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Flux emergence: flares and coronal mass ejections driven by dynamo action underneath the solar surface
Stockholm University, Nordic Institute for Theoretical Physics (Nordita). Stockholm University, Faculty of Science, Department of Astronomy. (AstroDynamo)
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
URN: urn:nbn:se:su:diva-70609OAI: diva2:482135
2011-05-06, FB52, Albanova Universitet Centrum, 13:22 (English)
Available from: 2012-01-25 Created: 2012-01-23 Last updated: 2012-01-25Bibliographically approved
List of papers
1. Dynamo-driven plasmoid ejections above a spherical surface
Open this publication in new window or tab >>Dynamo-driven plasmoid ejections above a spherical surface
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

magnetohydrodynamics (MHD), turbulence, Sun: dynamo, Sun: coronal mass ejections (CMEs), stars: magnetic field
National Category
Astronomy, Astrophysics and Cosmology
urn:nbn:se:su:diva-70037 (URN)10.1051/0004-6361/201117023 (DOI)000296554800046 ()
authorCount :3Available from: 2012-01-16 Created: 2012-01-16 Last updated: 2013-04-05Bibliographically approved
2. Surface appearance of dynamo-generated large-scale fields
Open this publication in new window or tab >>Surface appearance of dynamo-generated large-scale fields
2010 (English)In: Astronomy and Astrophysics, ISSN 0004-6361, E-ISSN 1432-0746, Vol. 523, A19- p.Article in journal (Refereed) Published
Abstract [en]

Aims: Twisted magnetic fields are frequently seen to emerge above the visible surface of the Sun. This emergence is usually associated with the rise of buoyant magnetic flux structures. Here we ask how magnetic fields from a turbulent large-scale dynamo appear above the surface if there is no magnetic buoyancy. Methods: The computational domain is split into two parts. In the lower part, which we refer to as the turbulence zone, the flow is driven by an assumed helical forcing function leading to dynamo action. Above this region, which we refer to as the exterior, a nearly force-free magnetic field is computed at each time step using the stress-and-relax method. Results: Twisted arcade-like field structures are found to emerge in the exterior above the turbulence zone. Strong current sheets tend to form above the neutral line, where the vertical field component vanishes. Time series of the magnetic field structure show recurrent plasmoid ejections. The degree to which the exterior field is force free is estimated as the ratio of the dot product of current density and magnetic field strength to their respective rms values. This ratio reaches values of up to 95% in the exterior. A weak outward flow is driven by the residual Lorentz force.

magnetohydrodynamics (MHD), turbulence, stars: magnetic field, Sun: dynamo, Sun: coronal mass ejections (CMEs)
National Category
Natural Sciences
urn:nbn:se:su:diva-50483 (URN)10.1051/0004-6361/201014287 (DOI)
Available from: 2011-01-05 Created: 2010-12-28 Last updated: 2013-04-15Bibliographically approved

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