Influence of a coronal envelope as a free boundary to global convective dynamo simulations

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

doi:10.1051/0004-6361/201526131

Cited by 35 publications

(61 citation statements)

References 90 publications

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“…Generally, Set E behaves similarly as Sets A-C, that is, a transition from oscillatory dynamos at intermediate Re M to quasi-stationary or irregularly varying solutions at high Re M . However, Run B1 of Warnecke et al (2016a) is similar to Run E2, but with Pr SGS = 2 instead of 1 shows EW. Set F is qualitatively different from the other sets similarly as for the differential rotation.…”

Section: Dynamo Modesmentioning

confidence: 72%

“…The simulations of Käpylä et al (2012), Augustson et al (2015), Käpylä et al (2016a), and Warnecke et al (2016a) showing equatorward migration and a region of negative radial shear at mid-latitudes have Pr SGS 1. This is in contrast to earlier simulations with Pr SGS < 1, which did not show equatorward migration (e.g., Brun et al 2004;Brown et al 2011;Nelson et al 2013) and had consistently positive gradients of Ω.…”

Section: Differential Rotation and Meridional Circulationmentioning

confidence: 94%

“…Distinguishing the contributions from the Λ effect and the turbulent viscosity is currently only possible using assumptions regarding either of the two coefficients and computing the other from the Reynolds stress (e.g., Käpylä et al 2014;Karak et al 2015;Warnecke et al 2016a). We do not attempt this here, but study the total turbulent stress T i j = Q i j + M i j and the anisotropy parameters realized in simulations with varying A4, page 8 of 18 Fig.…”

Section: Angular Momentum Transportmentioning

confidence: 99%

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Convection-driven spherical shell dynamos at varying Prandtl numbers

Käpylä

Olspert

et al. 2017

A&A

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Context. Stellar convection zones are characterized by vigorous high-Reynolds number turbulence at low Prandtl numbers. Aims. We study the dynamo and differential rotation regimes at varying levels of viscous, thermal, and magnetic diffusion. Methods. We perform three-dimensional simulations of stratified fully compressible magnetohydrodynamic convection in rotating spherical wedges at various thermal and magnetic Prandtl numbers (from 0.25 to 2 and from 0.25 to 5, respectively). Differential rotation and large-scale magnetic fields are produced self-consistently. Results. We find that for high thermal diffusivity, the rotation profiles show a monotonically increasing angular velocity from the bottom of the convection zone to the top and from the poles toward the equator. For sufficiently rapid rotation, a region of negative radial shear develops at mid-latitudes as the thermal diffusivity is decreased, corresponding to an increase of the Prandtl number. This coincides with and results in a change of the dynamo mode from poleward propagating activity belts to equatorward propagating ones. Furthermore, the clearly cyclic solutions disappear at the highest magnetic Reynolds numbers and give way to irregular sign changes or quasi-stationary states. The total (mean and fluctuating) magnetic energy increases as a function of the magnetic Reynolds number in the range studied here (5-151), but the energies of the mean magnetic fields level off at high magnetic Reynolds numbers. The differential rotation is strongly affected by the magnetic fields and almost vanishes at the highest magnetic Reynolds numbers. In some of our most turbulent cases, however, we find that two regimes are possible, where either differential rotation is strong and mean magnetic fields are relatively weak, or vice versa. Conclusions. Our simulations indicate a strong nonlinear feedback of magnetic fields on differential rotation, leading to qualitative changes in the behaviors of large-scale dynamos at high magnetic Reynolds numbers. Furthermore, we do not find indications of the simulations approaching an asymptotic regime where the results would be independent of diffusion coefficients in the parameter range studied here.

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Section: Dynamo Modesmentioning

confidence: 72%

Section: Differential Rotation and Meridional Circulationmentioning

confidence: 94%

Section: Angular Momentum Transportmentioning

confidence: 99%

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Convection-driven spherical shell dynamos at varying Prandtl numbers

Käpylä

Olspert

et al. 2017

A&A

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“…These studies suggest that the dynamo operating in a two-layer model becomes stronger and more easily excited than that in a one-layer model . Furthermore, in global simulations of a convectively driven dynamo, the presence of a coronal layer on top of the convection zone leads to spokelike differential rotation together with a near-surface shear layer (Warnecke et al 2013a(Warnecke et al , 2015, instead of otherwise mainly cylindrical contours of angular velocity. Mitra et al (2014) use a different two-layer setup in which turbulence is present in both layers, but in the lower layer it is driven helically, leading to large-scale dynamo action, while in the upper layer, it is driven nonhelically.…”

Section: Introductionmentioning

confidence: 99%

Bipolar region formation in stratified two-layer turbulence

Warnecke

Losada

Brandenburg

et al. 2016

A&A

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Aims. This work presents an extensive study of the previously discovered formation of bipolar flux concentrations in a two-layer model. We interpret the formation process in terms of negative effective magnetic pressure instability (NEMPI), which is a possible mechanism to explain the origin of sunspots. Methods. In our simulations, we use a Cartesian domain of isothermal stratified gas that is divided into two layers. In the lower layer, turbulence is forced with transverse nonhelical random waves, whereas in the upper layer no flow is induced. A weak uniform magnetic field is imposed in the entire domain at all times. In most cases, it is horizontal, but a vertical and an inclined field are also considered. In this study we vary the stratification by changing the gravitational acceleration, magnetic Reynolds number, strength of the imposed magnetic field, and size of the domain to investigate their influence on the formation process. Results. Bipolar magnetic structure formation takes place over a large range of parameters. The magnetic structures become more intense for higher stratification until the density contrast becomes around 100 across the turbulent layer. For the fluid Reynolds numbers considered, magnetic flux concentrations are generated at magnetic Prandtl number between 0.1 and 1. The magnetic field in bipolar regions increases with higher imposed field strength until the field becomes comparable to the equipartition field strength of the turbulence. A larger horizontal extent enables the flux concentrations to become stronger and more coherent. The size of the bipolar structures turns out to be independent of the domain size. A small imposed horizontal field component is necessary to generate bipolar structures. In the case of bipolar region formation, we find an exponential growth of the large-scale magnetic field, which is indicative of a hydromagnetic instability. Additionally, the flux concentrations are correlated with strong large-scale downward and converging flows. These findings imply that NEMPI is responsible for magnetic flux concentrations.

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“…Nonetheless, recent models have been used to estimate the effect of a nearly forcefree corona on controlling the emergence of flux from lower layers (Warnecke et al 2011), on the injection of magnetic twist in the heliosphere (Warnecke et al 2012) as well as on a global dynamo (e.g. Warnecke et al 2016), concluding that the presence of a corona cannot be neglected.…”

Section: Introductionmentioning

confidence: 99%

Analytic mean-fieldα²-dynamo with a force-free corona

Bonanno

Sordo

2017

A&A

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Context. Stellar dynamos are affected by boundary conditions imposed by stellar coronae. Under some approximations it is possible to find analytical solutions. Interior dynamo models often consider a current-free coronae without taking into account the constraints imposed by the presence of currents in the corona. Aims. We aim to analytically evaluate the effect of coronal currents and of an outer boundary condition on the efficiency of an α 2 dynamo. We intend to estimate the change in geometry and dinamo excitation numbers with respect to the current-free case. Methods. We analytically solve the turbulent dynamo induction equation for a homogeneous, non-mirror symmetric turbulence, in a spherical domain surrounded by a linear force-free corona with mean magnetic field B satisfying ∇ × B = βB. Results. The dynamo number is a decreasing function of β. Moreover, if the current is parallel to the field (β > 0) the dynamo number is smaller than in the force-free case. On the contrary (β < 0) the dynamo number is greater than in the force-free case. Conclusions. The presence of currents in the corona needs to be taken into account because it affects the condition for excitation of a dynamo.

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Influence of a coronal envelope as a free boundary to global convective dynamo simulations

Cited by 35 publications

References 90 publications

Convection-driven spherical shell dynamos at varying Prandtl numbers

Convection-driven spherical shell dynamos at varying Prandtl numbers

Bipolar region formation in stratified two-layer turbulence

Analytic mean-fieldα²-dynamo with a force-free corona

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Influence of a coronal envelope as a free boundary to global convective dynamo simulations

Cited by 35 publications

References 90 publications

Convection-driven spherical shell dynamos at varying Prandtl numbers

Convection-driven spherical shell dynamos at varying Prandtl numbers

Bipolar region formation in stratified two-layer turbulence

Analytic mean-fieldα2-dynamo with a force-free corona

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Resources

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Analytic mean-fieldα²-dynamo with a force-free corona