3D Solar Null Point Reconnection MHD Simulations
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Numerical MHD simulations of 3D reconnection events in the solar corona have improved enormously over the last few years, not only in resolution, but also in their complexity, enabling more and more realistic modeling. Various ways to obtain the initial magnetic field, different forms of solar atmospheric models as well as diverse driving speeds and patterns have been employed. This study considers differences between simulations with stratified and non-stratified solar atmospheres, addresses the influence of the driving speed on the plasma flow and energetics, and provides quantitative formulas for mapping electric fields and dissipation levels obtained in numerical simulations to the corresponding solar quantities. The simulations start out from a potential magnetic field containing a null-point, obtained from a Solar and Heliospheric Observatory (SOHO) Michelson Doppler Imager (MDI) magnetogram magnetogram extrapolation approximately 8 hours before a C-class flare was observed. The magnetic field is stressed with a boundary motion pattern similar to – although simpler than – horizontal motions observed by SOHO during the period preceding the flare. The general behavior is nearly independent of the driving speed, and is also very similar in stratified and non-stratified models, provided only that the boundary motions are slow enough. The boundary motions cause a build-up of current sheets, mainly in the fan-plane of the magnetic null-point, but do not result in a flare-like energy release. The additional free energy required for the flare could have been partly present in non-potential form at the initial state, with subsequent additions from magnetic flux emergence or from components of the boundary motion that were not represented by the idealized driving pattern.
KeywordsSun Corona Magnetic reconnection Magnetic null-point
We would like to especially thank Jacob Trier Frederiksen and Troels Haugbølle for valuable discussions and for their assistance with the simulations. We thank Guillaume Aulanier and Sophie Masson for providing us with their MHD data and driver information. We also thank the referee for useful comments and criticism. This work has been supported by the Niels Bohr International Academy and the SOLAIRE Research Training Network of the European Commission (MRTN-CT-2006-035484). The work of Å.N. was partially supported by the Danish Research Council for Independent Research (FNU) and the funding from the European Commission’s Seventh Framework Programme (FP7/2007-2013) under the grant agreement SWIFF (project n° 263340, www.swiff.eu ). The data for the magnetic field extrapolation were taken from the SOHO/MDI catalog. SOHO is a project of international cooperation between ESA and NASA. We furthermore acknowledge that the results in this paper have been achieved using resources at the Danish Center for Scientific Computing in Copenhagen, as well as PRACE and GCS/NIC Research Infrastructure resources on JUGENE and JUROPA based at Jülich in Germany.
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