amp; J. Reiter which comes from <a href="http://soi.stanford.edu/" rel="nofollow noreferrer">The Michelson Doppler Imager</a> aboard <a href="https://en.wikipedia.org/wiki/Solar_and_Heliospheric_Observatory" rel="nofollow noreferrer">SOHO</a>.</p>

<p><a href="https://i.sstatic.net/zAjOf.gif" rel="nofollow noreferrer"><img src="https://i.sstatic.net/zAjOf.gif" alt="enter image description here"></a></p>

<p><strong>above:</strong> Fig. 1c: Full disk Dopplergram. (full-scale version). Full-disk Dopplergram obtained with MDI on 9 July 1996. The large left-right variation is the signal from solar rotation. The "texture" is a combination of the motion from the sound waves that permeate the Sun and the large convective cells called "super-granulation". The bright feature in the lower right quadrant is an active region. <a href="https://soho.nascom.nasa.gov/publications/CDROM1/papers/rhodes/index.html" rel="nofollow noreferrer">Source</a></p>

<p><a href="https://i.sstatic.net/v9b6V.gif" rel="nofollow noreferrer"><img src="https://i.sstatic.net/v9b6V.gif" alt="enter image description here"></a></p>

<p><strong>above:</strong> Fig. 1b: Power spectrum obtained from 144 days the MDI Medium-l data for the modes averaged over the azimuthal order m. The power concentrates in ridges corresponding to solar acoustic (p) modes. The lowest weak ridge corresponds to the fundamental (f) mode. <a href="https://soho.nascom.nasa.gov/publications/CDROM1/papers/rhodes/index.html" rel="nofollow noreferrer">Source</a></p>

CC BY-SA 4.0
2019-03-11T10:45:51.363Z
2019-03-11T10:45:51.363Z
uhoh
<p>In the ABC News video <a href="https://www.youtube.com/watch?v=ubYhcVWmrbE" rel="nofollow noreferrer">New space discovery has astronomers buzzing</a> at about 02:30 <a href="https://en.wikipedia.org/wiki/Hakeem_M._Oluseyi" rel="nofollow noreferrer">Dr. Hakeem M. Oluseyi</a> explains that the shortness of <a href="https://en.wikipedia.org/wiki/Fast_radio_burst" rel="nofollow noreferrer">FRB</a>s (before dispersion) suggests a compact source size and illustrates this by using the Sun as an example. </p>

<p>I'd never though about this until just now, but the radius of the Sun is indeed about 2.3 light-seconds. It takes light from the edge of the Sun's disk over 2 seconds longer to reach us than light from the central area.</p>

<p>Are there any ways in which this light time difference is noticeable, or needs to be taken into account?</p>

CC BY-SA 4.0
2019-02-08T16:28:27.767Z
I wouldn't expect so, since that would require things to happen simultaneously at widely-separated points on the Sun's surface, and there's not a good way for that to happen. (The obvious way would be for something to happen in the center of the sun, and propagate out to reach the whole surface at once, but there's too much variable Sun-stuff in the way.) However, I am by no means an expert.
Trip Space-Parasite
CC BY-SA 4.0
2019-02-08T20:18:01.900Z
@TripSpace-Parasite see comments [here](https://astronomy.stackexchange.com/a/29500/7982) for a possible example of something that can "happen simultaneously at widely-separated points on the Sun's surface".
uhoh
CC BY-SA 4.0
2019-02-08T21:29:23.533Z
I stand (well, slouch) corrected!
Trip Space-Parasite
CC BY-SA 4.0
2019-02-08T21:50:23.013Z
@TripSpace-Parasite I'm by no means an expert either. It's speculation at this point but I have a hunch it will pan out.
uhoh
CC BY-SA 4.0
2019-02-08T23:55:22.487Z
No, no, it's a good point. Helioseismology is a thing, so obviously waves can propagate across and through the Sun pretty well. I hadn't thought of that.
Trip Space-Parasite
<the-sun><light>
Are there any ways in which the light time difference between the center and the edge of the solar disk is noticeable?
uhoh
# Are there any ways in which the light time difference between the center and the edge of the solar disk is noticeable?

(Asked by: username_0 on 2019-02-06)

In the ABC News video [New space discovery has astronomers buzzing](https://www.youtube.com/watch?v=ubYhcVWmrbE) at about 02:30 [Dr. Hakeem M. Oluseyi](https://en.wikipedia.org/wiki/Hakeem_M._Oluseyi) explains that the shortness of [FRB](https://en.wikipedia.org/wiki/Fast_radio_burst)s (before dispersion) suggests a compact source size and illustrates this by using the Sun as an example.

I'd never though about this until just now, but the radius of the Sun is indeed about 2.3 light-seconds. It takes light from the edge of the Sun's disk over 2 seconds longer to reach us than light from the central area.

Are there any ways in which this light time difference is noticeable, or needs to be taken into account?

**Tags:** the-sun, light

> **Comments:**
> **username_1**: I wouldn't expect so, since that would require things to happen simultaneously at widely-separated points on the Sun's surface, and there's not a good way for that to happen. (The obvious way would be for something to happen in the center of the sun, and propagate out to reach the whole surface at once, but there's too much variable Sun-stuff in the way.) However, I am by no means an expert.  
> **username_0**: @username_1 see comments [here](https://astronomy.stackexchange.com/a/29500/7982) for a possible example of something that can "happen simultaneously at widely-separated points on the Sun's surface".  
> **username_1**: I stand (well, slouch) corrected!  
> **username_0**: @username_1 I'm by no means an expert either. It's speculation at this point but I have a hunch it will pan out.  
> **username_1**: No, no, it's a good point. Helioseismology is a thing, so obviously waves can propagate across and through the Sun pretty well. I hadn't thought of that.  

---

## 1 answers

### Answer by username_0 on 2019-03-11. Score: 2 | Accepted answer

@Astrosnapper's [answer](https://astronomy.stackexchange.com/a/29500/7982) to *Helioseismology of the Sun, what is actually measured?* begins with:

> The two main methods to detect the solar *p*-mode oscillations, which have a period of about 5 minutes (so frequency of ~3 milliHertz) are line of sight Doppler shift and change in irradiance.

3 mHz is the peak of the power spectrum, but a 2D histogram of shows significant power at frequencies beyond 8 mHz, corresponding to a period of only 128 seconds.

What does this mean? A dopplergram is collected over an extended period of time, maps of line-of-sight velocity determined via doppler shift of a given spectral line, and fit to a model based on spherical harmonics. Below is an example of a dopplergram and a histogram of power as a function of frequency and spherical harmonic. The shading left-to-right is the doppler shift associated with the rotation of the Sun, I'm not sure if the orbital motion of the Earth and other kinematic effects have been subtracted.

I went with 8 mHz and calculated the time and phase delay due to the different distances of parts of the 695,700 km radius Sun at a given instant of time. (script):

[![enter image description here](https://i.sstatic.net/gwpl4.png)](https://i.sstatic.net/gwpl4.png)

I found some older data and a nice description [Measurements of Frequencies of Solar Oscillations from the MDI medium-l Program](https://soho.nascom.nasa.gov/publications/CDROM1/papers/rhodes/index.html) by E.J. Rhodes, Jr., A.G. Kosovichev, P.H. Scherrer, J. Schou & J. Reiter which comes from [The Michelson Doppler Imager](http://soi.stanford.edu/) aboard [SOHO](https://en.wikipedia.org/wiki/Solar_and_Heliospheric_Observatory).

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

**above:** Fig. 1c: Full disk Dopplergram. (full-scale version). Full-disk Dopplergram obtained with MDI on 9 July 1996. The large left-right variation is the signal from solar rotation. The "texture" is a combination of the motion from the sound waves that permeate the Sun and the large convective cells called "super-granulation". The bright feature in the lower right quadrant is an active region. [Source](https://soho.nascom.nasa.gov/publications/CDROM1/papers/rhodes/index.html)

[![enter image description here](https://i.sstatic.net/v9b6V.gif)](https://i.sstatic.net/v9b6V.gif)

**above:** Fig. 1b: Power spectrum obtained from 144 days the MDI Medium-l data for the modes averaged over the azimuthal order m. The power concentrates in ridges corresponding to solar acoustic (p) modes. The lowest weak ridge corresponds to the fundamental (f) mode. [Source](https://soho.nascom.nasa.gov/publications/CDROM1/papers/rhodes/index.html)


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