期刊
ASTROPHYSICAL JOURNAL
卷 731, 期 1, 页码 -出版社
IOP PUBLISHING LTD
DOI: 10.1088/0004-637X/731/1/69
关键词
convection; magnetohydrodynamics (MHD); stars: interiors; stars: magnetic field; stars: rotation; Sun: interior
资金
- NASA [NNG05G124G, NNX08AI57G, NNG05GN08H, NNH09AK14I]
- NSF [AST 09-02004, PHY 08-21899]
- Jeffrey L. Bishop fellowship at CITA
- CNRS/INSU (France)
- European Research Council [207430]
- PSC
- SDSC
- TACC
- NICS
- Div Atmospheric & Geospace Sciences
- Directorate For Geosciences [0929367] Funding Source: National Science Foundation
- Division Of Astronomical Sciences
- Direct For Mathematical & Physical Scien [0902004] Funding Source: National Science Foundation
- European Research Council (ERC) [207430] Funding Source: European Research Council (ERC)
Young solar-type stars rotate rapidly and many are magnetically active. Some appear to undergo magnetic cycles similar to the 22 yr solar activity cycle. We conduct simulations of dynamo action in rapidly rotating suns with the three-dimensional magnetohydrodynamic anelastic spherical harmonic (ASH) code to explore dynamo action achieved in the convective envelope of a solar-type star rotating at five times the current solar rotation rate. We find that dynamo action builds substantial organized global-scale magnetic fields in the midst of the convection zone. Striking magnetic wreaths span the convection zone and coexist with the turbulent convection. A surprising feature of this wreath-building dynamo is its rich time dependence. The dynamo exhibits cyclic activity and undergoes quasi-periodic polarity reversals where both the global-scale poloidal and toroidal fields change in sense on a roughly 1500 day timescale. These magnetic activity patterns emerge spontaneously from the turbulent flow and are more organized temporally and spatially than those realized in our previous simulations of the solar dynamo. We assess in detail the competing processes of magnetic field creation and destruction within our simulations that contribute to the global-scale reversals. We find that the mean toroidal fields are built primarily through an Omega-effect, while the mean poloidal fields are built by turbulent correlations which are not well represented by a simple alpha-effect. During a reversal the magnetic wreaths propagate toward the polar regions, and this appears to arise from a poleward propagating dynamo wave. As the magnetic fields wax and wane in strength and flip in polarity, the primary response in the convective flows involves the axisymmetric differential rotation which varies on similar timescales. Bands of relatively fast and slow fluid propagate toward the poles on timescales of roughly 500 days and are associated with the magnetic structures that propagate in the same fashion. In the Sun, similar patterns are observed in the poleward branch of the torsional oscillations, and these may represent poleward propagating magnetic fields deep below the solar surface.
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