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Dynamo-driven plasmoid ejections above a spherical surface
Stockholm University, Faculty of Science, Department of Astronomy.
Stockholm University, Faculty of Science, Department of Astronomy.
2011 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 534, A 11- p.Article in journal (Refereed) Published
Abstract [en]

Aims: We extend earlier models of turbulent dynamos with an upper, nearly force-free exterior to spherical geometry, and study how flux emerges from lower layers to the upper ones without being driven by magnetic buoyancy. We also study how this affects the possibility of plasmoid ejection. Methods: A spherical wedge is used that includes northern and southern hemispheres up to mid-latitudes and a certain range in longitude of the Sun. In radius, we cover both the region that corresponds to the convection zone in the Sun and the immediate exterior up to twice the radius of the Sun. Turbulence is driven with a helical forcing function in the interior, where the sign changes at the equator between the two hemispheres. Results: An oscillatory large-scale dynamo with equatorward migration is found to operate in the turbulence zone. Plasmoid ejections occur in regular intervals, similar to what is seen in earlier Cartesian models. These plasmoid ejections are tentatively associated with coronal mass ejections (CMEs). The magnetic helicity is found to change sign outside the turbulence zone, which is in agreement with recent findings for the solar wind. Movie is available in electronic form at http://www.aanda.org

Place, publisher, year, edition, pages
2011. Vol. 534, A 11- p.
Keyword [en]
magnetohydrodynamics (MHD), turbulence, Sun: dynamo, Sun: coronal mass ejections (CMEs), stars: magnetic field
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
URN: urn:nbn:se:su:diva-70037DOI: 10.1051/0004-6361/201117023ISI: 000296554800046OAI: oai:DiVA.org:su-70037DiVA: diva2:478726
Note
authorCount :3Available from: 2012-01-16 Created: 2012-01-16 Last updated: 2017-12-08Bibliographically approved
In thesis
1. Flux emergence: flares and coronal mass ejections driven by dynamo action underneath the solar surface
Open this publication in new window or tab >>Flux emergence: flares and coronal mass ejections driven by dynamo action underneath the solar surface
2011 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Helically shaped magnetic field structuresknown as coronal mass ejections (CMEs) are closely related to so-called eruptive flares. On the one hand, these events are broadly believed to be due tothe buoyant rise of magnetic flux tubes from the bottom of the convection zone to the photosphere where they form structures such as sunspots. On the other hand, models of eruptive flares and CMEs have no connection to the convection zone and the magnetic field generated bydynamo action. It is well known that a dynamo can produce helical structures and twisted magnetic fields as observed in the Sun. In this work we ask, how a dynamo-generated magnetic field appears above the surface without buoyancy force and how this field evolves inthe outer atmosphere of the Sun.

We apply a new approach of a two layer model, where the lower one represents the convection zone and the upper one the solar corona. The two layers are included in one single simulation domain. In the lower layer, we use a helical forcing function added to the momentum equation to create a turbulent dynamo. Due to dynamo action, a large-scale field is formed. As a first step we use a Cartesian cube. We solve the equations of the so-called force-free model in the upper layer to create nearly force-free coronal magnetic fields. In a second step we use a spherical wedge, which extends radially from 0.7 to 2 solar radii. We include density stratification due to gravity in anisothermal domain. The wedge includes both hemispheres of the Sun and we apply a helicalforcing with different signs in each hemisphere.

As a result, a large-scale field is generated by a turbulent dynamo acting underneath the surface. Due to the latitudinal variation of the helicity produced by the helical forcing, the dynamo is oscillating in the spherical wedge. Twisted magnetic fields emerge above the surface and form arch-like structures with strong current sheets. Plasmoids and CME-like structures are ejected recurrently into the outerlayers. In the spherical simulations we find that the magnetic helicity changes sign in the exterior, which is in agreement with recent analysis of the solar wind data.

Place, publisher, year, edition, pages
010 Publishers, 2011. 95 p.
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy; Theoretical Astrophysics; Space and Plasma Physics
Identifiers
urn:nbn:se:su:diva-70609 (URN)
Presentation
2011-05-06, FB52, Albanova Universitet Centrum, 13:22 (English)
Opponent
Supervisors
Available from: 2012-01-25 Created: 2012-01-23 Last updated: 2012-01-25Bibliographically approved
2. Combining Models of Coronal Mass Ejections and Solar Dynamos
Open this publication in new window or tab >>Combining Models of Coronal Mass Ejections and Solar Dynamos
2013 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Observations show that Coronal Mass Ejections (CMEs) are associated with twisted magnetic flux configurations. Conventionally, CMEs are modeled by shearing and twisting the footpoints of a certain distribution of magnetic flux at the solar surface and letting it evolve at the surface. Of course, the surface velocities and magnetic field patterns should ultimately be obtained from realistic simulations of the solar convection zone where the field is generated by dynamo action. Therefore, a unified treatment of the convection zone and the CMEs is needed. Numerical simulations of turbulent dynamos show that the amplification of magnetic fields can be catastrophically quenched at magnetic Reynolds numbers typical of the interior of the Sun. A strong flux of magnetic helicity leaving the dynamo domain can alleviate this quenching. In this sense, a realistic (magnetic) boundary condition is an important ingredient of a successful solar dynamo model. Using a two-layer model developed in this thesis, we combine a dynamo-active region with a magnetically inert but highly conducting upper layer which models the solar corona. In four steps we improve this setup from a forced to a convectively driven dynamo and from an isothermal to a polytropic stratified corona. The simulations show magnetic fields that emerge at the surface of the dynamo region and are ejected into the coronal part of the domain. Their morphological form allows us to associate these events with CMEs. Magnetic helicity is found to change sign in the corona to become consistent with recent helicity measurements in the solar wind. Our convection-driven dynamo model with a coronal envelope has a solar-like differential rotation with radial (spoke-like) contours of constant rotation rate, together with a solar-like meridional circulation and a near-surface shear layer. The spoke-like rotation profile is due to latitudinal entropy gradient which violates the Taylor--Proudman balance through the baroclinic term. We find mean magnetic fields that migrate equatorward in models both with and without the coronal layer. One remarkable result is that the dynamo action benefits substantially from the presence of a corona becoming stronger and more realistic. The two-layer model represents a new approach to describe the generation of coronal mass ejections in a self-consistent manner. On the other hand, it has important implications for solar dynamo models as it admits many magnetic features observed in the Sun.

Place, publisher, year, edition, pages
Stockholm: Department of Astronomy, Stockholm University, 2013. 119 p.
Keyword
Magnetohydrodynamics, convection, turbulence, solar dynamo, solar rotation, solar activity, coronal mass ejections
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-88896 (URN)978-91-7447-675-0 (ISBN)
Public defence
2013-05-31, sal FB52, Albanova University Center, Roslagstullsbacken 21, Stockholm, 13:15 (English)
Opponent
Supervisors
Note

At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 5: Manuscript; Paper 6: Manuscript.

Available from: 2013-05-08 Created: 2013-04-04 Last updated: 2013-04-29Bibliographically approved

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