Austrian Early Career Conference 2024

Contribution:
Talk

Authors:
Vartika Pandey; Philippe-A. Bourdin

Affiliations:
Institute of Physics, University of Graz

Title:
Settling motions in 1D stratified models of stellar coronae

Abstract:
1D MHD simulations are important to understand plasma's response to heating. They have provided us with important results in solar and stellar physics, e.g. inversion studies, spectro-polarimetric studies, loop models for flares etc. And of course 1D models provide better spatial resolution with less computational and time demands. Most of these 1D simulations require a model atmosphere to start and the ones available are not exact to our current understanding of the Sun and other stars.
Here, we present a 1D sample atmospheric model that spans from the solar interior to the outer corona of the Sun. We investigate the effect of resolution/grid distance on the numerical stability of a simulation and also emphasize the importance of parametric studies to get the simulation setup as realistic as possible. We also talk about various heat/energy transfer mechanisms and which one suits best to maintain realistic temperature and density profiles.
When a stratification is transferred to a new simulation setup the numerical and analytical derivatives are not identical so, the initial hydrodynamic equilibrium is inexact. To find an equilibrium, pressure imbalances need to settle and later, shock waves are generated. These observed motions in the vertical direction have amplitudes up to 10 km/s. This effect would stop us from comparing our model output with realistic observed Doppler shifts of about 2 km/s. Therefore, we settle our initial stratification with 1D-MHD simulation under realistic solar parameters, like mass diffusion, heat conduction, viscosity, radiative losses, upwards decreasing magnetic field pressure, and Spitzer heat conduction along the magnetic field. We implement an artificial heating function for the corona that resembles the heating in a self-consistent 3D model driven by observations. This way, we avoid the collapse of solar corona due to insufficient heating in the 1D case. We are able to maintain the high temperatures in the corona with our artificial heating function. Generally, it might sound right to use lower diffusion constants for finer grid resolutions, but we find actually more diffusion is needed to maintain stratification models numerically stable.
Thus we present a complete atmospheric stratification that spans from the interior to the outer corona and can be used as the initial condition for 3D-MHD simulations and is able to replace older stratifications like the VAL-C and FAL-C that have shortcomings like too low spatial extent or insufficient methods.