Solar oblateness variations in phase with the 22 year-magnetic cycle

The solar oblateness results from distortion processes due to several phenomena inside the Sun but also induced by the centrifugal potential of the surface rotation. This fundamental parameter is therefore of great scientific interest, but its measurements for more than a century are still very controversial, whether for its average value and/or its variations observed or not over time. Images acquired for almost the whole Cycle 24 during the roll calibration mode by the Helioseismic and Magnetic Imager (HMI) onboard the Solar Dynamic Observatory (SDO) are used for calculating solar oblateness. The average oblateness obtained is 8.8+/-0.8 milli-arcseconds in good agreement with measurements of the last two decades. Variations are observed in anti-phase with the solar activity during cycle 24 whereas they were in phase with activity of Cycle 23. More generally, the trend of both in phase variation during odd cycles and anti-phase variation during even cycles is also confirmed when revisiting past measurements. We give an overview of the main issues raised by solar oblateness, present the data used and processing method, and discuss the major results of this study.