The enigmatic chromosphere is the transition between the solar surface and the eruptive outer solar atmosphere. The chromosphere harbors and constrains the mass and energy loading processes that define the heating of the corona, the acceleration and the composition of the solar wind, and the energetics and triggering of solar outbursts. In spite of its importance, the chromosphere is arguably the least understood domain of solar physics. All at once it represents the transition from optically thick to thin radiation escape, from gas-pressure to magnetic-pressure domination, from neutral to ionised state, from MHD to plasma physics, and from near-equilibrium (“LTE”) to non-equilibrium conditions. Its physics is so complex that traditional methods relying on analytic analysis or simplified mechanisms do not work. It has become abundantly clear that only ab-initio numerical simulation, built on the same fundamental non-linear physics equations that the Sun obeys, and explicit
ly accompanied by sufficiently deep subsequent analysis of what occurs within each simulation, can deliver physical insight to understand how the chromosphere works.
I will report on recent results from such numerical simulations with the Bifrost code. The 3D radiation MHD equations are solved for a computational region extending from the convection zone into the corona for various initial magnetic field configurations. We include conduction along magnetic field lines, optically thin radiative losses in the corona, non-LTE radiative losses in the chromosphere, heating from incoming radiation from the corona and full radiative transfer including scattering in the photosphere. Effects of the hydrogen ionization balance being out of equilibrium and ion-neutral effects are also discussed.
