The solar dynamo begins near the surface.

Academic Article

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.
  • Authors

  • Vasil, Geoffrey M
  • Lecoanet, Daniel
  • Augustson, Kyle
  • Burns, Keaton J
  • Oishi, Jeff
  • Brown, Benjamin P
  • Brummell, Nicholas
  • Julien, Keith
  • Status

    Publication Date

  • May 2024
  • Published In

  • Nature  Journal
  • Digital Object Identifier (doi)

    Start Page

  • 769
  • End Page

  • 772
  • Volume

  • 629
  • Issue

  • 8013