MAGNETIC CYCLES IN A CONVECTIVE DYNAMO SIMULATION OF A YOUNG SOLAR-TYPE STAR

By Benjamin P. Brown, Mark S. Miesch, M. Browning, A. Brun, J. Toomre. 2011

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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 Ω-effect, while the mean poloidal fields are built by turbulent correlations which are not well represented by a simple α-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.

Subfields: Astronomy and Astrophysics, Molecular Biology

View Paper: https://www.semanticscholar.org/paper/a3505b85ba078e3e3f715c98e9c97a350710b569

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Citations in the corpus

Solar Dynamo Theory (2014, methodology): “…with angular velocity isocontours approximately aligned with the rotational axis, but now it is a pole-to-equator angular velocity difference markedly smaller than solar (cf. Käpylä et al. 2010, their figure 1; Brown et al. 2011, their figure 3c; and Racine et al. 2011, their figure 4).”
The Maunder minimum (1645–1715) was indeed a grand minimum: A reassessment of multiple datasets (2015, background): “Although modern solar-type dynamo models reproduce the “regular” part of the behaviour of the large-scale solar magnetic ﬁeld (Ghizaru et al. 2010; Brown et al. 2011; Schrinner et al. 2012; Gastine et al. 2012; Käpylä et al. 2012), it is still challenging to extend the integration time over several…”
CYCLIC MAGNETIC ACTIVITY DUE TO TURBULENT CONVECTION IN SPHERICAL WEDGE GEOMETRY (2012, background): “2010), and at even more rapid rotation, poleward migrating activity is found (Käpylä et al. 2010; Brown et al. 2011).”
MAGNETIC WREATHS AND CYCLES IN CONVECTIVE DYNAMOS (2012, methodology): “The wreaths in case D3 are discussed extensively by Brown et al. (2010) and additional wreaths are analyzed at somewhat faster rotation rate (5Ω⊙) by Brown et al. (2011).”