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Simulation of the Formation of a Solar Active Region  

Mark Cheung   Submitted: 2010-06-21 15:01

We present a radiative magnetohydrodynamics simulation of the formation of an Active Region on the solar surface. The simulation models the rise of a buoyant magnetic flux bundle from a depth of 7.5 Mm in the convection zone up into the solar photosphere. The rise of the magnetic plasma in the convection zone is accompanied by predominantly horizontal expansion. Such an expansion leads to a scaling relation between the plasma density and the magnetic field strength such that B propto varrho1/2. The emergence of magnetic flux into the photosphere appears as a complex magnetic pattern, which results from the interaction of the rising magnetic field with the turbulent convective flows. Small-scale magnetic elements at the surface first appear, followed by their gradual coalescence into larger magnetic concentrations, which eventually results in the formation of a pair of opposite polarity spots. Although the mean flow pattern in the vicinity of the developing spots is directed radially outward, correlations between the magnetic field and velocity field fluctuations allow the spots to accumulate flux. Such correlations result from the Lorentz-force driven, counter-streaming motion of opposite-polarity fragments. The formation of the simulated Active Region is accompanied by transient light bridges between umbrae and umbral dots. Together with recent sunspot modeling, this work highlights the common magnetoconvective origin of umbral dots, light bridges and penumbral filaments.

Authors: M. C. M. Cheung, M. Rempel, A. M. Title, M. Sch?ssler
Projects: Hinode/SOT

Publication Status: Accepted for publication in ApJ
Last Modified: 2010-06-22 15:20
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Solar surface emerging flux regions: a comparative study of radiative MHD modeling and Hinode SOT observations  

Mark Cheung   Submitted: 2008-06-23 14:31

We present results from numerical modeling of emerging flux regions on the solar surface. The modeling was carried out by means of 3D radiative MHD simulations of the rise of buoyant magnetic flux tubes through the convection zone and into the photosphere. Due to the strong stratification of the convection zone, the rise results in a lateral expansion of the tube into a magnetic sheet, which acts as a reservoir for small-scale flux emergence events at the scale of granulation. The interaction of the convective downflows and the rising magnetic flux undulates it to form serpentine field lines emerging into the photosphere. Observational characteristics including the pattern of emerging flux regions, the cancellation of surface flux and associated high speed downflows, the convective collapse of photospheric flux tubes, the appearance of anomalous darkenings, the formation of bright points and the possible existence of transient kilogauss horizontal fields are discussed in the context of new observations from the Hinode Solar Optical Telescope. Implications for the local helioseismology of emerging flux regions are also discussed.

Authors: M. C. M. Cheung, M. Schuessler, T.D. Tarbell & A. M. Title
Projects: Hinode/SOT

Publication Status: Accepted for publication in ApJ
Last Modified: 2008-09-23 20:59
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Magnetic flux emergence in granular convection: Radiative MHD simulations and observational signatures  

Mark Cheung   Submitted: 2007-02-25 10:03

We study the emergence of magnetic flux from the near-surface layers of the solar convection zone into the photosphere. To model magnetic flux emergence, we carried out a set of numerical radiative magnetohydrodynamics simulations. Our simulations take into account the effects of compressibility, energy exchange via radiative transfer, and partial ionization in the equation of state. All these physical ingredients are essential for a proper treatment of the problem. Furthermore, the inclusion of radiative transfer allows us to directly compare the simulation results with actual observations of emerging flux. We find that the interaction between the magnetic flux tube and the external flow field has an important influence on the emergent morphology of the magnetic field. Depending on the initial properties of the flux tube (e.g. field strength, twist, entropy etc.), the emergence process can also modify the local granulation pattern. The emergence of magnetic flux tubes with a flux of 1019 Mx disturbs the granulation and leads to the transient appearance of a dark lane, which is coincident with upflowing material. These results are consistent with observed properties of emerging magnetic flux. Accompanying mpeg animations: http://www.mps.mpg.de/homes/cheung/U6emergence.mpg http://www.mps.mpg.de/homes/cheung/U1emergence.mpg

Authors: M. C. M. Cheung, M. Schuessler & F. Moreno-Insertis
Projects: None

Publication Status: A&A accepted
Last Modified: 2007-02-26 11:04
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Abstracts by Author
Simulation of the Formation of a Solar Active Region
Solar surface emerging flux regions: a comparative study of radiative MHD modeling and Hinode SOT observations
Magnetic flux emergence in granular convection: Radiative MHD simulations and observational signatures

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