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Licentiate seminar: 3D non-LTE radiative transfer with a multigrid scheme
  Thesis defense

Wednesday 19 April 2017
from 13:00 to 15:00
at FA32
Speaker : Johan Pires Bjørgen (Institutet för solfysik, Stockholm University)
Abstract : Realistic numerical simulations of the solar atmosphere are required to understand the different phenomena observed on the solar surface. Model atmospheres produced by the simulations require subsequent synthesis of spectral lines to compare with observations. The spatial resolution of the numerical simulations keeps increasing to keep up with the expected increase in telescope size from 1-m class (e.g. SST) to 4-m class in 2019 (DKIST). Today, multilevel accelerated lambda iteration is the standard method to synthe- size non-LTE spectral lines in the solar and stellar community. The disadvantage of this method is that the computational work increases quadratically with decreasing grid spacing. In recent years, there has been a focus on multigrid methods for solving the non-LTE radiative transfer problem more efficiently. However, these methods have not been tested on realistic problems: three-dimensional (3D) MHD model atmospheres and strongly scattering lines. This thesis focuses on increasing the efficiency of non- LTE modeling of spectral lines in realistic solar models. Bjørgen and Leenaarts (2017) implemented a non-linear multigrid solver in an existing 3D non-LTE radiative transfer code. We showed that the method can handle realistic model atmospheres produced by radiative- MHD simulations. Choosing the correct parameters and components for multigrid are critical for a successful convergence. With multigrid we found a speed-up of a factor 4.5 − 6 compared to multilevel accelerated lambda iteration. The speed-up is expected to increase as the grid spacing of the model atmospheres gets smaller.

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