Change search
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Magnetic flux concentrations in a polytropic atmosphere
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.
2014 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 564, article id A2Article in journal (Refereed) Published
Abstract [en]

Context. Strongly stratified hydromagnetic turbulence has recently been identified as a candidate for explaining the spontaneous formation of magnetic flux concentrations by the negative effective magnetic pressure instability (NEMPI). Much of this work has been done for isothermal layers, in which the density scale height is constant throughout. Aims. We now want to know whether earlier conclusions regarding the size of magnetic structures and their growth rates carry over to the case of polytropic layers, in which the scale height decreases sharply as one approaches the surface. Methods. To allow for a continuous transition from isothermal to poly tropic layers, we employ a generalization of the exponential function known as the q-exponential. This implies that the top of the polytropic layer shifts with changing polytropic index such that the scale height is always the same at some reference height. We used both mean-field simulations (MPS) and direct numerical simulations (DNS) of forced stratified turbulence to determine the resulting flux concentrations in polytropic layers. Cases of both horizontal and vertical applied magnetic fields were considered. Results. Magnetic structures begin to form at a depth where the magnetic field strength is a small fraction of the local equipartition field strength with respect to the turbulent kinetic energy. Unlike the isothermal case where stronger fields can give rise to magnetic flux concentrations at larger depths, in the polytropic case the growth rate of NEMPI decreases for structures deeper down. Moreover, the structures that form higher up have a smaller horizontal scale of about four times their local depth. For vertical fields, magnetic structures of super-equipartition strengths are formed, because such fields survive downward advection that causes NEMPI with horizontal magnetic fields to reach premature nonlinear saturation by what is called the potato-sack effect. The horizontal cross-section of such structures found in DNS is approximately circular, which is reproduced with MFS of NEMPI using a vertical magnetic field. Conclusions. Results based on isothermal models can be applied locally to polytropic layers. For vertical fields, magnetic flux concentrations of super-equipartition strengths form, which supports suggestions that sunspot formation might be a shallow phenomenon.

Place, publisher, year, edition, pages
2014. Vol. 564, article id A2
Keywords [en]
magnetohydrodynamics (MHD), hydrodynamics, turbulence, Sun: dynamo
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
URN: urn:nbn:se:su:diva-103961DOI: 10.1051/0004-6361/201322315ISI: 000334671000002OAI: oai:DiVA.org:su-103961DiVA, id: diva2:722870
Funder
EU, European Research Council, 227952EU, European Research Council, 227915Swedish Research Council, 621-2011-5076Swedish Research Council, 2012-5797
Note

AuthorCount:4;

Available from: 2014-06-09 Created: 2014-05-27 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)
Opponent
Supervisors
Available from: 2018-12-18 Created: 2018-11-06 Last updated: 2018-12-18Bibliographically approved

Open Access in DiVA

No full text in DiVA

Other links

Publisher's full text

Search in DiVA

By author/editor
Rivero Losada, IllaBrandenburg, AxelRogachevskii, Igor
By organisation
Department of AstronomyNordic Institute for Theoretical Physics (Nordita)
In the same journal
Astronomy and Astrophysics
Astronomy, Astrophysics and Cosmology

Search outside of DiVA

GoogleGoogle Scholar

doi
urn-nbn

Altmetric score

doi
urn-nbn
Total: 20 hits
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf