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Combining Models of Coronal Mass Ejections and Solar Dynamos
Stockholm University, Faculty of Science, Department of Astronomy. (AstroDynamo)
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 [en]
Magnetohydrodynamics, convection, turbulence, solar dynamo, solar rotation, solar activity, coronal mass ejections
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
URN: urn:nbn:se:su:diva-88896ISBN: 978-91-7447-675-0 (print)OAI: oai:DiVA.org:su-88896DiVA: diva2:614450
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
List of papers
1. 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.

Keyword
magnetohydrodynamics (MHD), turbulence, stars: magnetic field, Sun: dynamo, Sun: coronal mass ejections (CMEs)
National Category
Natural Sciences
Identifiers
urn:nbn:se:su:diva-50483 (URN)10.1051/0004-6361/201014287 (DOI)
Available from: 2011-01-05 Created: 2010-12-28 Last updated: 2017-12-11Bibliographically approved
2. 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 http://www.aanda.org

Keyword
magnetohydrodynamics (MHD), turbulence, Sun: dynamo, Sun: coronal mass ejections (CMEs), stars: magnetic field
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-70037 (URN)10.1051/0004-6361/201117023 (DOI)000296554800046 ()
Note
authorCount :3Available from: 2012-01-16 Created: 2012-01-16 Last updated: 2017-12-08Bibliographically approved
3. Magnetic twist: a source and property of space weather
Open this publication in new window or tab >>Magnetic twist: a source and property of space weather
2012 (English)In: Journal of Space Weather and Space Climate, ISSN 2115-7251, E-ISSN 2115-7251, Vol. 2, A11- p.Article in journal (Refereed) Published
Abstract [en]

Aim: We present evidence for finite magnetic helicity density in the heliosphere and numerical models thereof, and relate it to the magnetic field properties of the dynamo in the solar convection zone.

Methods: We use simulations and solar wind data to compute magnetic helicity either directly from the simulations or indirectly using time series of the skew-symmetric components of the magnetic correlation tensor.

Results: We find that the solar dynamo produces negative magnetic helicity at small scales and positive at large scales. However, in the heliosphere these properties are reversed and the magnetic helicity is now positive at small scales and negative at large scales. We explain this by the fact that a negative diffusive magnetic helicity flux corresponds to a positive gradient of magnetic helicity, which leads to a change of sign from negative to positive values at some radius in the northern hemisphere.

Keyword
MHD, turbulence, solar activity, coronal mass ejection (CME), solar wind
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy
Identifiers
urn:nbn:se:su:diva-89103 (URN)10.1051/swsc/2012011 (DOI)000325007800011 ()
Available from: 2013-04-11 Created: 2013-04-11 Last updated: 2017-12-06Bibliographically approved
4. Ejections of Magnetic Structures Above a Spherical Wedge Driven by a Convective Dynamo with Differential Rotation
Open this publication in new window or tab >>Ejections of Magnetic Structures Above a Spherical Wedge Driven by a Convective Dynamo with Differential Rotation
2012 (English)In: Solar Physics, ISSN 0038-0938, E-ISSN 1573-093X, Vol. 280, no 2, 299-319 p.Article in journal (Refereed) Published
Abstract [en]

We combine a convectively driven dynamo in a spherical shell with a nearly isothermal density-stratified cooling layer that mimics some aspects of a stellar corona to study the emergence and ejections of magnetic field structures. This approach is an extension of earlier models, where forced turbulence simulations were employed to generate magnetic fields. A spherical wedge is used which consists of a convection zone and an extended coronal region to a parts per thousand aEuro parts per thousand 1.5 times the radius of the sphere. The wedge contains a quarter of the azimuthal extent of the sphere and 150(a similar to) in latitude. The magnetic field is self-consistently generated by the turbulent motions due to convection beneath the surface. Magnetic fields are found to emerge at the surface and are ejected to the coronal part of the domain. These ejections occur at irregular intervals and are weaker than in earlier work. We tentatively associate these events with coronal mass ejections on the Sun, even though our model of the solar atmosphere is rather simplistic.

Keyword
Magnetic fields, corona, Coronal mass ejections, theory, Interior, convective zone, Turbulence, Helicity current
National Category
Astronomy, Astrophysics and Cosmology
Identifiers
urn:nbn:se:su:diva-82995 (URN)10.1007/s11207-012-0108-4 (DOI)000309865800002 ()
Note

AuthorCount:4;

Available from: 2012-12-03 Created: 2012-12-03 Last updated: 2017-12-07Bibliographically approved
5. Solar-like differential rotation in a convective dynamo with a coronal envelope
Open this publication in new window or tab >>Solar-like differential rotation in a convective dynamo with a coronal envelope
(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
Magnetohydrodynamics, convection, turbulence, Sun: dynamo, Sun: rotation, Sun: activity
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy; Space and Plasma Physics
Identifiers
urn:nbn:se:su:diva-88888 (URN)
Available from: 2013-04-05 Created: 2013-04-04 Last updated: 2016-07-01Bibliographically approved
6. Effects of strong stratification on equatorward dynamo wave propagation
Open this publication in new window or tab >>Effects of strong stratification on equatorward dynamo wave propagation
Show others...
(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
Magnetohydrodynamics, convection, turbulence, Sun: dynamo, rotation, activity
National Category
Astronomy, Astrophysics and Cosmology
Research subject
Astronomy; Space and Plasma Physics
Identifiers
urn:nbn:se:su:diva-88891 (URN)
Available from: 2013-04-12 Created: 2013-04-04 Last updated: 2016-07-01Bibliographically approved

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