Cyclic Magnetic Activity Due to Turbulent Convection in Spherical Wedge Geometry

Käpylä, Petri J.; Mantere, M. J.; Brandenburg, Axel

doi:10.1088/2041-8205/755/1/l22

Cited by 166 publications

(264 citation statements)

References 23 publications

(28 reference statements)

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“…The resolution is one of the keys to access the solar surface. We note that some studies using the finite difference have been done in the stellar or solar context using a moderate ratio of the speed of sound and the convection velocity with adjustments of the radiative flux and the stratification (Käpylä et al 2011(Käpylä et al , 2012. Although this type of approach provides insights for the maintenance of the mean flow and the magnetic field, proper reproduction using the solar parameters, such as the stratification, the luminosity, the partial ionization effect and the rotation, and direct comparison with actual observations cannot be achieved.…”

Section: Reduced Speed Of Sound Techniquementioning

confidence: 99%

High-Resolution Calculations of the Solar Global Convection With the Reduced Speed of Sound Technique. I. The Structure of the Convection and the Magnetic Field Without the Rotation

Hotta

Rempel

Yokoyama

2014

ApJ

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High-resolution calculations of the solar global convection with the reduced speed of sound technique: I. The structure of the convection and the magnetic field without the rotation. The purpose of this study is to achieve multi-scale convection, including scales from 10 Mm (smaller than supergranulation) up to ∼ 200 Mm (global scale). We accomplish this by approaching the solar surface up to 0.99R ⊙ using unprecedented resolution, studying in particular the influence of the location of the upper boundary on the convective structure in the deeper parts of the convection zone. In addition, we use our approach to study the transport and the generation of the magnetic field by turbulent convection in the absence of rotation, i.e., we study the operation of a small-scale dynamo in the highly stratified convection zone.The paper is organized as follows: We provide an overview of the reduced speed of sound technique in §2, introduce our numerical setting in §3, and show the results in §4,present the properties based on the obtained spatial distributions in §4.1, discuss the

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Section: Reduced Speed Of Sound Techniquementioning

confidence: 99%

High-Resolution Calculations of the Solar Global Convection With the Reduced Speed of Sound Technique. I. The Structure of the Convection and the Magnetic Field Without the Rotation

Hotta

Rempel

Yokoyama

2014

ApJ

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“…Furthermore, numerical simulations of convective dynamos produce diffuse magnetic fields throughout the convection zone (e.g. Ghizaru et al 2010;Käpylä et al 2012b;Yadav et al 2015;Augustson et al 2015), which could act as the seed field for NEMPI.…”

Section: Introductionmentioning

confidence: 99%

Magnetic flux concentrations from turbulent stratified convection

Käpylä

Brandenburg

Kleeorin

et al. 2016

A&A

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Context. The formation of magnetic flux concentrations within the solar convection zone leading to sunspot formation is unexplained. Aims. We study the self-organization of initially uniform sub-equipartition magnetic fields by highly stratified turbulent convection. Methods. We perform simulations of magnetoconvection in Cartesian domains representing the uppermost 8.5−24 Mm of the solar convection zone with the horizontal size of the domain varying between 34 and 96 Mm. The density contrast in the 24 Mm deep models is more than 3 × 10 3 or eight density scale heights, corresponding to a little over 12 pressure scale heights. We impose either a vertical or a horizontal uniform magnetic field in a convection-driven turbulent flow in set-ups where no small-scale dynamos are present. In the most highly stratified cases we employ the reduced sound speed method to relax the time step constraint arising from the high sound speed in the deep layers. We model radiation via the diffusion approximation and neglect detailed radiative transfer in order to concentrate on purely magnetohydrodynamic effects. Results. We find that super-equipartition magnetic flux concentrations are formed near the surface in cases with moderate and high density stratification, corresponding to domain depths of 12.5 and 24 Mm. The size of the concentrations increases as the box size increases and the largest structures (20 Mm horizontally near the surface) are obtained in the models that are 24 Mm deep. The field strength in the concentrations is in the range of 3-5 kG, almost independent of the magnitude of the imposed field. The amplitude of the concentrations grows approximately linearly in time. The effective magnetic pressure measured in the simulations is positive near the surface and negative in the bulk of the convection zone. Its derivative with respect to the mean magnetic field, however, is positive in most of the domain, which is unfavourable for the operation of the negative effective magnetic pressure instability (NEMPI). Simulations in which a passive vector field is evolved do not show a noticeable difference from magnetohydrodynamic runs in terms of the growth of the structures. Furthermore, we find that magnetic flux is concentrated in regions of converging flow corresponding to large-scale supergranulation convection pattern. Conclusions. The linear growth of large-scale flux concentrations implies that their dominant formation process is a tangling of the large-scale field rather than an instability. One plausible mechanism that can explain both the linear growth and the concentration of the flux in the regions of converging flow pattern is flux expulsion. A possible reason for the absence of NEMPI is that the derivative of the effective magnetic pressure with respect to the mean magnetic field has an unfavourable sign. Furthermore, there may not be sufficient scale separation, which is required for NEMPI to work.

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“…In the past few years, many such simulations, although distinct in their computational design, have achieved the production of axisymmetric large-scale magnetic fields undergoing more or less regular polarity reversals, some even producing sustained solar-like cycles (Augustson et al 2014;Brown et al 2010Brown et al , 2011Ghizaru et al 2010;Käpylä et al 2010Käpylä et al , 2012Käpylä et al , 2013Racine et al 2011). Although they still lack some potentially important field regeneration mechanisms -most notably perhaps the surface decay of active regions and associated buildup of a surface dipole moment, -they represent unique virtual laboratory experiments allowing us to investigate the dynamics of convectively-driven magnetic cycles.…”

Section: Introductionmentioning

confidence: 99%

Characteristics of magnetic solar-like cycles in a 3D MHD simulation of solar convection

Passos

Charbonneau

2014

A&A

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We analyse the statistical properties of the stable magnetic cycle unfolding in an extended 3D magnetohydrodynamic simulation of solar convection produced with the EULAG-MHD code. The millennium simulation spans over 1650 years, in the course of which forty polarity reversals take place on a regular ∼40 yr cadence, remaining well-synchronized across solar hemispheres. In order to characterize this cycle and facilitate its comparison with measures typically used to represent solar activity, we build two proxies for the magnetic field in the simulation mimicking the solar toroidal field and the polar radial field. Several quantities that characterize the cycle are measured (period, amplitudes, etc.) and correlations between them are computed. These are then compared with their observational analogs. From the typical Gnevyshev-Ohl pattern, to hints of Gleissberg modulation, the simulated cycles share many of the characteristics of their observational analogs even though the simulation lacks poloidal field regeneration through active region decay, a mechanism nowadays often considered an essential component of the solar dynamo. Some significant discrepancies are also identified, most notably the in-phase variation of the simulated poloidal and toroidal large-scale magnetic components, and the low degree of hemispheric coupling at the level of hemispheric cycle amplitudes. Possible causes underlying these discrepancies are discussed.

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Cyclic Magnetic Activity Due to Turbulent Convection in Spherical Wedge Geometry

Cited by 166 publications

References 23 publications

High-Resolution Calculations of the Solar Global Convection With the Reduced Speed of Sound Technique. I. The Structure of the Convection and the Magnetic Field Without the Rotation

High-Resolution Calculations of the Solar Global Convection With the Reduced Speed of Sound Technique. I. The Structure of the Convection and the Magnetic Field Without the Rotation

Magnetic flux concentrations from turbulent stratified convection

Characteristics of magnetic solar-like cycles in a 3D MHD simulation of solar convection

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