standard semi-empirical models of solar atmosphere much better than the standard LTE multi-group opacity scheme with four frequency bins . Furthermore, the opacity and source function are set to zero at the base of the transition region, or the first grid cell with $T > 20$ kK, in order to avoid contributions from the corona in the downward directed rays.

A slope-limited diffusion scheme was chosen so that overshoots are minimized in locations where the gradients can be significant, like near the flare ribbons. The problem is however not completely removed. For example, it results in cold pockets at the base of the transition region in some areas of the computational domain. Finally, to relax the time-step constraints, the Boris correction and hyperbolic heat conduction are used. For more details about the code, see @2017ApJ...834...10R. Figure \[fig:atmos\_photosphere\] displays the model atmosphere. Panel (a) shows the intensity in the continuum at 5000 Å, with granulation and a pair of sunspot-like structures (but note the lack of penumbra around the spots), separated by 50 Mm. Panel (b) displays the vertical field strength, revealing that the upper sunspot is in fact a $\delta$-spot. In panel (c) we show the lowest height where the 