No Iron "Surface" for the Sun
The best explanation for this anomaly is the the sun has a solid surface.
Sun's constant size surprises scientists
(PhysOrg.com) -- A group of astronomers led by the University of Hawaii's Dr. Jeff Kuhn has found that in recent times the sun's size has been remarkably constant. Its diameter has changed by less than one part in a million over the last 12 years. ...
Here is the research paper that is the basis of the news report.
One solar cycle of solar astrometry with MDI/SOHO
Emilio, Kuhn & Bush; Solar and Stellar Variability: Impact on Earth and Planets, Proceedings of the International Astronomical Union, IAU Symposium, Volume 264, p. 21-32, February 2010
Abstract:
In this work we describe the method and results of precise solar astrometry made with the Michelson Doppler Imager (MDI), on board the Solar and Heliospheric Observatory (SOHO), during one complete solar cycle. We measured an upper limit to the solar radius variation, the absolute solar radius value and the solar shape. Our results are 22 mas peak-to-peak upper limit for the solar radius variation over the solar cycle, the absolute radius was measured as 959.28 ± 0.15 arcsec at 1 AU and the difference between polar and equatorial solar radii in 1997 was 5 km and about three times larger in 2001.
And here is the response from
Cuddles:
Nice try, but you seem to have missed a rather important point. If you heat a metre long rod of steel by a single degree*, it will increase in length by around 1.15*10-5. That's a fair bit more than one part in a million. So your claim of a solid iron surface on the Sun would actually result in much, much bigger variations in size than are observed, and so is already known to be completely inconsistent with reality.
*Note that this is at 25oC. Oddly enough, no-one's ever measured the thermal expansion coefficient at 6000o.
I have to agree, although I think the problem for a solid surface is actually rather larger than one would expect simply from this response alone. As has already been pointed out numerous times, a solid or "rigid" iron surface are both strongly ruled out by elementary thermodynamics, a branch of physics which
brantc &
Mozina seem to hold in utter contempt. But it stands unchallenged anyway.
However, compare the results reported above to the results reported here:
On the Variability of the Apparent Solar Radius
Chapman, Dobias & Walton; Astrophysical Journal 681(2): 1689-1702, July 2008
Abstract:
Full-disk photometric solar images at a wavelength of 672.3 nm have been obtained daily since 1986 using the CFDT1 (Cartesian Full Disk Telescope No. 1). An analysis of these images from 1986 through the end of 2004 December has shown a peak-to-peak variation in the geocentric north-south solar radius of 0.136+/-0.01, approximately in phase with the solar cycle. The multiple correlation coefficient squared is R2=0.0404 (R=0.2). While this correlation coefficient is small, due to the large number of data points (N=4042), the level of significance is less than 0.02. The radius had a maximum value near the times of maximum activity for solar cycles 22 and 23.
The
SOHO MDI Instrument observes the sun at a wavelength of 676.78 nm, the center wavelength of a neutral Nickel absorption feature in the photosphere. This wavelength is not too far from the 672.3 nm reported here. Yet the ground based measurements show a variability in apparent solar radius of nearly 14%. These variations are geocentric, so changes due to the changing Earth-sun distance have been removed. These observations appear to me to be inconsistent with the observations reported above by Emilio, Kuhn & Bush. So I will withhold an ultimate judgement until this apparent conflict is resolved.
However, it should be fairly obvious that the solar radius is not fixed, but still variable even in the research pointed to by
brantc. One would not normally appeal to a solid surface to explain a smaller than expected cycle of expansion & contraction. But with a little physical insight, as
Cuddles gives us, normal thermal expansion would actually imply a larger than observed change in radius. But of course there are no measurements of thermal expansion of iron at 6000 Kelvins, since iron is a vapor at such temperatures. So one must wonder exactly why a solid iron surface is supposed to be the "best explanation" for anything that is observed to happen at temperatures where physics requires iron to be a vapor. Hence, I think the rejection of the claim by
brantc is in order.
But see also this ...
Variability of the solar shape (before space dedicated missions)
Rozelot, Damiani & Lefebvre; journal of Atmospheric and Solar-Terrestrial Physics 71(17-18): 1683-1694, December 2009
Abstract:
Shrinking or expansion of the solar shape and irradiance variations are ultimately related to solar activity. We give here a review on existing ground-based or space solar radius measurements, extending the concept to shape changes. We show how helioseismology results allow us to look at the variations below the surface, where changes are not uniform, putting in evidence a new shallow layer, the leptocline, which is the seat of solar asphericities, radius variations with the 11-yr cycle and the cradle of complex physical processes: partial ionization of the light elements, opacities changes, superadiabaticity, strong gradient of rotation and pressure. Based on such physical grounds, we show why it is important to get accurate measurements from scheduled dedicated space missions: PICARD, SDO, DynaMICCS, ASTROMETRIA, SPHERIS. Such measurements will provide us a unique opportunity to study in detail the relationship between global solar properties and changes in the Sun's interior.
Here we see a recent example from a long list of helioseismology research which quite conclusively proves that there cannot be any solid surface below the visible photosphere of the sun. Curiously,
Mozina claims quite the contrary, that helioseismology does reveal a solid, or at least "rigid" surface below the visible photosphere, commonly citing
Lefebvre & Kosovichev, 2005 as a justification, despite the obvious fact that the paper actually
directly rules out a solid or rigid surface of any kind.
So, we have both thermodynamics and helioseismology ruling out a solid or rigid surface. We have
brantc claiming a "solid" surface explains expansion and contraction better than a non-solid surface. And we have
Mozina insisting that the data from helioseismology which rules out a rigid surface actually requires it (and he is apparently the only living person who interprets the data in that way). Any reasonably intelligent person must be quite exasperated by such insane devotion to the physically impossible.