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Bipolar region formation in stratified two-layer turbulence
Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Max-Planck-Institut für Sonnensystemforschung, Germany.
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). University of Colorado, USA.
Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Ben-Gurion University of the Negev, Israel.
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Number of Authors: 52016 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 589, article id A125Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
2016. Vol. 589, article id A125
Keywords [en]
magnetohydrodynamics (MHD), turbulence, sunspots, starspots, Sun: magnetic fields
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
URN: urn:nbn:se:su:diva-131541DOI: 10.1051/0004-6361/201525880ISI: 000375318300137OAI: oai:DiVA.org:su-131541DiVA, id: diva2:945431
Available from: 2016-07-01 Created: 2016-06-21 Last updated: 2018-11-06Bibliographically approved
In thesis
1. Formation of solar bipolar regions: Magnetic flux concentrations from suction of the negative effective magnetic pressure instability
Open this publication in new window or tab >>Formation of solar bipolar regions: Magnetic flux concentrations from suction of the negative effective magnetic pressure instability
2018 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Sunspots stand out on the visible solar surface. They appear as dark structures evolving and changing over time. They host energetic and violent events, like coronal mass ejections and flares, and concentrate strong magnetic fields. Hundreds of years of studies provide a record of sunspot cycles, as reported by the well-known butterfly diagram, as well as some of their general observational properties, such as size, maximum field strength, and lifetime. However, we lack a general theory that explains how the magnetic field cluster in the spots and how it evolves over time.

This thesis studies the negative effective magnetic pressure instability (NEMPI) as a mechanism able to form such magnetic flux concentrations and thus magnetic spots. A weak magnetic field suppresses the turbulence locally and reduces the turbulent pressure. The resulting contraction concentrates the field further, which reduces the turbulent pressure even more, and so on. We study the conditions where NEMPI is excited, trying to reproduce some of the complexities of the solar environment. We focus on the effects of rotation, the change of stratification, and the influence of a simplified corona. We solve the magnetohydrodynamic equations using both direct numerical simulations and mean-field simulations of strongly stratified turbulence in a weak magnetic field.

Even slow rotation with a Coriolis number of 0.01 can suppress the instability. Higher values of rotation lead to dynamo action, increasing the magnetic field in a new coupled dynamo-NEMPI system. In the solar case, the dependence of NEMPI on rotation constrains the depth where the instability can operate: since the Coriolis number is very small in the uppermost layers of the Sun, NEMPI can only be a shallow phenomenon. Changing the type of stratification from isothermal to polytropic pushes the instability further to the upper parts of the computational domain. Unlike the isothermal case, in the polytropic cases the density scale height is no longer constant, but the stratification decreases deeper down, making it increasingly difficult for NEMPI to operate.

A corona changes dramatically the semblance of flux concentrations. A bipolar region is formed, instead of a single spot. It develops at the interface between the turbulent and the non-turbulent layers, forming a loop-like structure in the coronal layer. The bipoles move apart and finally decay and disappear. We study the structure in a wide range of parameters and test the physical conditions of its appearance. Higher stratification and imposed field strength intensify the magnetic structures, which reach even equipartition values, until a plateau and subsequent decrease occur. The increase of the domain size strengthens the maximum magnetic field and gives more coherence to the spots, keeping their sizes. We measure a strong large-scale downward and converging flows associated with the concentration of flux. Finally, we also include rotation in the two-layer model, confirming the previous results: slow rotation suppresses the formation of bipolar regions. A stronger imposed magnetic field alleviates the suppression somewhat and strengthens the structures.

These studies demonstrate the viability of NEMPI to form magnetic flux concentrations in both monopolar and bipolar structures. We find that NEMPI can only develop in the uppermost layers, where the local Coriolis number is small and the stratification strong.

Place, publisher, year, edition, pages
Stockholm: Department of Astronomy, Stockholm University, 2018. p. 70
Keywords
Magnetohydrodynamics (MHD), turbulence, dynamo, Sun: magnetic fields, Sun: rotation, Sun: activity, Sun: dynamo
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-161765 (URN)978-91-7797-434-5 (ISBN)978-91-7797-435-2 (ISBN)
Public defence
2019-01-08, sal FB42, AlbaNova universitetscentrum, Roslagstullsbacken 21, Stockholm, 13:00 (English)
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Available from: 2018-12-18 Created: 2018-11-06 Last updated: 2018-12-18Bibliographically approved

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