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Mean-field and direct numerical simulations of magnetic flux concentrations from vertical field
Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
Stockholm University, Faculty of Science, Department of Astronomy. Stockholm University, Nordic Institute for Theoretical Physics (Nordita).
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2014 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 562, A53Article in journal (Refereed) Published
Abstract [en]

Context. Strongly stratified hydromagnetic turbulence has previously been found to produce magnetic flux concentrations if the domain is large enough compared with the size of turbulent eddies. Mean-field simulations (MFS) using parameterizations of the Reynolds and Maxwell stresses show a large-scale negative effective magnetic pressure instability and have been able to reproduce many aspects of direct numerical simulations (DNS) regarding growth rate, shape of the resulting magnetic structures, and their height as a function of magnetic field strength. Unlike the case of an imposed horizontal field, for a vertical one, magnetic flux concentrations of equipartition strength with the turbulence can be reached, resulting in magnetic spots that are reminiscent of sunspots. Aims. We determine under what conditions magnetic flux concentrations with vertical field occur and what their internal structure is. Methods. We use a combination of MFS, DNS, and implicit large-eddy simulations (ILES) to characterize the resulting magnetic flux concentrations in forced isothermal turbulence with an imposed vertical magnetic field. Results. Using DNS, we confirm earlier results that in the kinematic stage of the large-scale instability the horizontal wavelength of structures is about 10 times the density scale height. At later times, even larger structures are being produced in a fashion similar to inverse spectral transfer in helically driven turbulence. Using ILES, we find that magnetic flux concentrations occur for Mach numbers between 0.1 and 0.7. They occur also for weaker stratification and larger turbulent eddies if the domain is wide enough. Using MFS, the size and aspect ratio of magnetic structures are determined as functions of two input parameters characterizing the parameterization of the effective magnetic pressure. DNS, ILES, and MFS show magnetic flux tubes with mean-field energies comparable to the turbulent kinetic energy. These tubes can reach a length of about eight density scale heights. Despite being <= 1% equipartition strength, it is important that their lower part is included within the computational domain to achieve the full strength of the instability. Conclusions. The resulting vertical magnetic flux tubes are being confined by downflows along the tubes and corresponding inflow from the sides, which keep the field concentrated. Application to sunspots remains a viable possibility.

Place, publisher, year, edition, pages
2014. Vol. 562, A53
Keyword [en]
sunspots, Sun: magnetic fields, turbulence, magnetic fields, hydrodynamics
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
URN: urn:nbn:se:su:diva-102981DOI: 10.1051/0004-6361/201322681ISI: 000332161800105OAI: oai:DiVA.org:su-102981DiVA: diva2:714413
Note

AuthorCount:5;

Available from: 2014-04-28 Created: 2014-04-25 Last updated: 2017-12-05Bibliographically approved
In thesis
1. Origin of solar surface activity and sunspots
Open this publication in new window or tab >>Origin of solar surface activity and sunspots
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Sunspots and active regions are two of the many manifestations of the solar magnetic field. This field plays an important role in causing phenomena such as coronal mass ejections, flares, and coronal heating. Therefore, it is important to study the origin of sunspots and active regions and determine the underlying mechanism which creates them. It is believed that flux tubes rising from the bottom of the convection zone can create sunspots. However, there are still unanswered questions about this model. In particular, flux tubes are expected to expand as they rise, hence their strength weakens and some sort of reamplification mechanism must complement this model to match the observational properties of sunspots. To compensate for the absence of such an amplification mechanism, the field strength of the flux tubes, when at the bot- tom of the convection zone, must be far stronger than present dynamo models can explain.

In the last few years, there has been significant progress toward a new model of magnetic field concentrations based on the negative effective mag- netic pressure instability (NEMPI) in a highly stratified turbulent plasma. NEMPI is a large-scale instability caused by a negative contribution to the total mean-field pressure due to the suppression of the total turbulent pressure by a large-scale magnetic field. In this thesis, I study for the first time NEMPI in the presence of a dynamo-generated magnetic field in both spherical and Carte- sian geometries. The results of mean-field simulations in spherical geometry show that NEMPI and the dynamo instability can act together at the same time such that we deal with a coupled system involving both NEMPI and dynamo effects simultaneously. I also consider a particular two-layer model which was previously found to lead to the formation of bipolar magnetic structures with super-equipartition strength in the presence of a dynamo-generated field. In this model, the turbulence is forced in the entire domain, but the forcing is made helical in the lower part of the domain, and non-helical in the upper part. The study of such a system in spherical geometry showed that, when the stratification is strong enough, intense bipolar regions form and, as time passes, they expand, merge and create giant structures. To understand the underlying mechanism of the formation of such intense, long-lived bipolar structures with a sharp boundary, we performed a systematic numerical study of this model in plane parallel geometry by varying the magnetic Reynolds number, the scale separation ratio, and Coriolis number. Finally, I investigate the formation of the current sheet between bipolar regions and reconnection of oppositely orientated magnetic field lines and demonstrate that for large Lundquist numbers, S, the reconnection rate is nearly independent of S – in agreement with recent studies in identical settings.

Place, publisher, year, edition, pages
Stockholm: Department of Astronomy, Stockholm University, 2016. 58 p.
Keyword
Magneto-hydrodynamics, solar physics, dynamo theory
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-128774 (URN)978-91-7649-370-0 (ISBN)
Public defence
2016-05-20, FD5, Albanova University Center, Roslagstullsbacken 21, Stockholm, 13:00 (English)
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Available from: 2016-04-27 Created: 2016-04-04 Last updated: 2017-02-20Bibliographically approved

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