A coronal loop in a box: From energy generation to coronal emission
von Cosima Alexandra Breu
Datum der mündl. Prüfung:2022-03-31
Betreuer:Prof. Dr. Hardi Peter
Gutachter:Prof. Dr. Hardi Peter
Gutachter:Prof. Dr. Laurent Gizon
EnglischThe corona is the outer atmosphere of the Sun. It contains an abundance of bright arcs that consist of hot plasma trapped by the magnetic field. These coronal loops are heated to temperatures far surpassing the solar surface temperature. The heating of this hot coronal plasma is one of the major open problems in stellar astrophysics. Several mechanisms have been proposed, which can be divided roughly into models based on the slow braiding of magnetic field lines and subsequent impulsive energy release in the corona, the dissipation of magnetohydrodynamic waves, and flux emergence and cancellation and reconnection at the loop footpoints. We conduct 3D radiative magnetohydrodynamic simulations with the MURaM code to study the generation and transport of energy and observable signatures of heating. To combine the computational efficiency of idealized loop models with the more realistic driving of coronal heating and mass flows in simulations that include part of the convection zone, we model the coronal loop as a ”straightened-out” magnetic flux tube in a Cartesian box connected to a shallow convection zone layer at each footpoint. This way, our model is driven self-consistently by magnetoconvection, while the structure across the loop cross-section is well resolved. Gray radiative transfer in the photosphere and chromosphere, optically thin losses in the corona and field-aligned Spitzer heat conduction are taken into account in the model. The interaction of granulation with the magnetic field leads to a Poynting flux into the atmosphere. The model reproduces the bursty heating found in previous coronal simulations. In response to the energy input, the coronal loop develops a complex structure of small-scale current sheets and flows. The emission synthesized from the loop model shows substructure reproducing observed strand widths. In the next step, we have a closer look at the magnetic coupling between the photosphere and the corona. Vortex motions are abundant within magnetic concentrations in the photosphere and have been found to contribute to chromospheric heating. We find that some of these vortex tubes reach coronal heights and carry a strong Poynting flux beyond the transition region. Regions with enhanced swirling strength show increased Poynting flux and heating rates. The influence of vortex flows on the atmospheric structure is largest in the chromosphere and low corona, where vortices are overdense and twisted magnetic field lines lead to an upwards directed Lorentz force. With increasing height, vortex flows get deformed until the original rotational motion is not discernible anymore. The relation between vortices and coronal emission is complex. While the rotational motion can lead to gradients in the magnetic and velocity field that show increased viscous and resistive dissipation and lead to brightening of the plasma, coronal emission depends both on temperature and density, which is not significantly increased in vortices in the coronal part of the plasma. Therefore, there is no one-to-one correspondence between vortex tubes andcoronal strands. Eventually, we calculate synthetic spectral line profiles of coronal emission lines to study the response of coronal emission to heating and and plasma motions. We find values for the nonthermal line broadening close to observational values both perpendicular and parallel to the loop axis. Despite being on the same order of magnitude, nonthermal line broadening parallel and perpendicular to the line of sight attributed to different processes. Perpendicular to the magnetic guide field, the velocity field is governed by small-scale twisting and shearing motions. For a line of sight parallel to the magnetic field, evaporation in response to heating events causes an increase in nonthermal broadening. For a line of sight perpendicular to the magnetic guide field, we reproduce the independence of the nonthermal line broadening of the resolution of the observing instrument. This model provides a new comprehensive view of processes governing the structure and evolution of coronal loops. In particular, the complex magnetic structure drives the energy flux into the upper atmosphere. Small vortex structures extend from the photosphere into the corona and the internal turbulent-like motions lead to a fine-structuring of the loop.
Keywords: magnetohydrodynamics (MHD)- methods: numerical; Sun: UV radiation; Sun: magnetic fields; Sun: corona