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Magnetic Reconnection and Hyperbolic Flux Tubes

The Sun is one of the most interesting objects for plasma physicists. Especially its outer regions – the photosphere and corona – provide a variety of unsolved questions. From solar observations and theoretical investigations it became clear that current sheet formation is the key to answer many questions. In this project we investigate, using magneto hydrodynamical simulations, the creation of thin
current sheets inside a hyperbolic flux tube. The general magnetic setup, based on observed solar magnetic structures, is sheared by photospheric flows, which trigger a pinching of the flux tube. The questions to be answered in this project are the role of the kind of the photospheric flows and of the quasi-static evolution in the pinching process.

Hyperbolic Flux Tubes

Thin current sheet formation in simple hyperbolic flux tube (HFT) configurations are investigated through time-dependent numerical magneto-hydrodynamical (MHD) simulations. The results are directly compared to the estimates from the simplified quasi-stationary analytical model by Titov et al., APJ (2003). Our simulations support key elements of the analytical theory: Hyperbolic plasma flow in the HFT center result from shear boundary motion and lead to time-exponential growth of the current density there. However, the time scale
estimates by Titov et al. need corrections for two important effects in order to agree with the fully time-dependent evolution: The effective shear length in the HFT center is not determined by the boundary shear length itself, but rather by its mapping along the field lines, which can differ significantly from the former one. Further, the plasma velocity decays faster with distance from the driver boundaries towards the HFT center than linearly, as assumed in the analytical treatment.

These MHD simulations are using the highly scalable computational framework racoon and are carried out on JSC's high performance computer systems.