IfA-08-162

A Large Excess in Apparent Solar Oblateness Due to Surface Magnetism

Martin D. Fivian
Space Sciences Laboratory, University of California, Berkeley
mfivian@ssl.berkeley.edu

Hugh S. Hudson
Space Sciences Laboratory, University of California, Berkeley

Robert P. Lin
Space Sciences Laboratory and Physics Department, University of California, Berkeley

H. Jabran Zahid
Space Sciences Laboratory, University of California, Berkeley and Institute for Astronomy, University of Hawaii at Manoa

To appear in Science

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.