The solar dynamo begins near the surface

Geoffrey M. Vasil (), Daniel Lecoanet, Kyle Augustson, Keaton J. Burns, Jeffrey S. Oishi, Benjamin P. Brown, Nicholas Brummell and Keith Julien
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Geoffrey M. Vasil: University of Edinburgh
Daniel Lecoanet: Northwestern University
Kyle Augustson: Northwestern University
Keaton J. Burns: Massachusetts Institute of Technology
Jeffrey S. Oishi: Bates College
Benjamin P. Brown: University of Colorado Boulder
Nicholas Brummell: University of California Santa Cruz
Keith Julien: University of Colorado Boulder

Nature, 2024, vol. 629, issue 8013, 769-772

Abstract: 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. 1). Moreover, longitudinal flows called torsional oscillations closely shadow sunspot migration, undoubtedly sharing a common cause2. 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 layer3,4. Within this zone, inwardly increasing differential rotation coupled with a poloidal magnetic field strongly implicates the magneto-rotational instability5,6, prominent in accretion-disk theory and observed in laboratory experiments7. 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 models8 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 amplitudes9. State-of-the-art numerical simulations corroborate these estimates and reproduce hemispherical magnetic current helicity laws10. 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.

Date: 2024
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DOI: 10.1038/s41586-024-07315-1

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