Some of the strongest lines in the solar UV spectrum come from singly ionized carbon, at wavelengths of 1334-1335 Å. The C II 1335 lines will be central observables with the NASA/SMEX mission Interface Region Imaging Spectrograph (IRIS). The lines are formed in the upper chromosphere/lower transition region and show great promise for the
diagnostics of the dynamic upper chromosphere.
The C II lines are formed in the optically thick, non-LTE regime and 3D numerical models are important for the deciphering of the diagnostic potential. To make 3D non-LTE radiative transfer computationally feasible it is crucial to have a model atom with as few levels as possible while retaining the main physical processes.
In this work we develop such a model atom and we study the formation of the C II lines to explore the diagnostic potential of these lines.