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

Solar oblateness results from distortion processes due to several phenomena inside of the Sun, but it can also be induced by the centrifugal potential of surface rotation. This fundamental parameter is of great scientific interest, yet for more than a century its measurements have remained a controversial topic, whether because of its average value or its variations observed (or not) over time. Special images acquired for almost the whole of Cycle 24 by the Helioseismic and Magnetic Imager on board the Solar Dynamic Observatory are used for calculating solar oblateness. The average oblateness obtained is 8.8 ± 0.8 mas, in good agreement with measurements over the last two decades. Variations are observed in anti-phase with the solar activity during Cycle 24, whereas they were in phase with activity during 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 22 yr magnetic cycle; it has extreme values during the polarity reversals, with a maximum swelling during odd cycles and the opposite for even ones. This oscillation could resolve the controversy surrounding past measurements.