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We use 3D radiation magnetohydrodynamic models to investigate how thethermodynamic quantities in the simulation are encoded in observablequantities, thus exploring the diagnostic potential of the 133.5 nm lines. Wefind that the line core intensity is correlated with the temperature at theformation height but the correlation is rather weak, especially when the linesare strong. The line core Doppler shift is a good measure of the line-of-sightvelocity at the formation height. The line width is both dependent on the widthof the absorption profile (thermal and non-thermal width) and an opacitybroadening factor of 1.2-4 due to the optically thick line formation with alarger broadening for double peak profiles. The 133.5 nm lines can be formedboth higher and lower than the core of the Mg II k line depending on the amountof plasma in the 14-50 kK temperature range. More plasma in this temperaturerange gives a higher 133.5 nm formation height relative to the Mg II k linecore. The synthetic line profiles have been compared with IRIS observations.The derived parameters from the simulated line profiles cover the parameterrange seen in observations but on average the synthetic profiles are toonarrow. We interpret this discrepancy as a combination of a lack of plasma atchromospheric temperatures in the simulation box and too small non-thermalvelocities. The large differences in the distribution of properties between thesynthetic profiles and the observed ones show that the 133.5 nm lines arepowerful diagnostics of the upper chromosphere and lower transition region.

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