Recently, I came across spectacular pictures of solar granulation like this:

enter image description here


The photograph is impressive because it has a resolution of less than 0.1 arc seconds, which corresponds to about 50 km.

I wonder whether this is in contradiction with the standard solar model that assumes a thickness of the photosphere of about 300 kilometers. After all, it's the whole photosphere that emits the light.

How can the sum of fast and highly turbulent processes, with a vertical extension of several hundred kilometers, result in a picture with a spatial resolution of some tens of kilometers?

There have been claims by a non-mainstream researcher, Pierre-Marie Robitaille, that the sun consists of liquid metallic hydrogen, not a gas. Sounds exotic, but the granulation pictures seem to provide some evidence.

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    $\begingroup$ I'm not sure where you get 50 km. The APOD source from which the image really comes from says each granule is about 1000 km across and that the sunspots are typically bigger than earth. $\endgroup$ – Kyle Kanos Sep 29 '14 at 20:44
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    $\begingroup$ The resolution is much smaller than the structure. arxiv.org/abs/1403.6896 says 0.1 arcseconds. A lower value of 0.07 was reported on a recent meeting. $\endgroup$ – ClassicalPhysicist Sep 29 '14 at 21:06
  • $\begingroup$ ooo, I misunderstood what you were saying. I read it as saying the image represented 50 km across, but that's just the resolution scale length. I presume that adaptive optics is the key reason. $\endgroup$ – Kyle Kanos Sep 29 '14 at 21:13
  • $\begingroup$ Yes, adaptive optics is used, but it can fix the distubances an accurate picture has undergone after the light was emitted, but not the inherent blurriness due to the origin of the picture in the whole photosphere. $\endgroup$ – ClassicalPhysicist Sep 29 '14 at 21:18
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    $\begingroup$ What makes you believe that you can see metallic hydrogen in this picture? If I showed you the picture of a molten lead sea from 150 million km distance, what would it look like? $\endgroup$ – CuriousOne Sep 29 '14 at 21:27

The resolution is much smaller than the granulation pattern. i.e. The granules are well resolved and are of order 500-1000 km in diameter, not 50 km. The sunspot is of order the diameter of the Earth.

Granulation is caused by convective cells rising and falling such that they just poke up into the bottom of the photosphere - i.e. that region where most of the continuum light comes from at around optical depths of 0.5-1. This is no coincidence as it is the escape of radiation that stifles the convective instability.

The size of the convective cells is given by some small multiple of the pressure scale height, which is given by $kT/\mu g$, where $\mu$ is the mean molecular weight (of order $1.67\times10^{-27}$ kg), $T$ is the gas temperature (of order $10^{4}$ K) and $g$ is the local gravity (about 300 m/s$^2$). i.e. the cells should be a small multiple of 275 km. Which they are.


Trying to read between the lines I think what you are really asking is why can we see any spatial structure that is smaller than this? You use the term "resolution", but that is a property of the camera, not the Sun.

There are two reasons: (i) The argument on size scales above is an order of magnitude argument. Structure does occur on smaller scales - a spatial Fourier transform would show a dominant scale at 500-1000km, but turbulence cascades structure to smaller scales too. For instance lower pressure downflows could be squeezed between uprising cells. There are also magnetic fields threading the region that also play a role (seen clearly in the periphery of the sunspot). (ii) This is just a snapshot. If you were to average many pictures taken over a time longer than a few convective cell turnover times (the cells have peculiar velocities of order 1 km/s so turnover times of ~5 minutes) then the picture would be much more blurred.

The OP's concern appears to be mainly that they think any small scale structure in the picture should be washed out by turbulent motion and the absorption/re-emission of light in higher layers of the atmosphere. However, once granulation reaches an optical depth of about 2/3 then most of the light escapes directly with no further interaction with gas above it. The continuum itself is formed over a layer (a plane parallel approximation is a bit crude) which is much smaller than the size of the granulation cells (perhaps 100km vs 500-1000km).

Here is a time lapse movie taken at Big Bear observatory over a couple of hours, clearly showing the convective motions at the base of the photosphere.

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  • $\begingroup$ You obviously misunderstood the question. The size of the granulation pattern is irrelevant here. Your statement that we see something which is below the photosphere - by definition not visible - seems to make little sense. $\endgroup$ – ClassicalPhysicist Sep 29 '14 at 22:38
  • $\begingroup$ Yet again, you are attributing statements to me that I did not make. Granulation is caused by convective cells rising and falling below the photosphere. If you're going to hand out downvotes on that basis, you won't be getting many answers to your questions... $\endgroup$ – Rob Jeffries Sep 29 '14 at 22:41
  • $\begingroup$ Sorry, your answer missed the point for the two reasons I explained... now you paid back by downvoting the question... well let's close our discussion. $\endgroup$ – ClassicalPhysicist Sep 29 '14 at 22:50
  • $\begingroup$ Comment to Edit: (ii) you say: "If you were to average many pictures taken over a time..." This is what practically happens since the picture is an average from all different depths of the photosphere. It SHOULD be blurred, and it's not. This is a problem. $\endgroup$ – ClassicalPhysicist Oct 1 '14 at 21:42
  • $\begingroup$ The granule pattern is seen best in continuum light formed at the bottom of the photosphere - basically directly viewing the tops of the convective cells. If you take a picture in a deep photospheric absoprtion line formed higher up in the photosphere, indeed the pattern is not so distinct and the temperature variations are less. $\endgroup$ – Rob Jeffries Oct 1 '14 at 22:27

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