Measurements of the shape of the Sun, and in particular the solar oblateness, have been published and debated for nearly a century. An unexpected capability of the Michelson Doppler Imager aboard the Solar and Heliospheric Observatory (SoHO) has been the ability to precisely determine the solar limb shape and brightness. The stable thermal environment of the SoHO experiment at the L1 Earth-Sun Lagrange point has allowed NSO scientists, in collaboration with Stanford and Lockheed experiment PIs, to obtain the most accurate measurement of the solar oblateness and hexadecapole shape terms. From an experiment performed last March where the SoHO spacecraft was rotated in fixed angular increments around the telescope axis, the difference between solar equatorial and polar radii associated with the static oblateness was derived to be 8.07 ± 0.58 milliarcsec. For the first time a significant hexadecapole (l = 4) solar shape distortion of 1.4 ± 0.54 milliarcsec was also determined. The measured oblateness tends to rule out the possibility of a rapidly rotating solar core although it is not yet clear if the derived hexadecapole shape is consistent with "standard solar models." An extra bonus from the solar limb astrometry experiment has been a precise determination of solar latitudinal color temperature variations. The March experiment also confirmed earlier ground-based experiments that indicated that the solar photosphere is about 1 degree hotter near the poles and equator than it is near mid latitudes.
In a related analysis, the time variable limb shape (on timescales from minutes to hours) appears to be dominated only by statistical noise sources down to limb shape change amplitudes of a few microarcseconds. Analysis of an early one-month sample of SoHO/MDI data rules out the possibility of solar g-modes with corresponding surface velocity amplitudes larger than a few mm/s.
Jeff Kuhn