Magnetic flux concentrations from dynamo-generated fields
Sarah Jabbari,Sarah Jabbari,Axel Brandenburg,Axel Brandenburg,I. R. Losada,I. R. Losada,Nathan Kleeorin,Nathan Kleeorin,Nathan Kleeorin,Igor Rogachevskii,Igor Rogachevskii,Igor Rogachevskii +11 more
Reads0
Chats0
TLDR
In this paper, the authors examined quantitatively whether the increase in the growth rate of negative effective magnetic pressure instability can be explained by an alpha(2) mean field dynamo, and whether both NEMPI and the dynamo instability can operate at the same time.Abstract:
Context The mean field theory of magnetized stellar convection gives rise to two distinct instabilities; the large-scale dynamo instability, operating in the bulk of the convection zone and a negative effective magnetic pressure instability (NEMPI) operating in the strongly stratified surface layers. The latter might be important in connection with magnetic spot formation. However, as follows from theoretical analysis, the growth rate of NEMPI is suppressed with increasing rotation rates. On the other hand, recent direct numerical simulations (DNS) have shown a subsequent increase in the growth rate. Aims. We examine quantitatively whether this increase in the growth rate of NEMPI can be explained by an alpha(2) mean field dynamo, and whether both NEMPI and the dynamo instability can operate at the same time. Methods. We use both DNS and mean field simulations (MFS) to solve the underlying equations numerically either with or without an imposed horizontal held, We use the test-field method to compute relevant dynamo coefficients. Results. DNS show that magnetic flux concentrations are still possible up to rotation rates above which the large-scale dynamo effect produces mean magnetic fields. The resulting DNS growth rates are quantitatively reproduced with MPS. As expected for weak or vanishing rotation, the growth rate of NEMPI increases with increasing gravity, but there is a correction term for strong gravity and large turbulent magnetic diffusivity. Conclusions. Magnetic flux concentrations are still possible for rotation rates above which dynamo action takes over For the solar rotation rate, the corresponding turbulent turnover time is about 5 h, with dynamo action commencing in the layers beneath.read more
Citations
More filters
Journal ArticleDOI
On the Formation of Active Regions
Robert F. Stein,Åke Nordlund +1 more
TL;DR: In this article, a simulation was performed where uniform, untwisted, horizontal magnetic field of 1 kG strenght was advected into the bottom of a computational domain 48 Mm wide by 20 Mm deep.
Nonlinear Evolution of Kink Unstable Magnetic Flux Tubes
TL;DR: In this paper, the authors study the kink flux tube problem with a three-dimensional numerical model containing only the most basic features of a kink-unstable flux tube.
Journal ArticleDOI
Magnetic flux concentrations from turbulent stratified convection
Petri J. Käpylä,Petri J. Käpylä,Petri J. Käpylä,Axel Brandenburg,Nathan Kleeorin,Nathan Kleeorin,Maarit J. Käpylä,Igor Rogachevskii,Igor Rogachevskii +8 more
TL;DR: In this article, the authors studied the formation of magnetic flux concentrations within the solar convection zone leading to sunspot formation and found that magnetic flux is concentrated in regions of converging flow corresponding to large-scale supergranulation convection pattern.
Journal ArticleDOI
Intense bipolar structures from stratified helical dynamos
Dhrubaditya Mitra,Axel Brandenburg,Axel Brandenburg,Nathan Kleeorin,Nathan Kleeorin,Nathan Kleeorin,Igor Rogachevskii,Igor Rogachevskii,Igor Rogachevskii +8 more
TL;DR: In this paper, the authors performed direct numerical simulations of the equations of magnetohydrodynamics with external random forcing and in the presence of gravity. And they showed that magnetic flux concentrations can exceed the turbulent kinetic energy by even a factor of 10.
References
More filters
Journal ArticleDOI
The Magnetic Polarity of Sun-Spots
Journal ArticleDOI
The inverse cascade and nonlinear alpha-effect in simulations of isotropic helical hydromagnetic turbulence
TL;DR: In this paper, a numerical model of isotropic homogeneous turbulence with helical forcing is investigated, which produces strong dynamo action with an additional large-scale field on the scale of the box (at wavenumber k = 1; forcing is at k = 5).