On solar oblateness measurements during the current solar cycle 24
Résumé
The rotation of the Sun on itself involves a flatness of the Polar Regions. The solar oblateness results from the rotation of the whole solar interior and the distribution of its mass according to the depth. Thus, possible diagnostic of the internal rotation is provided by the solar oblateness. The solar oblateness also places constraints on general relativity. Indeed, the modern era of measurements of the solar oblateness began in the 1960s with Dicke’s measurements, which were useful in understanding the perihelion precession of Mercury’s orbit, one of the classical tests of general relativity. Thus, for various reasons, it is necessary to better know the solar oblateness value and to study its dependence with the solar activity. Based on measurements collected from various instruments over the past 50 years, the measured solar equator-to-pole radius difference converges towards 8 mas (near 5.8 km). Now, with space era, we felt it was possible to obtain very accurate measurements of the solar equator-to-pole radius difference and its evolution over time. Thus, we developed an original method to estimate the solar equator-to-pole radius difference from two solar space missions (Solar Dynamics Observatory and PICARD). When analysing the solar radius versus angle data, we observed an anti-correlation between the limb brightness and the radius determined from the inflection point. The apparent radius was smaller if an active region was near the limb. The bright active regions were confined to low latitudes and never occur at the poles. The exact cause of this anti-correlation needs still to be understood but it is clear that it may cause an artefact in the determination of the solar oblateness leading to a negative bias, even if the more active regions were eliminated from the analysis. In this talk, we describe the method, and then present current results about solar oblateness variations after five years of solar observations (from 2010 to 2015) and linkages between measurements and harsh space environment.