FORWARD MODELING OF THE CORONA OF THE SUN AND SOLAR-LIKE STARS: FROM A THREE-DIMENSIONAL MAGNETOHYDRODYNAMIC MODEL TO SYNTHETIC EXTREME-ULTRAVIOLET SPECTRA

摘要

A forward model is described in which we synthesize spectra from an ab initio three-dimensional MHD simulation of an outer stellar atmosphere, where the coronal heating is based on braiding of magnetic flux due to photo-spheric footpoint motions. We discuss the validity of assumptions such as ionization equilibrium and investigate the applicability of diagnostics like the differential emission measure inversion. We find that the general appearance of the synthesized corona is similar to the solar corona and that, on a statistical basis, integral quantities such as average Doppler shifts or differential emission measures are reproduced remarkably well. The persistent redshifts in the transition region, which have puzzled theorists since their discovery, are explained by this model as caused by the flows induced by the heating through braiding of magnetic flux. While the model corona is only slowly evolving in intensity, as is observed, the amount of structure and variability in Doppler shift is very large. This emphasizes the need for fast coronal spectroscopic observations, as the dynamical response of the corona to the heating process manifests itself in a comparably slow evolving coronal intensity but rapid changes in Doppler shift.