The solar chromosphere, spanning the region between the photosphere and the transition to the corona, remains one of the least understood parts of the Sun. This is partly because observing the chromosphere and interpreting these observations is full of pitfalls. Also, the simulation of the chromosphere is complex, as the particle densities and collisional rates are too low to maintain local thermodynamic equilibrium (LTE). Additionally, the recombination rates of hydrogen are larger than the dynamical timescales and the populations must be solved in non-equilibrium (NE). Realistic simulations of the chromosphere must treat the magneto-hydrodynamics, time-dependant atomic and molecular chemistry, and radiation transfer simultaneously.
The MURaM radiation-MHD code has previously been used for investigation of the connection between the solar photosphere and corona, ranging from small-scale dynamo generated ‘quiet’ sun fields to sunspots and complex active regions. Until now these simulations have been performed in LTE, greatly limiting their realism in the solar chromosphere. We have extended MURaM to include NLTE effects following the prescriptions used in the Bifrost code. The low viscocity and resistivity of the MURaM code leads to turbulent convection in the photosphere with kilo-Gauss mixed-polarity magnetic fields. This results in a dynamic chromosphere with strong shocks and a finely structured magnetic field. We discuss the implications of this new model towards observations of chromospheric spectral lines.