Forward modeling of stellar coronae: from a 3D MHD model to synthetic EUV spectra

TL;DR

This paper presents a forward modeling approach that synthesizes EUV spectra from a 3D MHD simulation of a stellar corona, successfully reproducing many observed features and explaining persistent redshifts as flows caused by magnetic flux braiding.

Contribution

It introduces a novel method to generate synthetic spectra from ab-initio 3D MHD models, linking coronal heating mechanisms to observable spectral signatures.

Findings

Synthesized corona resembles the solar corona in appearance.

Integral quantities like Doppler shifts and emission measures are accurately reproduced.

Redshifts in the transition region are explained by magnetic flux braiding-induced flows.

Abstract

A forward model is described in which we synthesize spectra from an ab-initio 3D MHD simulation of an outer stellar atmosphere, where the coronal heating is based on braiding of magnetic flux due to photospheric 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 spectroscopy, 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.