Physics
Scientific paper
Sep 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008espm...12.2.90v&link_type=abstract
"12th European Solar Physics Meeting, Freiburg, Germany, held September, 8-12, 2008. Online at http://espm.kis.uni-freiburg.de/,
Physics
Scientific paper
The quiescent solar corona is regularly modified by very fast ejections of coronal material and magnetic field (CME) that occur preferably above active regions. Most CME models require the formation of twisted magnetic field structures (flux ropes) before or during such events.
Unfortunately, the coronal magnetic field is not directly measurable at present, and therefore it is difficult to verify the validity of different CME models.
In order to remove this obstacle, the extrapolation of photospheric magnetic field measurements can be used to reconstruct the missing coronal information. As an application of our magneto-frictional code, we present an extrapolation of a measured magnetogram where a flux rope is found.
In such applications it is necessary to estimate how well our extrapolation code can reproduce all aspects of highly nonlinear structures such as flux ropes. This is of course possible only using test fields.
The Titov and Demoulin force-free equilibrium (Titov and Demoulin, Astr. and Astrophys. 351, 707, (1999), hereafter TD) models a semi-circular, 3D current-carrying flux rope by means of a current ring embedded in a potential field. The parameters of the TD model can be adjusted to create both stable and kink- and torus-unstable configurations.
Its solar relevance was confirmed by the quantitative reproduction of some specific CME features (see e.g., Toeroek and Kliem, Astroph. J. Lett. 630 L97 (2005)).
Therefore, the TD solution is by far the most realistic analytical equilibrium available to date for the modeling of solar active regions.
Employing the TD equilibrium as a test-field, we show that the magnetofrictional extrapolation code can reproduce the energy and the twist of the magnetic field within a percent accuracy.
This information is essential for the reconstruction of coronal fields involved in eruptions because the twist is, together with the height profile of the overlying potential field, the most important stability parameter -- at least as long as the TD equilibrium is a good model of the considered active region.
Perfectly reproduced are also X-type magnetic topology features, sometimes referred to as Hyperbolic Flux Tubes, which are regarded to be essential to the physics of CMEs and flares because they are preferred locations for the formation of current sheets.
On the other hand, we also show how the scale-height of the potential field that is used as initial condition in the extrapolation influences the quality of the reconstructed field: different initial conditions reproduce correctly the twist and the topology, but less accurately the height and the shape of the flux rope.
Consequently, care must be taken when comparing the shapes of soft X-ray and EUV loops, especially those in the nearly potential field overlying filaments, with the field lines obtained from the extrapolation of the corresponding magnetogram.
Kliem Bernhard
Toeroek Tibor
Valori Gherardo
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