On the Cause of Solar-Like Equatorward Migration in Global Convective Dynamo Simulations

Warnecke, Jörn; Käpylä, Petri J.; Käpylä, Maarit J.; Brandenburg, Axel

doi:10.1088/2041-8205/796/1/l12

Cited by 54 publications

(97 citation statements)

References 30 publications

(63 reference statements)

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“…More precisely, the sign of the latitudinal propagation of a dynamo wave is opposite to the sign required for both the equatorward migration of low-latitude magnetic structures and the poleward propagation of magnetic field as seen in Figure 10. So, although the Parker-Yoshimura mechanism seems to be at work in other convective dynamo simulations (Racine et al 2011;Warnecke et al 2014), it does not appear to be operating here.…”

Section: Kinematic Versus Nonlinear Dynamo Wavesmentioning

confidence: 71%

“…Augustson et al (2014) demonstrated that the kinematic expression in Equation (19) with a defined using only the kinetic helicity fails to explain the equatorward propagation seen in this dynamo simulation. The current helicity can in principle reverse the sign of the scalar α-effect (Warnecke et al 2014). For instance, it has recently been shown that the direction of the propagation of the magnetic field through a cycle can depend upon the Parker-Yoshimura mechanism (e.g., Racine et al 2011;Käpylä et al 2013;Warnecke et al 2014).…”

Section: Kinematic Versus Nonlinear Dynamo Wavesmentioning

confidence: 99%

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Grand Minima and Equatorward Propagation in a Cycling Stellar Convective Dynamo

Augustson

Brun

Miesch

et al. 2015

ApJ

113

145

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The 3D MHD Anelastic Spherical Harmonic code, using slope-limited diffusion, is employed to capture convective and dynamo processes achieved in a global-scale stellar convection simulation for a model solar-mass star rotating at three times the solar rate. The dynamo-generated magnetic fields possesses many timescales, with a prominent polarity cycle occurring roughly every 6.2 years. The magnetic field forms large-scale toroidal wreaths, whose formation is tied to the low Rossby number of the convection in this simulation. The polarity reversals are linked to the weakened differential rotation and a resistive collapse of the large-scale magnetic field. An equatorial migration of the magnetic field is seen, which is due to the strong modulation of the differential rotation rather than a dynamo wave. A poleward migration of magnetic flux from the equator eventually leads to the reversal of the polarity of the high-latitude magnetic field. This simulation also enters an interval with reduced magnetic energy at low latitudes lasting roughly 16 years (about 2.5 polarity cycles), during which the polarity cycles are disrupted and after which the dynamo recovers its regular polarity cycles. An analysis of this grand minimum reveals that it likely arises through the interplay of symmetric and antisymmetric dynamo families. This intermittent dynamo state potentially results from the simulation's relatively low magnetic Prandtl number. A mean-field-based analysis of this dynamo simulation demonstrates that it is of the α-Ω type. The timescales that appear to be relevant to the magnetic polarity reversal are also identified.

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Section: Kinematic Versus Nonlinear Dynamo Wavesmentioning

confidence: 71%

Section: Kinematic Versus Nonlinear Dynamo Wavesmentioning

confidence: 99%

Grand Minima and Equatorward Propagation in a Cycling Stellar Convective Dynamo

Augustson

Brun

Miesch

et al. 2015

ApJ

113

145

View full text Add to dashboard Cite

show abstract

“…It has been argued that such dynamo solutions are Parker waves that are driven by the strong DR produced in the convection zone (for example, see Busse & Simitev 2006;Warnecke et al 2014). Indeed, such cyclic dynamo solutions are always accompanied by a larger DR (Busse & Simitev 2006;Browning 2008;Yadav et al 2013a) as compared to their quasi-steady dipoledominant counterparts where DR is highly quenched (Aubert 2005;Yadav et al 2013aYadav et al , 2015b.…”

Section: Resultsmentioning

confidence: 99%

Magnetic Cycles in a Dynamo Simulation of Fully Convective M-Star Proxima Centauri

Yadav

Christensen

Wolk

et al. 2016

ApJL

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The recent discovery of an Earth-like exoplanet around Proxima Centauri has shined a spot light on slowly rotating fully convective M-stars. When such stars rotate rapidly (period 20 days), they are known to generate very high levels of activity that is powered by a magnetic field much stronger than the solar magnetic field. Recent theoretical efforts are beginning to understand the dynamo process that generates such strong magnetic fields. However, the observational and theoretical landscape remains relatively uncharted for fully convective M-stars that rotate slowly. Here, we present an anelastic dynamo simulation designed to mimic some of the physical characteristics of Proxima Centauri, a representative case for slowly rotating fully convective M-stars. The rotating convection spontaneously generates differential rotation in the convection zone that drives coherent magnetic cycles where the axisymmetric magnetic field repeatedly changes polarity at all latitudes as time progress. The typical length of the "activity" cycle in the simulation is about nine years, in good agreement with the recently proposed activity cycle length of about seven years for Proxima Centauri. Comparing our results with earlier work, we hypothesis that the dynamo mechanism undergoes a fundamental change in nature as fully convective stars spin down with age.

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“…In the previous numerical studies of compressible thermal convection with Pr larger than unity, the thermal diffusion has been treated isotropically (Warnecke et al 2014;O'Mara et al 2016;Käpylä et al 2017a), and thus, all of the thermal effects described above are simultaneously included. However, recent MHD simulations revealed that the small-scale magnetism tends to make the small-scale motions highly anisotropic, suggesting that the SGS turbulent transport coefficients should also be anisotropic: More precisely, a main decrease in κ H is inferred (Hotta et al 2015a).…”

Section: Effects Of the Decrease In The Effective Thermal Diffusivitymentioning

confidence: 99%

Convective Velocity Suppression via the Enhancement of the Subadiabatic Layer: Role of the Effective Prandtl Number

Bekki

Hotta

Yokoyama

2017

ApJ

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It has recently been recognized that the convective velocities achieved in the current solar convection simulations might be over-estimated. The newly-revealed effects of the prevailing small-scale magnetic field within the convection zone may offer possible solutions to this problem. The small-scale magnetic fields can reduce the convective amplitude of small-scale motions through the Lorentz-force feedback, which concurrently inhibits the turbulent mixing of entropy between upflows and downflows. As a result, the effective Prandtl number may exceed unity inside the solar convection zone. In this paper, we propose and numerically confirm a possible suppression mechanism of convective velocity in the effectively high-Prandtl number regime. If the effective horizontal thermal diffusivity decreases (the Prandtl number accordingly increases), the subadiabatic layer which is formed near the base of the convection zone by continuous depositions of low entropy transported by adiabatically downflowing plumes is enhanced and extended. The global convective amplitude in the high-Prandtl thermal convection is thus reduced especially in the lower part of the convection zone via the change in the mean entropy profile which becomes more subadiabatic near the base and less superadiabatic in the bulk.

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On the Cause of Solar-Like Equatorward Migration in Global Convective Dynamo Simulations

Cited by 54 publications

References 30 publications

Grand Minima and Equatorward Propagation in a Cycling Stellar Convective Dynamo

Grand Minima and Equatorward Propagation in a Cycling Stellar Convective Dynamo

Magnetic Cycles in a Dynamo Simulation of Fully Convective M-Star Proxima Centauri

Convective Velocity Suppression via the Enhancement of the Subadiabatic Layer: Role of the Effective Prandtl Number

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