Internationale Wissenschaftliche Jahrestagung der Astronomischen Gesellschaft, Freiburg, Germany, 2003:

AN 324, Suppl.Issue 3, 66 (2003)

Sven Wedemeyer (Institut für Theoretische Physik und Astrophysik, Universität Kiel, 24098 Kiel, Germany),
Bernd Freytag (Department for Astronomy and Space Physics, Uppsala University, Box 515, 75120 Uppsala, Sweden),
Matthias Steffen (Astrophysikalisches Institut Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany),
Hans-Günter Ludwig ( Lund Observatory, Box 43, 22100 Lund, Sweden),
Hartmut Holweger (Institut für Theoretische Physik und Astrophysik, Universität Kiel, 24098 Kiel, Germany),

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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