The solar dynamo begins near the surface.
basic_science · Level V
Where this comes from
- Record sourced from PubMed, PMID 38778233.
- Also identified by DOI 10.1038/s41586-024-07315-1 and PMC identifier 11111411.
- Licence recorded as CC BY.
- The licence permits redistribution, so the abstract is shown in full and the full text is available from the publisher.
Abstract
The magnetic dynamo cycle of the Sun features a distinct pattern: a propagating region of sunspot emergence appears around 30° latitude and vanishes near the equator every 11 years (ref. <sup>1</sup>). Moreover, longitudinal flows called torsional oscillations closely shadow sunspot migration, undoubtedly sharing a common cause<sup>2</sup>. Contrary to theories suggesting deep origins of these phenomena, helioseismology pinpoints low-latitude torsional oscillations to the outer 5-10% of the Sun, the near-surface shear layer<sup>3,4</sup>. Within this zone, inwardly increasing differential rotation coupled with a poloidal magnetic field strongly implicates the magneto-rotational instability<sup>5,6</sup>, prominent in accretion-disk theory and observed in laboratory experiments<sup>7</sup>. Together, these two facts prompt the general question: whether the solar dynamo is possibly a near-surface instability. Here we report strong affirmative evidence in stark contrast to traditional models<sup>8</sup> focusing on the deeper tachocline. Simple analytic estimates show that the near-surface magneto-rotational instability better explains the spatiotemporal scales of the torsional oscillations and inferred subsurface magnetic field amplitudes<sup>9</sup>. State-of-the-art numerical simulations corroborate these estimates and reproduce hemispherical magnetic current helicity laws<sup>10</sup>. The dynamo resulting from a well-understood near-surface phenomenon improves prospects for accurate predictions of full magnetic cycles and space weather, affecting the electromagnetic infrastructure of Earth.