Surface appearance of dynamo-generated large-scale fields

J. Space Weather Space Clim.

Mitra

2012

Aims: We present evidence for finite magnetic helicity density in the heliosphere and numerical models thereof, and relate it to the magnetic field properties of the dynamo in the solar convection zone. Methods: We use simulations and solar wind data to compute magnetic helicity either directly from the simulations or indirectly using time series of the skew-symmetric components of the magnetic correlation tensor. Results: We find that the solar dynamo produces negative magnetic helicity at small scales and positive at large scales. However, in the heliosphere these properties are reversed and the magnetic helicity is now positive at small scales and negative at large scales. We explain this by the fact that a negative diffusive magnetic helicity flux corresponds to a positive gradient of magnetic helicity, which leads to a change of sign from negative to positive values at some radius in the northern hemisphere.

Section: Discussionmentioning

confidence: 99%

“…This was done by Warnecke & Brandenburg (2010) who used a dynamo that was driven by turbulence that in turn was driven by a forcing function in the momentum equation. To imitate the effects of stratification and rotation that are known to produce helicity, they used a forcing function that was itself helical.…”

Section: Plasmoid Ejections In Cartesian Modelsmentioning

confidence: 99%

Magnetic twist: a source and property of space weather

J. Space Weather Space Clim.

Mitra

2012

“…The main difference between these two layers is the presence of the forcing function f (x, y, z, t) in the lower layer, which is called the turbulent layer. For a smooth transition between the two layers, we apply a modulation of the forcing function similar to Warnecke & Brandenburg (2010),…”

Section: Modelmentioning

confidence: 99%

Bipolar region formation in stratified two-layer turbulence

Losada

et al. 2016

A&A

Self 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.

“…This approach has been successful in generating plasmoids reminiscent of coronal mass ejections (Warnecke et al 2011). In that work, a simplified corona is combined with a large-scale dynamo that is either generated by forced turbulence in a Cartesian domain (Warnecke & Brandenburg 2010) or in spherical coordinates (Warnecke et al 2011), or through self-consistent turbulent convection (Warnecke et al 2012b). This approach leads to a more realistic "boundary condition" at the interface between the dynamo and corona without using, for example, radiative transfer.…”

Section: Introductionmentioning

confidence: 99%

Bipolar Magnetic Structures Driven by Stratified Turbulence With a Coronal Envelope

Losada

et al. 2013

ApJ

We report the spontaneous formation of bipolar magnetic structures in direct numerical simulations of stratified forced turbulence with an outer coronal envelope. The turbulence is forced with transverse random waves only in the lower (turbulent) part of the domain. Our initial magnetic field is either uniform in the entire domain or confined to the turbulent layer. After about 1-2 turbulent diffusion times, a bipolar magnetic region of vertical field develops with two coherent circular structures that live during one turbulent diffusion time, and then decay during 0.5 turbulent diffusion times. The resulting magnetic field strengths inside the bipolar region are comparable to the equipartition value with respect to the turbulent kinetic energy. The bipolar magnetic region forms a loop-like structure in the upper coronal layer. We associate the magnetic structure formation with the negative effective magnetic pressure instability in the two-layer model.