A changing solar shape

The Sun's shape is sensitive to the influence of gravity, rotation, and local turbulence and magnetic fields in its outer atmosphere. A careful measurement of this shape has long been sought to better understand the solar structure and its change during the 11 yr solar cycle. Numerous disparate measurements of the solar oblateness or the fractional difference between equatorial and polar radii have been difficult to interpret, in part because this quantity is much smaller than terrestrial atmospheric seeing and most instrumental noise sources. In 1997 the Michelson Doppler Imager (MDI) aboard the Solar and Heliospheric Observatory (SOHO) obtained a precise measurement of the oblateness from above the atmosphere by utilizing a spacecraft roll procedure to remove instrumental influences. In 2001 this technique was repeated, and we report here on the detection of a time-variable solar shape from these data. The changing oblateness we find from 1997 to 2001 is smaller than the apparent discrepancy between earlier ground-based observations, but is significantly larger than MDI's astrometric measurement uncertainty. The shape change appears to be anticorrelated with the observed helioseismic variability. This fact and our MDI measurements suggest that the outer solar atmosphere expands nonhomologously during the cycle. It is possible that solar cycle changes in the turbulent pressure in the outer atmosphere can account for both the optical limb change and the helioseismic acoustic global solar shape change.