3D Solar Null Point Reconnection MHD Simulations
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3D Solar Null Point Reconnection MHD Simulations G. Baumann · K. Galsgaard · Å. Nordlund
Received: 8 December 2011 / Accepted: 10 October 2012 / Published online: 2 November 2012 © Springer Science+Business Media Dordrecht 2012
Abstract 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. Keywords Sun · Corona · Magnetic reconnection · Magnetic null-point
Advances in European Solar Physics Guest Editors: Valery M. Nakariakov, Manolis K. Georgoulis, and Stefaan Poedts G. Baumann () · K. Galsgaard · Å. Nordlund Niels Bohr Institute, Juliane Maries Vej 30, 2100 København Ø, Denmark e-mail: [email protected]
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1. Introduction There have been different attempts to initialize the magnetic field of the photosphere and corona for numerical simulations; amongst others by elimination of the complex observed small scale structure by the use of several photospheric magnetic monopole sources (Priest, Bungey, and Titov, 1997), by flux emergence experiments (Archontis et al., 2004), as well as by extrapolation (e.g. Masson et al., 2009) of solar magnetograms, e.g. from SOHO/MDI. The latter type has typically been used together with potential extrapolations, for simplicity reasons as well as due to the limited availability of vector magnetograms. As potential magnetic fields contain no free m
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