July 31, 2014

The figure of the Sun

The Sun’s rotation on its axis causes a flattening of its polar regions. This is the result of the rotation of the Sun’s interior and the distribution of its mass as a function of depth. Observations show a flattening that extends from 8.8 10 -6 (measured by SDS (Solar Disk Sextant) from a stratospheric balloon, by Lydon and Sofia, 1996) to 1.1 10 -5 measured from the ground (Bursa, 1986). The MDI instrument on the SOHO spacecraft has measured it as 9.8 10 -6 (Kuhn, 1998). These values show that the difference in diameter between the equator and poles extends from 17 to 22 mas, so we need to measure it more precisely and study to what extent it is related to solar activity. Ground-based measurements have also shown a deviation from the ellipsoid. Rozelot et al. (2003) used a heliometer on the Pic du Midi in the French Pyrenees to measure an increase in diameter between the heliographic latitudes of 10° and 30° of the order of 20 mas, against an almost constant value above 50°. This result agrees with the measurements obtained by Reis-Neto et al. (2003). However, neither MDI in orbit nor SDS from a stratospheric balloon measured such values. Several explanations have been postulated for this disagreement.