Bipolar magnetic spots from dynamos in stratified spherical shell turbulence

Sarah Jabbari, Axel Brandenburg, Nathan Kleeorin, Dhrubaditya Mitra, Igor Rogachevskii

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11 Citations (Scopus)

Abstract

Recent work by Mitra et al. (2014) has shown that in strongly stratified forced two-layer turbulence with helicity and corresponding large-scale dynamo action in the lower layer, and nonhelical turbulence in the upper, a magnetic field occurs in the upper layer in the form of sharply bounded bipolar magnetic spots. Here we extend this model to spherical wedge geometry covering the northern hemisphere up to 75° latitude and an azimuthal extent of 180°. The kinetic helicity and therefore also the large-scale magnetic field are strongest at low latitudes. For moderately strong stratification, several bipolar spots form that eventually fill the full longitudinal extent. At early times, the polarity of spots reflects the orientation of the underlying azimuthal field, as expected from Parker's Ω-shaped flux loops. At late times their tilt changes such that there is a radial field of opposite orientation at different latitudes separated by about 10°. Our model demonstrates the spontaneous formation of spots of sizes much larger than the pressure scale height. Their tendency to produce filling factors close to unity is argued to be reminiscent of highly active stars. We confirm that strong stratification and strong scale separation are essential ingredients behind magnetic spot formation, which appears to be associated with downflows at larger depths.

Original languageEnglish
Article number166
Number of pages11
JournalThe Astrophysical Journal
Volume805
Issue number2
DOIs
Publication statusPublished - 1 Jun 2015
Externally publishedYes

Keywords

  • dynamo
  • magnetohydrodynamics (MHD)
  • Sun: activity
  • turbulence

Cite this

Jabbari, S., Brandenburg, A., Kleeorin, N., Mitra, D., & Rogachevskii, I. (2015). Bipolar magnetic spots from dynamos in stratified spherical shell turbulence. The Astrophysical Journal, 805(2), [166]. https://doi.org/10.1088/0004-637X/805/2/166