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The formation of IRIS diagnostics VIII. IRIS observations in the C II 133.5 nm multiplet

The C II 133.5 nm multiplet has been observed by NASA's Interface Region Imaging Spectrograph (IRIS) in unprecedented spatial resolution. The aims of this work are to characterize these new observations of the C II lines, place them in context with previous work, and to identify any additional value the C II lines bring when compared with other spectral lines. We make use of wide, long exposure IRIS rasters covering the quiet Sun and an active region. Line properties such as velocity shift and width are extracted from individual spectra and analyzed. The lines have a variety of shapes (mostly single-peak or double-peak), are strongest in active regions and weaker in the quiet Sun. The ratio between the 133.4 nm and 133.5 nm components is always less than 1.8, indicating that their radiation is optically thick in all locations. Maps of the C II line widths are a powerful new diagnostic of chromospheric structures, and their line shifts are a robust velocity diagnostic. Compared with earlier quiet Sun observations, we find similar absolute intensities and mean line widths, but smaller red shifts; this difference can perhaps be attributed to differences in spectral resolution and spatial coverage. The C II intensity maps are somewhat similar to those of transition region lines, but also share some features with chromospheric maps such as those from the Mg II k line, indicating that they are formed between the upper chromosphere and transition region. C II intensity, width, and velocity maps can therefore be used to gather additional information about the upper chromosphere.