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Solar-like differential rotation in a convective dynamo with a coronal envelope
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, Faculty of Science, Department of Astronomy.
(English)Manuscript (preprint) (Other academic)
Abstract [en]

We report on the results of four convective dynamo simulations with an souter coronal layer. The magnetic field is self-consistently generated by the convectivemotions beneath the surface. Above the convection zone we include a polytropic layerthat extends to 1.6 solar radii. The temperature increases in this regionto ≈8 times the value at the surface, corresponding to ≈1.2 times the value at the bottom of the spherical shell. We associate this region with the solar corona. We find a solar-like differential rotation with radial contours of constant rotation rate, together with a solar-like meridionalcirculation and a near-surface shear layer. This spoke-like rotation profile is caused by a non-zero latitudinalentropy gradient which violates the Taylor-Proudman balance via thebaroclinic term. The lower density stratification compared with the Sun leads to anequatorward return flow above the surface. The mean magnetic field is in most of the casesoscillatory with equatorward migration in one case. In other cases the equatorward migration is overlaid by stationary oreven poleward migrating mean fields.

Keyword [en]
Magnetohydrodynamics, convection, turbulence, Sun: dynamo, Sun: rotation, Sun: activity
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy; Space and Plasma Physics
Identifiers
URN: urn:nbn:se:su:diva-88888OAI: oai:DiVA.org:su-88888DiVA: diva2:614410
Available from: 2013-04-05 Created: 2013-04-04 Last updated: 2016-07-01Bibliographically approved
In thesis
1. 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

Open Access in DiVA

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