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Published Online October 2, 2008
Science DOI: 10.1126/science.1160863

Reports

Submitted on May 22, 2008
Accepted on September 24, 2008

A Large Excess in Apparent Solar Oblateness Due to Surface Magnetism

Martin D. Fivian 1*, Hugh S. Hudson 1, Robert P. Lin 2, H. Jabran Zahid 3

1 Space Sciences Laboratory, University of California, Berkeley, Berkeley, CA, USA.
2 Space Sciences Laboratory, University of California, Berkeley, Berkeley, CA, USA.; Physics Department, University of California, Berkeley, Berkeley, CA, USA.
3 Space Sciences Laboratory, University of California, Berkeley, Berkeley, CA, USA.; Institute for Astronomy, University of Hawaii, Manoa.

* To whom correspondence should be addressed.
Martin D. Fivian , E-mail: mfivian{at}ssl.berkeley.edu

The shape of the Sun subtly reflects its rotation and internal flows. The surface rotation rate, about 2 km s-1 at the equator, predicts an oblateness (equator-pole radius difference) of 7.8 mas (milli arc sec), or about 0.001%. Observations from the RHESSI (The Reuven Ramaty High-Energy Solar Spectroscopic Imager) satellite show unexpectedly large flattening relative to the expectation from surface rotation. This excess is dominated by the quadrupole term and gives a total oblateness of 10.77 ± 0.44 mas. The position of the limb correlates with a sensitive EUV proxy, the 284 Å limb brightness. We relate the larger radius values to magnetic elements in the enhanced network, and use the correlation to correct for it as a systematic error term in the oblateness measurement. The corrected oblateness of the nonmagnetic Sun is 8.01 ± 0.14 mas, near the value expected from rotation.


THIS ARTICLE HAS BEEN CITED BY OTHER ARTICLES:
Comment on "A Large Excess in Apparent Solar Oblateness Due to Surface Magnetism".
J. R. Kuhn, M. Emilio, and R. Bush (2009)
Science 324, 1143
   Abstract »    Full Text »    PDF »
Response to Comment on "A Large Excess in Apparent Solar Oblateness Due to Surface Magnetism".
M. D. Fivian, H. S. Hudson, R. P. Lin, and H. J. Zahid (2009)
Science 324, 1143
   Abstract »    Full Text »    PDF »


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Science. ISSN 0036-8075 (print), 1095-9203 (online)