Numerical simulations and observations of Mg II in the solar chromosphere

The Mg II h&k lines are amongst the best diagnostic tools of the upper solar chromosphere. This region of the atmosphere is of particular interest as it is the lowest region of the Sun's atmosphere where the magnetic field is dominant in the energetics and dynamics, defining its structure. While highly successful in the photosphere and lower to mid chromosphere, numerical models have produced syntheticMg II lines that do not match the observations well. We present a number of large scale models with magnetic field topologies representative of the quiet Sun, ephemeral flux regions and plage, and also models where the numerical resolution is high and where we go beyond the MHD paradigm. The results of this study show models with a much improved correspondence with \iris\ observations both in terms of intensities and widths, especially underscoring the importance of chromospheric mass loading and of capturing the magnetic field topology and evolution in simulations. This comes in addition to the importance of capturing the generation of small scale velocity fields and including non-equilibrium ionization and ion-neutral interaction effects. Understanding and modeling all these effects and their relative importance is necessary in order to reproduce observed spectral features.