  Because of the complex physics that governs the formation of chromospheric
lines, interpretation of solar chromospheric observations is difficult. The
origin and characteristics of many chromospheric features are, because of this,
unresolved. We focus here on studying two prominent features: long fibrils and
flare ribbons. To model them, we use a 3D MHD simulation of an active region
which self-consistently reproduces both of them. We model the H$\alpha$, Mg II
k, Ca II K, and Ca II 8542 {\AA} lines using the 3D non-LTE radiative transfer
code Multi3D.
  This simulation reproduces long fibrils that span between the
opposite-polarity sunspots and go up to 4 Mm in height. They can be traced in
all lines due to density corrugation. Opposite to previous studies, H$\alpha$,
Mg II h&k, and Ca II H&K, are formed at similar height in this model. Magnetic
field lines are aligned with the H$\alpha$ fibrils, but the latter holds to a
lesser extent for the Ca II 8542 {\AA} line.
  The simulation shows structures in the H$\alpha$ line core that look like
flare ribbons. The emission in the ribbons is caused by a dense chromosphere
and a transition region at high column mass. The ribbons are visible in all
chromospheric lines, but least prominent in Ca II 8542 {\AA} line. In some
pixels, broad asymmetric profiles with a single emission peak are produced,
similar to the profiles observed in flare ribbons. They are caused by a deep
onset of the chromospheric temperature rise and large velocity gradients.
  The simulation produces long fibrils similar to what is seen in observations.
It also produces structures similar to flare ribbons despite the lack of
non-thermal electrons in the simulation. The latter suggests that thermal
conduction might be a significant agent in transporting flare energy to the
chromosphere in addition to non-thermal electrons.

Solar and Stellar Astrophysics