Bipolar Magnetic Structures Driven by Stratified Turbulence With a Coronal Envelope

Warnecke, Jörn; Losada, I. R.; Brandenburg, Axel; Kleeorin, N.; Rogachevskii, I.

doi:10.1088/2041-8205/777/2/l37

Cited by 46 publications

(75 citation statements)

References 42 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…In the present work, we extend the studies of Warnecke et al (2013b) concerning the detailed dependence on density stratification (Sect. 3.1), magnetic Reynolds number (Sect.…”

Section: Introductionmentioning

confidence: 72%

“…The model is essentially the same as that of Warnecke et al (2013b), but in this work we vary the stratification, the imposed magnetic field, and the magnetic Reynolds number. We use a Cartesian domain (x, y, z), which has the size…”

Section: Modelmentioning

confidence: 99%

“…Warnecke et al (2013b) were for the first time able to produce bipolar magnetic regions with NEMPI using a two-layer setup with a weak imposed horizontal magnetic field. Turbulence is driven by a forcing function within the lower layer, while in the upper unforced layer, called the coronal envelope, all motions are a consequence of overshooting and magnetic field tension.…”

Section: Introductionmentioning

confidence: 99%

See 2 more Smart Citations

Bipolar region formation in stratified two-layer turbulence

Warnecke

Losada

Brandenburg

et al. 2016

A&A

View full text Add to dashboard Cite

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.

show abstract

“…In the present work, we extend the studies of Warnecke et al (2013b) concerning the detailed dependence on density stratification (Sect. 3.1), magnetic Reynolds number (Sect.…”

Section: Introductionmentioning

confidence: 72%

Section: Modelmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

Bipolar region formation in stratified two-layer turbulence

Warnecke

Losada

Brandenburg

et al. 2016

A&A

View full text Add to dashboard Cite

show abstract

“…Brandenburg et al 2010Brandenburg et al , 2012Kemel et al 2012b;Käpylä et al 2012a, and references therein). Further numerical studies have confirmed the existence of NEMPI in direct numerical simulations (DNS) of forced turbulence with weak imposed horizontal (Brandenburg et al 2011) and vertical ) magnetic fields, and in a two-layer system with an upper unforced coronal layer and a lower forced layer (Warnecke et al 2013(Warnecke et al , 2016. With NEMPI, even uniform, sub-equipartition magnetic fields can lead to flux concentrations if there is sufficient scale separation between the forcing scale and the size of the domain in highly stratified turbulence.…”

Section: Introductionmentioning

confidence: 86%

“…However, in highly stratified simulations when potential or vertical field conditions were applied, the studies of Stein & Nordlund (2012) and Warnecke et al (2013) found the possibility of bipolar-region formation. Motivated by these results we apply a vertical field condition in most of the current models.…”

Section: Imposed Horizontal Fieldmentioning

confidence: 99%

Magnetic flux concentrations from turbulent stratified convection

Käpylä

Brandenburg

Kleeorin

et al. 2016

A&A

View full text Add to dashboard Cite

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.

show abstract