the horizontal grid spacing of the simulation decreases from $48$ km to $31$ km.

@2012ApJ...749..136L [@2013ApJ...772...90L] and computed synthetic images of the chromosphere in [H$\alpha$]{}, [K]{}, and [k]{} from the quiet Sun simulation by . Their line-core images showed ordered fibrils stretching between two opposite magnetic polarities separated by about 8 Mm, which also limited the fibril length to 8 Mm. @2017ApJ...839...22H computed H$\alpha$ and [h]{} for a flux emergence simulation, also computed with the Bifrost code. Their H$\alpha$ line-core image showed some thick unordered fibrils with a maximum length of 12 Mm, while the corresponding [h]{} image does not show a clear fibril structure.

The current MURaM simulation is the first simulation including a chromosphere that contains the spatial scales of an entire active region, and succeeds in reproducing long fibrils connecting the opposite polarities of an active region as seen in observations.

In the following subsection we analyse some of the structures seen in the synthetic intensity images in more detail.

Synthetic fibrils {#sec:synthetic_fibrils}
-----------------

The horizontal domain size of the current MURaM simulation is 98 Mm${}\times{}$49 Mm. This large size allows for $\sim$40 Mm long magnetic loops in the chromosphere that connect the two sunspots. The synthetic 