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The formation of IRIS diagnostics VI. The Diagnostic Potential of the C II Lines at 133.5 nm in the Solar Atmosphere

We use 3D radiation magnetohydrodynamic models to investigate how the thermodynamic quantities in the simulation are encoded in observable quantities, thus exploring the diagnostic potential of the 133.5 nm lines. We find that the line core intensity is correlated with the temperature at the formation height but the correlation is rather weak, especially when the lines are strong. The line core Doppler shift is a good measure of the line-of-sight velocity at the formation height. The line width is both dependent on the width of the absorption profile (thermal and non-thermal width) and an opacity broadening factor of 1.2-4 due to the optically thick line formation with a larger broadening for double peak profiles. The 133.5 nm lines can be formed both higher and lower than the core of the Mg II k line depending on the amount of plasma in the 14-50 kK temperature range. More plasma in this temperature range gives a higher 133.5 nm formation height relative to the Mg II k line core. The synthetic line profiles have been compared with IRIS observations. The derived parameters from the simulated line profiles cover the parameter range seen in observations but on average the synthetic profiles are too narrow. We interpret this discrepancy as a combination of a lack of plasma at chromospheric temperatures in the simulation box and too small non-thermal velocities. The large differences in the distribution of properties between the synthetic profiles and the observed ones show that the 133.5 nm lines are powerful diagnostics of the upper chromosphere and lower transition region.