Magnetic flux concentrations from turbulent stratified convection

Käpylä, Petri J.; Brandenburg, Axel; Kleeorin, N.; Käpylä, Maarit J.; Rogachevskii, I.

doi:10.1051/0004-6361/201527731

Cited by 27 publications

(27 citation statements)

References 70 publications

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“…Although a modest resolution of 288 grid points was used, each scale height is covered by more than 40 grid points. This differs from the case of convection where the density (and pressure) scale height varies strongly as a function of depth and imposes much more restrictive constraints on the grid size (see, e.g., Käpylä et al 2016).…”

Section: Anisotropy Of Turbulencementioning

confidence: 99%

Effects of small‐scale dynamo and compressibility on the Λ effect

Käpylä

2019

Astron Nachr

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The Λ effect describes a rotation‐induced nondiffusive contribution to the Reynolds stress. It is commonly held responsible for maintaining the observed differential rotation of the Sun and other late‐type stars. Here, the sensitivity of the Λ effect to small‐scale magnetic fields and compressibility is studied by means of forced turbulence simulations either with anisotropic forcing in fully periodic cubes or in density‐stratified domains with isotropic forcing. Effects of small‐scale magnetic fields are studied in cases where the magnetic fields are self‐consistently generated by a small‐scale dynamo. The results show that small‐scale magnetic fields lead to a quenching of the Λ effect which is milder than in cases where also a large‐scale field is present. The effect of compressibility on the Λ effect is negligible in the range of Mach numbers from 0.015 to 0.8. Density stratification induces a marked anisotropy in the turbulence and a vertical Λ effect if the forcing scale is roughly two times larger than the density scale height.

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Section: Anisotropy Of Turbulencementioning

confidence: 99%

Effects of small‐scale dynamo and compressibility on the Λ effect

Käpylä

2019

Astron Nachr

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“…Leaving the flux emergence out, Kitiashvili et al (2010) showed that uniformly distributed field can be intensified and form a flux concentration corresponding to a pore. A series of works by Warnecke et al (2013Warnecke et al ( , 2016, Brandenburg et al (2013), and Käpylä et al (2016) investigated the intensification of pre-imposed weak field by Negative Effective Magnetic Pressure Instability (NEMPI, Brandenburg et al 2011) in more idealized models. It is very likely that the effects found in this models are also present in more complex realistic simulations.…”

Section: On the Formation Of Sunspotsmentioning

confidence: 99%

Emergence of Magnetic Flux Generated in a Solar Convective Dynamo. I. The Formation of Sunspots and Active Regions, and The Origin of Their Asymmetries

Chen

Rempel

Fan

2017

ApJ

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We present a realistic numerical model of sunspot and active region formation based on the emergence of flux bundles generated in a solar convective dynamo. To this end we use the magnetic and velocity fields in a horizontal layer near the top boundary of the solar convective dynamo simulation to drive realistic radiative-magnetohydrodynamic simulations of the upper most layers of the convection zone. The main results are: (1) The emerging flux bundles rise with the mean speed of convective upflows, and fragment into small-scale magnetic elements that further rise to the photosphere, where bipolar sunspot pairs are formed through the coalescence of the small-scale magnetic elements. (2) Filamentary penumbral structures form when the sunspot is still growing through ongoing flux emergence. In contrast to the classical Evershed effect, the inflow seems to prevail over the outflow in a large part of the penumbra. (3) A well formed sunspot is a mostly monolithic magnetic structures that is anchored in a persistent deep-seated downdraft lane. The flow field outside the spot shows a giant vortex ring that comprises of an inflow below 15 Mm depth and an outflow above 15 Mm depth. (4) The sunspots successfully reproduce the fundamental properties of the observed solar active regions, including the more coherent leading spots with a stronger field strength, and the correct tilts of bipolar sunspot pairs. These asymmetries can be linked to the intrinsic asymmetries in the magnetic and flow fields adapted from the convective dynamo simulation.

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“…1 of Käpylä et al 2013). Another possibility would be to apply the so-called reduced sound speed method where the sound speed is artificially changed so that the Mach number does not become too small at the base of the convection zone (Hotta et al 2012;Käpylä et al 2016b).…”

Section: Initial and Boundary Conditionsmentioning

confidence: 99%

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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Magnetic flux concentrations from turbulent stratified convection

Cited by 27 publications

References 70 publications

Effects of small‐scale dynamo and compressibility on the Λ effect

Effects of small‐scale dynamo and compressibility on the Λ effect

Emergence of Magnetic Flux Generated in a Solar Convective Dynamo. I. The Formation of Sunspots and Active Regions, and The Origin of Their Asymmetries

Convection-driven spherical shell dynamos at varying Prandtl numbers

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