Variations of Solar Oblateness with the 22 yr Magnetic Cycle Explain Apparently Inconsistent Measurements

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. Special images acquired for almost the whole Cycle 24 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 for Cycle 23. More generally, the trend of both in phase variation during odd cycles and anti-phase variation during even cycles is confirmed when revisiting past measurements. Therefore, it is possible that the sun initiates a physical process resulting in a pulsation with the 22yr magnetic cycle having extreme values during the polarity reversals with a maximum swelling during odd cycles and vice versa for even ones. This oscillation could explain the controversy surrounding past measurements.