Convective mechanism of amplification and structuring of magnetic fields

Getling Alexander V., a.getling@mail.ru, Institute of Nuclear Physics, Lomonosov Moscow State University, Moscow, 119991, Russia
Kolmychkov Vyacheslav V., ksv@keldysh.ru, Keldysh Institute of Applied Mathematics, Russian Academy of Sciences, Moscow, 125047 Russia
Mazhorova Olga S., olgamazhor@mail.ru, Keldysh Institute of Applied Mathematics, Russian Academy of Sciences, Moscow, 125047 Russia

Abstract
The convective mechanism of amplification and structuring of magnetic fields, the idea of which traces back to the simple kinematic model considered by Tverskoi (1966), can likely act in the solar subphotospheric layers. It is capable of forming bipolar magnetic configurations (and also a diversity of more complex ones), being directly related to the very topology of the cellular flow and largely independent of the flow scale. Getling and Ovchinnikov (2002), after the original hypothesis by Tverskoi, suggested that especially large and energy-containing convection cells located under the photospheric surface and exceeding typical supergranules in their size can produce the magnetic fields of bipolar magnetic regions and sunspot groups. Such a convective mechanism (which can be referred to as a local dynamo) does not require strong initial magnetic fields and does not depend on any assumptions other than the presence of cellular convection (which is actually observed on the Sun). We numerically simulate MHD convection of a Rayleigh–Benard type assuming that the magnetic field is initially uniform and horizontal (or inclined). We do not aim at reproducing the process in detail but rather investigate its essential features using simplified formulations of the problem. We specify various static temperature profiles to comprehend how they affect the flow and magnetic-field patterns. The computational domain measures 8 x 8 x 1. In the parameter-space region explored, the simulated convection patterns are generally neither regular nor stationary, although a cellular structure of the flow is preserved. To study the dynamics of the flow and magnetic field, we analyse both the distribution of the physical quantities over certain horizontal sections of the layer and the motion of corks, or “marked” fluid particles. It can be seen from our calculations that bipolar magnetic configurations do not necessarily develop at the centres of polygonal convection cells, and the convergence of the flow is most important for this process, irrespective of whether the convergence is located in the upper or in the lower half of the layer (in the downflow or upflow zones, respectively). In either case, the convergence acts as the centre of a cell with one direction of circulation or another. It is worth keeping in mind that the development of active regions originates either at the nodes of the supergranular network or (Title 2006) in the central parts of supergranules. It is remarkable that, in our simulations, the enhanced local magnetic fields are typically bipolar. In particular, we can see that the magnetic field lines are actually wound by the circulatory motion of the fluid if the flow remains relatively stable in its structure; the repeated passage of the corks through the flow-convergence zones indicates that the fluid experiences several turnovers in the cells and a substantial amplification of the magnetic field due to the winding of magnetic field lines (rather than sweeping of their vertical portions) is possible. This feature is illustrated by perspective plots of the cork trajectories. The strength of the magnetic field increases by a factor of several hundreds during the computed run. Thus, quasi-ordered convection appears plausibly to be a producer of strong magnetic fields in the near-photospheric layers.