Evidence for a solar distortion rotating with a period of 12.2 days

Abstract

Most of the day-to-day fluctuation (originally believed to be error) in the Princeton solar oblateness observations is well described as the signal due to a ∼10 km distortion of the solar photosphere rotating rigidly with a period of 12.22±0.12 days (sidereal). This ‘solar-rotator’ signal is well-defined by the observations and seems clearly significant statistically. Owing to the apparent precession of the solar spin-axis on the sky, the signal is not strictly periodic and has a wave-form that evolves with time in a predictable way. The previously discussed enigmatic periodicity of the oblateness residuals, for which no non-solar sources were found, is eliminated when the solar-rotator signal is subtracted. The residual errors are then found to be normally distributed and uncorrelated. Including the solar-rotator signal in the least-square fit increases the implied static oblateness by 16% to Δr = 45.8±3.3 millisec (equatorial excess radius). Both the static oblateness signal and the solar-rotator signal might be interpreted phenomenonologically as ‘brightness’ signals due to a rigidly rotating non-uniform distribution of temperature in the upper photosphere. However, such a description encounters physical difficulties when the requirements of energy and momentum balance are considered. Including the excess static oblateness yields a distortion in the form of an ellipoid whose major axis is tilted 85° from the solar axis. Possible conflicts with other recent observations are discussed.