Magnetic flux concentrations in a polytropic atmosphere

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Fältnamn	Indikatorer	Metadata
000		05052naa a2200469 4500
001		oai:DiVA.org:su-103961
003		SwePub
008		140527s2014 \| \|\|\|\|\|\|\|\|\|\|\|000 \|\|eng\|
009		oai:DiVA.org:kth-145589
024	7	a https://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-1039612 URI
024	7	a https://doi.org/10.1051/0004-6361/2013223152 DOI
024	7	a https://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-1455892 URI
040		a (SwePub)sud (SwePub)kth
041		a engb eng
042		9 SwePub
072	7	a ref2 swepub-contenttype
072	7	a art2 swepub-publicationtype
100	1	a Rivero Losada, Illau KTH,Stockholms universitet,Institutionen för astronomi,Nordiska institutet för teoretisk fysik (Nordita),Nordic Institute for Theoretical Physics NORDITA4 aut0 (Swepub:kth)u1xuy3wa
245	1 0	a Magnetic flux concentrations in a polytropic atmosphere
264		c 2014-03-26
264	1	b EDP Sciences,c 2014
338		a print2 rdacarrier
500		a AuthorCount:4;
500		a QC 20140523
520		a 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.
650	7	a NATURVETENSKAPx Fysikx Astronomi, astrofysik och kosmologi0 (SwePub)103052 hsv//swe
650	7	a NATURAL SCIENCESx Physical Sciencesx Astronomy, Astrophysics and Cosmology0 (SwePub)103052 hsv//eng
653		a magnetohydrodynamics (MHD)
653		a hydrodynamics
653		a turbulence
653		a Sun: dynamo
653		a Astronomy
653		a astronomi
700	1	a Brandenburg, Axelu KTH,Stockholms universitet,Institutionen för astronomi,Nordiska institutet för teoretisk fysik (Nordita),Nordic Institute for Theoretical Physics NORDITA4 aut0 (Swepub:kth)u1os89mi
700	1	a Kleeorin, Nathanu KTH,Stockholms universitet,Nordiska institutet för teoretisk fysik (Nordita),Ben-Gurion University of the Negev, Israel; N. I. Lobachevsky State University of Nizhny Novgorod, Russia,Nordic Institute for Theoretical Physics NORDITA4 aut0 (Swepub:kth)u10pszpw
700	1	a Rogachevskii, Igoru KTH,Stockholms universitet,Nordiska institutet för teoretisk fysik (Nordita),Ben-Gurion University of the Negev, Israel; N. I. Lobachevsky State University of Nizhny Novgorod, Russia,Nordic Institute for Theoretical Physics NORDITA4 aut0 (Swepub:kth)u1d6noho
710	2	a Stockholms universitetb Institutionen för astronomi4 org
773	0	t Astronomy and Astrophysicsd : EDP Sciencesg 564q 564x 0004-6361x 1432-0746
856	4	u https://www.aanda.org/articles/aa/pdf/2014/04/aa22315-13.pdf
856	4 8	u https://urn.kb.se/resolve?urn=urn:nbn:se:su:diva-103961
856	4 8	u https://doi.org/10.1051/0004-6361/201322315
856	4 8	u https://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-145589

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