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Effects of strong stratification on equatorward dynamo wave propagation
Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Stockholm University, Faculty of Science, Department of Astronomy.
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(English)Manuscript (preprint) (Other academic)
Abstract [en]

We present results from simulations of rotating magnetized  turbulent convection in spherical wedge geometry representing parts  of the latitudinal and longitudinal extents of a star.  Here we consider a set of runs for which the density stratification is  varied, keeping the  Reynolds and Coriolis numbers at similar values. In the case of weak  stratification we find quasi-steady solutions for moderate rotation and oscillatory dynamos with poleward migration of activity belts  for more rapid rotation. For stronger stratification a similar transition as a function of the Coriolis number is found, but with an equatorward migrating branch near the equator. We test the domain size dependence of our results for a rapidly rotating run with equatorward migration by varying the longitudinal  extent of our wedge. The energy of the axisymmetric mean magnetic field decreases as the domain size increases and we find that an  m=1 mode is excited for a full 2π φ-extent, reminiscent of the  field configurations deduced from observations of rapidly rotating late-type stars.

Keyword [en]
Magnetohydrodynamics, convection, turbulence, Sun: dynamo, rotation, activity
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy; Space and Plasma Physics
URN: urn:nbn:se:su:diva-88891OAI: diva2:614414
Available from: 2013-04-12 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.
Magnetohydrodynamics, convection, turbulence, solar dynamo, solar rotation, solar activity, coronal mass ejections
National Category
Astronomy, Astrophysics and Cosmology
Research subject
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)

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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Warnecke, JörnBrandenburg, Axel
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