The Precise Solar Shape and Its Variability

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Science  16 Aug 2012:
DOI: 10.1126/science.1223231

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The precise shape of the Sun has not been convincingly determined, despite half a century of modern photoelectric observations. The expected deviation of the solar limb shape from a perfect circle is very small, but such asphericity is sensitive to the Sun's otherwise invisible interior conditions as well as the solar atmosphere. From a long-running, space-based experiment, we show that, when analyzed with sufficiently high spatial resolution, the Sun's oblate shape is remarkably constant and almost completely unaffected by the solar cycle variability seen on its surface. The solar oblateness is substantially lower than theoretical expectations by an amount that could be accounted for by a slower differential rotation in the outer few percent of the Sun.

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