Numerical simulations of the solar atmosphere

Abstract

In this thesis several aspects of the solar atmosphere are investigated using numerical simulations. Simulations and observations of reversed solar granulation are compared. It is concluded that reversed granulation is a hydrodynamical process and is a consequence of convection reversal. Images are synthesized from simulations of solar magnetoconvection to study the quality of different proxy-magnetometry diagnostics. It is concluded that the blue wing of the Halpha line is the best proxy-magnetometry diagnostic, followed by the blue wing of Hbeta, which has slightly smaller contrast but higher diffraction-limited resolution. The effect of non-equilibrium ionization of hydrogen on the structure of the solar chromosphere is investigated. An algorithm for computation of non-equilibrium ionization is described, and is used in multi-dimensional numerical simulations of the solar atmosphere. It is concluded that the ionization fraction in the chromosphere is rather constant at about 0.1%-1% , irrespective of the gas temperature. Because the population of the lower level of the Halpha line is coupled to the continuum population, Halpha opacity is retained in cool parts of the model chromosphere. This is in stark contrast to models employing instantaneous equilibrium hydrogen ionization.