Modelling the chromospheric background pattern of the non-magnetic Sun
Performing time-dependent three-dimensional (non-magnetic) radiation
hydrodynamic simulations with CO5BOLD we are able to
investigate the generation, propagation, and dissipation of acoustic waves
in a self-consistent way.
The waves are excited by convective motions underneath the photosphere, move
upwards and steepen into hydrodynamic shocks in the model chromosphere where
they are an ubiquitous phenomenon.
Fourier spectra of the vertical velocity are strongly dominated by an
oscillation mode with a period near 5~min in the photosphere and below.
In the chromosphere, however, its contribution to the total power decreases
significantly with increasing height.
In contrast, the integrated power contribution for periods in the range
140 - 250 s (a ``3-min band'') grows with height until it becomes the major
contributor to the total power above the low chromosphere.
Interaction of the propagating shock waves finally results in a highly dynamical
and spatially intermittent structure of the model chromosphere.
In horizontal temperature slices it appears as network of hot matter and enclosed
cool regions which evolve on time scales of order twenty seconds.
The spatial scales are comparable to those of the underlying granulation.
The pattern is qualitatively very similar to the ``chromospheric background pattern'' which is revealed by high-resolution observations of the Sun (see Fig.1 and also, e.g., Lites et al. 1999, Krijger et al. 2001). The Ca II H observation shown in Fig. 1 was kindly provided by P. Sütterlin (2003). It was taken with the Dutch Open Telescope (DOT) with a filter of 1.6 A width which thus probes a large height range.
The mesh-like pattern of enhanced emission is interpreted as result of acoustic wave interference like it is the case in the presented hydrodynamical simulations. For a qualitative comparison we confront the observation in Fig. 1 with an intensity-like image calculated on basis of the simulation. For that, Planck functions were calculated from the temperature distribution on various mass iso-surfaces which cover the chromospheric height range likely contributing to the observed intensity. The image finally shows the sum of the Planck functions, resembling the Ca~II~H emission in a rough approximation, which is remarkably similar to the observation if one considers the simplicity of the approach.
Fig.1: Chromospheric background pattern: Integrated Planck functions from the hydrodynamical model (left) and observed Ca II H image (right). The observational data was kindly provided by P. Sütterlin.
Krijger, J. M., Rutten, R. J., Lites, B.~W., Straus, T., Shine, R. A., Tarbell, T. D., 2001, ApJ 379, 1052-1082
Lites, B. W., Rutten, R. J., Berger, T. E., 1999, ApJ 517, 1013
Sütterlin}, P., 2003, priv. comm.
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