Proceedings of the International Scientific Conference on Chromospheric and Coronal Magnetic Fields, 30 August - 2 September 2005, Max Planck Institute for Solar System Research, Katlenburg-Lindau, Germany, eds. D. Innes, A. Lagg, S. Solanki, D. Danesy, ESA SP-596, 396-401, 2005

Werner Schaffenberger (Kiepenheuer-Institut für Sonnenphysik, Schöneckstrasse 6, 79104 Freiburg, Germany)
Sven Wedemeyer-Böhm (Kiepenheuer-Institut für Sonnenphysik, Schöneckstrasse 6, 79104 Freiburg, Germany)
Oskar Steiner (Kiepenheuer-Institut für Sonnenphysik, Schöneckstrasse 6, 79104 Freiburg, Germany)
Bernd Freytag (Los Alamos National Laboratory/Depart. of Phys. and Astron. at Michigan State University, USA)

Magnetohydrodynamic simulation from the convection zone to the chromosphere

We have carried out a three-dimensional magnetohydrodynamic simulation of the integral layers from the convection zone to the chromosphere. The simulation starts with a homogeneous vertical magnetic field of a flux density of 10 G superposed on a previously computed, relaxed model of thermal convection. This flux density ought to mimic magnetoconvection in a network-cell interior. The three-dimensional computational domain extends from 1400 km below the surface of optical depth unity to 1400 km above it and it has a horizontal dimension of 4800× 4800 km. Thus, for the first time it became possible to extend simulations of magnetoconvection of the surface layers into the chromosphere. The magnetic field concentrates in narrow sheets near the surface of optical depth unity with field strengths up to approximately 1 kG. Below the surface the field disperses again but partially remains concentrated in flux tubes with a strength of a few hundred Gauss. The chromospheric magnetic field is marked by strong dynamics with a continuous reshuffling of magnetic flux on a time scale much shorter than in the photosphere or in the convection zone. The formation of weak flux tubes prevails again but on a spatial scale much larger than the width of the sheets near the surface and with a slight tendency to be located in between the flux concentrations at the surface. Highly dynamic filaments of stronger than average magnetic field are a ubiquitous phenomenon in the chromosphere. They form in the compression zone behind and along propagating shock fronts, that continue to be an integral part of chromospheric dynamics as already seen in the hydrodynamic simulations of the chromosphere by Wedemeyer et al. (2004), A&A, 414, 1121. These magnetic filaments that have a field strength of not more than a few tens of Gauss, rapidly move with the shock fronts and quickly dissolve or form with them. Over all, the picture of flux concentrations that strongly expand through the photosphere into a more homogeneous chromospheric field remains valid. That field fills the entire chromosphere and leads to a surface of plasma beta=1 around a height of 1000 km. However, the chromospheric magnetic field experiences a much more vigorous dynamics than previously thought.

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