History of Solar Oblateness

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(Section 2 of new Nugget)
(Beginning of Section 3, more links)
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([http://sprg.ssl.berkeley.edu/~tohban/nuggets/?page=article&article_id=19 a], [http://sprg.ssl.berkeley.edu/~tohban/nuggets/?page=article&article_id=85 b]) thanks to the small optical telescopes used for solar aspect sensing.
([http://sprg.ssl.berkeley.edu/~tohban/nuggets/?page=article&article_id=19 a], [http://sprg.ssl.berkeley.edu/~tohban/nuggets/?page=article&article_id=85 b]) thanks to the small optical telescopes used for solar aspect sensing.
These observations are [http://en.wikipedia.org/wiki/The_Three_Princes_of_Serendip serendipitous], as are those of [http://sohowww.nascom.nasa.gov/ SOHO/MDI] and soon those of [http://hmi.stanford.edu/ SDO/HMI].
These observations are [http://en.wikipedia.org/wiki/The_Three_Princes_of_Serendip serendipitous], as are those of [http://sohowww.nascom.nasa.gov/ SOHO/MDI] and soon those of [http://hmi.stanford.edu/ SDO/HMI].
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But there is also to be a dedicated space observatory actually <i>designed</i> for solar global observations, [http://www.tor.viarouge.net/PICARD/index.htm Picard], and we expect that it will make the definitive measurements over the next few years following its anticipated launch in June 2010.
+
Other dedicated space observatories are being actually <i>designed</i> for solar global observations, and we expect that these will make definitive measurements over the next few years following its anticipated launch in June 2010.
 +
In related fields we note the tremendous success of the photometry/[http://en.wikipedia.org/wiki/Asteroseismology asteroseismology] missions [http://www.astro.ubc.ca/MOST/ MOST], [http://132.149.11.177/COROT/GP_satellite.htm Corot], and [http://kepler.nasa.gov/ Kepler].  
The purpose of this Nugget is to show off some of the historical overview from [1], and to remind Nugget readers of the basic physics of the oblateness measurement.  
The purpose of this Nugget is to show off some of the historical overview from [1], and to remind Nugget readers of the basic physics of the oblateness measurement.  
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[http://en.wikipedia.org/wiki/Astrometry Astrometry] is one of the classical branches of astronomy, and precise determinations of the shape of the Sun make use of highly specialized techniques with similar fundamental problems of measurement.
[http://en.wikipedia.org/wiki/Astrometry Astrometry] is one of the classical branches of astronomy, and precise determinations of the shape of the Sun make use of highly specialized techniques with similar fundamental problems of measurement.
The oblateness of the Sun is normally defined as the (normalized) difference between equatorial and polar radii, and so it is a differential measurement as opposed to the absolute determination of the radius (or diameter), which is obviously much harder.
The oblateness of the Sun is normally defined as the (normalized) difference between equatorial and polar radii, and so it is a differential measurement as opposed to the absolute determination of the radius (or diameter), which is obviously much harder.
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[http://www.tor.viarouge.net/PICARD/index.htm Picard] will make the first optimized absolute measurements from space, and the oblateness (and other shape features) an interesting and highly important byproduct of the measurement.
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The RHESSI, SOHO, and now SDO measurements cannot be regarded as absolute and are restricted to shape alone, but we look forward to optimized and more direct measurements that can precisely determine the radius as well as the shape.
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The non-optimized RHESSI, SOHO, and now SDO measurements cannot be regarded as absolute and are restricted to shape alone.
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[[Image:126fig1.jpg|right|thumb|250px|Figure 1: Cross-section of the Sun through its rotation axis, showing its basically circular shape (red) with a small distortion induced by the rotation (blue).
 +
The radial scale has been magnified enormously as indicated.
 +
The points show actual RHESSI data as described in the text.]]
The shape of the Sun, or any star, reflects what is going on inside it.
The shape of the Sun, or any star, reflects what is going on inside it.
Most trivially it is rotating, and so an equatorial bulge should appear.
Most trivially it is rotating, and so an equatorial bulge should appear.
 +
This would be the main source of non-circularity sketched in Figure 1.
The Sun rotates slowly, and so this bulge is small (by one prediction, 7.98 mas, where the common unit mas is a milli-arc-sec, or closely 1 part per million of the
The Sun rotates slowly, and so this bulge is small (by one prediction, 7.98 mas, where the common unit mas is a milli-arc-sec, or closely 1 part per million of the
full radius).
full radius).
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recent [http://sprg.ssl.berkeley.edu/~tohban/wiki/index.php/Cycle_24_-_don%27t_panic_yet%21 anomalous minimum].
recent [http://sprg.ssl.berkeley.edu/~tohban/wiki/index.php/Cycle_24_-_don%27t_panic_yet%21 anomalous minimum].
This could well have detectable effects at the surface of the Sun other than the subtle motions of magnetic elements that trace out the flow associated with the cycle.
This could well have detectable effects at the surface of the Sun other than the subtle motions of magnetic elements that trace out the flow associated with the cycle.
 +
 +
== A history of the measurements ==
 +
 +
Early measurements of solar oblateness used the classical techniques of astrometry such as the measurement carefully exposed photographic plates, or visual observations through transit measurements or via eyepieces specifically designed for this purpose.
 +
The [http://en.wikipedia.org/wiki/Astrolabe astrolabe] can also be adapted for measurements of the solar diameter, and can be instrumented with modern detectors as well.
 +
The modern era of solar oblateness measurements can be said to have begun with [http://en.wikipedia.org/wiki/Robert_H._Dicke Dicke], who wished to make measurements sufficiently accurate to test rival theories of [http://en.wikipedia.org/wiki/General_relativity general relativity].
 +
Dicke's ground-based telescope contained several innovations designed to make the measurement differential in nature and as immune as possible to [http://www.schoolsobservatory.org.uk/astro/tels/seeing.shtml "seeing"] conditions.
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The device largely succeeded in this, especially after it was moved from Princeton, New Jersey, to a better site in California.
 +
It also inspired other instrumental development.
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== Conclusions ==
== References ==
== References ==
-
[http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6VHB-4YJ4NKR-4&_user=121723&_coverDate=03%2F06%2F2010&_alid=1307462546&_rdoc=2&_fmt=high&_orig=search&_cdi=6062&_sort=r&_docanchor=&view=c&_ct=5&_acct=C000009999&_version=1&_urlVersion=0&_userid=121723&md5=0f904d940638a417b1641698fd30b82f A brief history of the solar oblateness. A review]
+
[1] [http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6VHB-4YJ4NKR-4&_user=121723&_coverDate=03%2F06%2F2010&_alid=1307462546&_rdoc=2&_fmt=high&_orig=search&_cdi=6062&_sort=r&_docanchor=&view=c&_ct=5&_acct=C000009999&_version=1&_urlVersion=0&_userid=121723&md5=0f904d940638a417b1641698fd30b82f A brief history of the solar oblateness. A review]

Revision as of 17:53, 25 April 2010


Nugget
Number: 126
1st Author: Hugh Hudson
2nd Author: Jean-Pierre Rozelot
Published: 2010 April 26
Next Nugget: TBD
Previous Nugget: The Masuda Flare Revisited
List all



Contents

Introduction

Exact measurements of the shape of the Sun have a history extending well back into the 19th century (for full details, see reference [1]), and RHESSI is playing a small role in this continuing history (see two earlier nuggets (a, b) thanks to the small optical telescopes used for solar aspect sensing. These observations are serendipitous, as are those of SOHO/MDI and soon those of SDO/HMI. Other dedicated space observatories are being actually designed for solar global observations, and we expect that these will make definitive measurements over the next few years following its anticipated launch in June 2010. In related fields we note the tremendous success of the photometry/asteroseismology missions MOST, Corot, and Kepler.

The purpose of this Nugget is to show off some of the historical overview from [1], and to remind Nugget readers of the basic physics of the oblateness measurement. RHESSI has just completed its observations through the remarkable recent solar minimum and we see no particular reason why its data should not continue well into the operational lifetimes of SDO and Picard.

Why is the oblateness interesting?

Astrometry is one of the classical branches of astronomy, and precise determinations of the shape of the Sun make use of highly specialized techniques with similar fundamental problems of measurement. The oblateness of the Sun is normally defined as the (normalized) difference between equatorial and polar radii, and so it is a differential measurement as opposed to the absolute determination of the radius (or diameter), which is obviously much harder. The RHESSI, SOHO, and now SDO measurements cannot be regarded as absolute and are restricted to shape alone, but we look forward to optimized and more direct measurements that can precisely determine the radius as well as the shape.

Figure 1: Cross-section of the Sun through its rotation axis, showing its basically circular shape (red) with a small distortion induced by the rotation (blue). The radial scale has been magnified enormously as indicated. The points show actual RHESSI data as described in the text.

The shape of the Sun, or any star, reflects what is going on inside it. Most trivially it is rotating, and so an equatorial bulge should appear. This would be the main source of non-circularity sketched in Figure 1. The Sun rotates slowly, and so this bulge is small (by one prediction, 7.98 mas, where the common unit mas is a milli-arc-sec, or closely 1 part per million of the full radius). More rapidly rotating stars can have substantial bulge, which leads to the interesting effect known as the von Zeipel theorem which implies that there is also a substantial dimming of the surface brightness at the equator of such a star. In the case of the Sun both rotational ellipticity and rotational dimming are tiny and therefore a wonderful challenge for observers.

Many other potential mechanisms could change the shape of the Sun. The surface is known to rotate differentially in the sense that the equator region has an angular velocity greater than the polar regions. There must be internal differential rotation as well, although it is increasingly well constrained by helioseismology, and what we see at the surface may conceal mass internal mass distributions that can affect the shape. Such effects are related to tests of relativity theory, from which Einstein famously explained the anomalous perihelion precession of Mercury. This was an amazing coup of 19th-century astrometry. Finally there are links to the solar cycle; for example a north-south meridional circulation is required to explain the 11-year cycle - a hot research topic nowadays because of the recent anomalous minimum. This could well have detectable effects at the surface of the Sun other than the subtle motions of magnetic elements that trace out the flow associated with the cycle.

A history of the measurements

Early measurements of solar oblateness used the classical techniques of astrometry such as the measurement carefully exposed photographic plates, or visual observations through transit measurements or via eyepieces specifically designed for this purpose. The astrolabe can also be adapted for measurements of the solar diameter, and can be instrumented with modern detectors as well. The modern era of solar oblateness measurements can be said to have begun with Dicke, who wished to make measurements sufficiently accurate to test rival theories of general relativity. Dicke's ground-based telescope contained several innovations designed to make the measurement differential in nature and as immune as possible to "seeing" conditions. The device largely succeeded in this, especially after it was moved from Princeton, New Jersey, to a better site in California. It also inspired other instrumental development.


Conclusions

References

[1] A brief history of the solar oblateness. A review

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