Evidence for the Magneto-rotational Instability in the Solar Magnetic Cycle*

The Sun’s magnetic cycle displays a pattern of propagating sunspots, starting around 30^○ latitude and annihilating near the equator 11 years later. Relative longitudinal flows, called torsional oscillations, track sunspot migration and undoubtedly share a common cause. Notably, helioseismology reveals that low-latitude torsional oscillations only occur within the outer 5–10% in radius, coinciding with an inwardly increasing angular velocity called the Near-Surface Shear Layer (NSSL). Negative differential rotation gradient of sufficient strength with a polar magnetic field signifies the Magneto-Rotational Instability (MRI)—crucial in astrophysical accretion disks. Together, these two facts address the general question: where and how is the solar dynamo operating? Here, we provide evidence that the MRI operates within the NSSL and is essential to understanding the solar dynamo.