(Los Alamos National Laboratory/Depart. of Phys. and Astron. at Michigan State University, USA)
Magnetohydrodynamic simulation from the convection zone to the
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
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