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Astron. Astrophys. 337, 928-939 (1998)

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Convective intensification of solar surface magnetic fields: results of numerical experiments

U. Grossmann-Doerth, M. Schüssler and O. Steiner

Kiepenheuer-Institut für Sonnenphysik, Schöneckstr. 6, D-79104 Freiburg, Germany (ugd@kis.uni-freiburg.de; msch@kis.uni-freiburg.de; steiner@kis.uni-freiburg.de)

Received 27 February 1998 / Accepted 3 June 1998

Abstract

The concentration of magnetic flux by convective flows in the solar surface layers is studied by means of two-dimensional numerical simulations with radiative transfer. We follow the evolution of an initially homogeneous, vertical magnetic field, starting from an evolved state of simulated solar granulation. The results of three simulation runs with initial field strengths, [FORMULA], of 100 G, 200 G, and 400 G, respectively, are shown. In all cases, horizontal convective flows rapidly sweep magnetic flux into the intergranular downflow channels. The field is further amplified up to kilogauss values by partial evacuation due to a strongly accelerated downflow within the magnetic structure. The value of the field strength reached at a given depth and the size of the flux concentrations grows with the initial field strength (i.e., the amount of magnetic flux within the computional box). In the case of [FORMULA] G, the downflow within the flux concentration becomes so strong that it `bounces' off the high density plasma in the deeper layers; the resulting upflow leads to a strong, upward moving shock and to the dispersal of the flux sheet after a lifetime of about 200 s. In the cases with less magnetic flux ([FORMULA]G, 200 G), the downflow is less vigorous and the flux concentrations persist to the end of the simulation (about 5 minutes). Radiation diagnostics in the continuum and in spectral lines predicts observable signatures of the intensification process. The accelerated downflow leads to a conspicuous Doppler shift and a negative area asymmetry of Stokes V-profiles of spectral lines, while the intensification of the magnetic field may be detectable through the `magnetic line ratio' method in the visible and by direct Zeeman splitting of magnetically sensitive lines in the infrared wavelength ranges.

Key words: Sun: magnetic fields – Sun: photosphere – Sun: atmosphere – Sun: granulation – Sun: faculae, plages – MHD

Send offprint requests to: M. Schüssler

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© European Southern Observatory (ESO) 1998

Online publication: August 27, 1998
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