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Competition of rotation and stratification in flux concentrations
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).
Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Ben-Gurion University of the Negev, Israel; N. I. Lobachevsky State University of Nizhny Novgorod, Russia.
Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Ben-Gurion University of the Negev, Israel; N. I. Lobachevsky State University of Nizhny Novgorod, Russia.ORCID iD: 0000-0001-7308-4768
2013 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 556, article id A83Article in journal (Refereed) Published
Abstract [en]

Context. In a strongly stratified turbulent layer, a uniform horizontal magnetic field can become unstable and spontaneously form local flux concentrations due to a negative contribution of turbulence to the large-scale (mean-field) magnetic pressure. This mechanism, which is called negative effective magnetic pressure instability (NEMPI), is of interest in connection with dynamo scenarios in which most of the magnetic field resides in the bulk of the convection zone and not at the bottom, as is often assumed. Recent work using mean-field hydromagnetic equations has shown that NEMPI becomes suppressed at rather low rotation rates with Coriolis numbers as low as 0.1. Aims. Here we extend these earlier investigations by studying the effects of rotation both on the development of NEMPI and on the effective magnetic pressure. We also quantify the kinetic helicity resulting from direct numerical simulations (DNS) with Coriolis numbers and strengths of stratification comparable to values near the solar surface and compare it with earlier work at smaller scale separation ratios. Further, we estimate the expected observable signals of magnetic helicity at the solar surface. Methods. To calculate the rotational effect on the effective magnetic pressure we consider both DNS and analytical studies using the tau approach. To study the effects of rotation on the development of NEMPI we use both DNS and mean-field calculations of the three-dimensional hydromagnetic equations in a Cartesian domain. Results. We find that the growth rates of NEMPI from earlier mean-field calculations are well reproduced with DNS, provided the Coriolis number is below 0.06. In that case, kinetic and magnetic helicities are found to be weak and the rotational effect on the effective magnetic pressure is negligible as long as the production of flux concentrations is not inhibited by rotation. For faster rotation, dynamo action becomes possible. However, there is an intermediate range of rotation rates where dynamo action on its own is not yet possible, but the rotational suppression of NEMPI is being alleviated. Conclusions. Production of magnetic flux concentrations through the suppression of turbulent pressure appears to be possible only in the uppermost layers of the Sun, where the convective turnover time is less than two hours.

Place, publisher, year, edition, pages
2013. Vol. 556, article id A83
Keywords [en]
magnetohydrodynamics (MHD), hydrodynamics, turbulence, Sun: dynamo
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
URN: urn:nbn:se:su:diva-94587DOI: 10.1051/0004-6361/201220939ISI: 000323893500083OAI: oai:DiVA.org:su-94587DiVA, id: diva2:654805
Funder
EU, European Research Council, 227952Swedish Research Council, 621-2011-5076Swedish Research Council, 2012-5797EU, European Research Council, 227915
Note

AuthorCount:4;

Available from: 2013-10-08 Created: 2013-10-07 Last updated: 2020-03-05Bibliographically 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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