  To predict line emission in the solar atmosphere requires models which are
fundamentally different depending on whether the emission is from the
chromosphere or the corona. At some point between the two regions, there must
be a change between the two modelling regimes. Recent extensions to the coronal
modelling for carbon and oxygen lines in the solar transition region have shown
improvements in the emission of singly- and doubly-charged ions, along with
Li-like ions. However, discrepancies still remain, particularly for
singly-charged ions and intercombination lines. The aim of this work is to
explore additional atomic processes that could further alter the charge state
distribution and the level populations within ions, in order to resolve some of
the discrepancies. To this end, excitation and ionisation caused by both the
radiation field and by atom-ion collisions have been included, along with
recombination through charge transfer. The modelling is carried out using
conditions which would be present in the quiet Sun, which allows an assessment
of the part atomic processes play in changing coronal modelling, separately
from dynamic and transient events taking place in the plasma. The effect the
processes have on the fractional ion populations are presented, as well as the
change in level populations brought about by the new excitation mechanisms.
Contribution functions of selected lines from low charge states are also shown,
to demonstrate the extent to which line emission in the lower atmosphere could
be affected by the new modelling.
