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Origin of solar surface activity and sunspots
Stockholm University, Faculty of Science, Department of Astronomy. NORDITA. (Solar group)
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 [en]
Magneto-hydrodynamics, solar physics, dynamo theory
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
URN: urn:nbn:se:su:diva-128774ISBN: 978-91-7649-370-0 (print)OAI: oai:DiVA.org:su-128774DiVA: diva2:916647
Public defence
2016-05-20, FD5, Albanova University Center, Roslagstullsbacken 21, Stockholm, 13:00 (English)
Opponent
Supervisors
Available from: 2016-04-27 Created: 2016-04-04 Last updated: 2017-02-20Bibliographically approved
List of papers
1. Surface flux concentrations in a spherical alpha 2 dynamo
Open this publication in new window or tab >>Surface flux concentrations in a spherical alpha 2 dynamo
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2013 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 556, A106Article in journal (Refereed) Published
Abstract [en]

Context. In the presence of strong density stratification, turbulence can lead to the large-scale instability of a horizontal magnetic field if its strength is in a suitable range (around a few percent of the turbulent equipartition value). This instability is related to a suppression of the turbulent pressure so that the turbulent contribution to the mean magnetic pressure becomes negative. This results in the excitation of a negative effective magnetic pressure instability (NEMPI). This instability has so far only been studied for an imposed magnetic field. Aims. We want to know how NEMPI works when the mean magnetic field is generated self-consistently by an alpha(2) dynamo, whether it is affected by global spherical geometry, and whether it can influence the properties of the dynamo itself. Methods. We adopt the mean-field approach, which has previously been shown to provide a realistic description of NEMPI in direct numerical simulations. We assume axisymmetry and solve the mean-field equations with the Pencil Code for an adiabatic stratification at a total density contrast in the radial direction of approximate to 4 orders of magnitude. Results. NEMPI is found to work when the dynamo-generated field is about 4% of the equipartition value, which is achieved through strong alpha quenching. This instability is excited in the top 5% of the outer radius, provided the density contrast across this top layer is at least 10. NEMPI is found to occur at lower latitudes when the mean magnetic field is stronger. For weaker fields, NEMPI can make the dynamo oscillatory with poleward migration. Conclusions. NEMPI is a viable mechanism for producing magnetic flux concentrations in a strongly stratified spherical shell in which a magnetic field is generated by a strongly quenched alpha effect dynamo.

Keyword
sunspots, Sun: dynamo, turbulence, magnetohydrodynamics (MHD), hydrodynamics
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-94586 (URN)10.1051/0004-6361/201321353 (DOI)000323893500106 ()
Funder
EU, European Research Council, 227952EU, European Research Council, 227915
Note

AuthorCount:5;

Available from: 2013-10-08 Created: 2013-10-07 Last updated: 2017-12-06Bibliographically approved
2. Magnetic flux concentrations from dynamo-generated fields
Open this publication in new window or tab >>Magnetic flux concentrations from dynamo-generated fields
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2014 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 568, A112Article in journal (Refereed) Published
Abstract [en]

Context The mean field theory of magnetized stellar convection gives rise to two distinct instabilities; the large-scale dynamo instability, operating in the bulk of the convection zone and a negative effective magnetic pressure instability (NEMPI) operating in the strongly stratified surface layers. The latter might be important in connection with magnetic spot formation. However, as follows from theoretical analysis, the growth rate of NEMPI is suppressed with increasing rotation rates. On the other hand, recent direct numerical simulations (DNS) have shown a subsequent increase in the growth rate. Aims. We examine quantitatively whether this increase in the growth rate of NEMPI can be explained by an alpha(2) mean field dynamo, and whether both NEMPI and the dynamo instability can operate at the same time. Methods. We use both DNS and mean field simulations (MFS) to solve the underlying equations numerically either with or without an imposed horizontal held, We use the test-field method to compute relevant dynamo coefficients. Results. DNS show that magnetic flux concentrations are still possible up to rotation rates above which the large-scale dynamo effect produces mean magnetic fields. The resulting DNS growth rates are quantitatively reproduced with MPS. As expected for weak or vanishing rotation, the growth rate of NEMPI increases with increasing gravity, but there is a correction term for strong gravity and large turbulent magnetic diffusivity. Conclusions. Magnetic flux concentrations are still possible for rotation rates above which dynamo action takes over For the solar rotation rate, the corresponding turbulent turnover time is about 5 h, with dynamo action commencing in the layers beneath.

Keyword
sunspots, dynamo, turbulence, magnetohydrodynamics (MHD), hydrodynamics
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-108385 (URN)10.1051/0004-6361/201423499 (DOI)000341185900042 ()
Note

AuthorCount:5;

Available from: 2014-10-23 Created: 2014-10-22 Last updated: 2017-12-05Bibliographically approved
3. Mean-field and direct numerical simulations of magnetic flux concentrations from vertical field
Open this publication in new window or tab >>Mean-field and direct numerical simulations of magnetic flux concentrations from vertical field
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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.

Keyword
sunspots, Sun: magnetic fields, turbulence, magnetic fields, hydrodynamics
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-102981 (URN)10.1051/0004-6361/201322681 (DOI)000332161800105 ()
Note

AuthorCount:5;

Available from: 2014-04-28 Created: 2014-04-25 Last updated: 2017-12-05Bibliographically approved
4. BIPOLAR MAGNETIC SPOTS FROM DYNAMOS IN STRATIFIED SPHERICAL SHELL TURBULENCE
Open this publication in new window or tab >>BIPOLAR MAGNETIC SPOTS FROM DYNAMOS IN STRATIFIED SPHERICAL SHELL TURBULENCE
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2015 (English)In: Astrophysical Journal, ISSN 0004-637X, E-ISSN 1538-4357, Vol. 805, no 2, 166Article in journal (Refereed) Published
Abstract [en]

Recent work by Mitra et al. (2014) has shown that in strongly stratified forced two-layer turbulence with helicity and corresponding large-scale dynamo action in the lower layer, and nonhelical turbulence in the upper, a magnetic field occurs in the upper layer in the form of sharply bounded bipolar magnetic spots. Here we extend this model to spherical wedge geometry covering the northern hemisphere up to 75 degrees latitude and an azimuthal extent of 180 degrees. The kinetic helicity and therefore also the large-scale magnetic field are strongest at low latitudes. For moderately strong stratification, several bipolar spots form that eventually fill the full longitudinal extent. At early times, the polarity of spots reflects the orientation of the underlying azimuthal field, as expected from Parker's Omega-shaped flux loops. At late times their tilt changes such that there is a radial field of opposite orientation at different latitudes separated by about 10 degrees. Our model demonstrates the spontaneous formation of spots of sizes much larger than the pressure scale height. Their tendency to produce filling factors close to unity is argued to be reminiscent of highly active stars. We confirm that strong stratification and strong scale separation are essential ingredients behind magnetic spot formation, which appears to be associated with downflows at larger depths.

Keyword
magnetohydrodynamics (MHD), Sun: activity, dynamo, turbulence
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-119160 (URN)10.1088/0004-637X/805/2/166 (DOI)000356715400081 ()
Available from: 2015-08-12 Created: 2015-07-29 Last updated: 2017-12-04Bibliographically approved

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