Chromospheric Modelling

Observations show clearly that even the very quietest part of the solar chromosphere is very structured and dynamic. Diagnostics from this region is difficult because spectral lines are neither formed in LTE nor under optically thin conditions. The gas goes from being dominated by the gas pressure in the photosphere to being dominated by the magnetic pressure in the upper chromosphere. The ionization balance of important elements is furthermore out of equilibrium. A proper understanding of the observations thus demand complex modelling that ideally should include non-LTE radiative transfer in 3D including the dynamic driving in the convection zone, non-equilibrium ionization, magnetic fields, the transition region and the corona. We will discuss various aspects of this very complex problem and report on recent progress on the way to fully comprehensive models in 3D extending from the convection zone to the corona.