2
$\begingroup$

In the ABC News video New space discovery has astronomers buzzing at about 02:30 Dr. Hakeem M. Oluseyi explains that the shortness of FRBs (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?

Improve this question
$\endgroup$
5
  • 1
    $\begingroup$ 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. $\endgroup$
    – Trip Space-Parasite
    Commented Feb 8, 2019 at 16:28
  • $\begingroup$ @TripSpace-Parasite see comments here for a possible example of something that can "happen simultaneously at widely-separated points on the Sun's surface". $\endgroup$
    – uhoh
    Commented Feb 8, 2019 at 20:18
  • 1
    $\begingroup$ I stand (well, slouch) corrected! $\endgroup$
    – Trip Space-Parasite
    Commented Feb 8, 2019 at 21:29
  • $\begingroup$ @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. $\endgroup$
    – uhoh
    Commented Feb 8, 2019 at 21:50
  • 1
    $\begingroup$ 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. $\endgroup$
    – Trip Space-Parasite
    Commented Feb 8, 2019 at 23:55

1 Answer 1

Reset to default
2
$\begingroup$

@Astrosnapper's answer 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

I found some older data and a nice description Measurements of Frequencies of Solar Oscillations from the MDI medium-l Program by E.J. Rhodes, Jr., A.G. Kosovichev, P.H. Scherrer, J. Schou & J. Reiter which comes from The Michelson Doppler Imager aboard SOHO.

enter image description here

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

enter image description here

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

Improve this answer
$\endgroup$

You must log in to answer this question.

Not the answer you're looking for? Browse other questions tagged .