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The C I 135.58 line is located in the wavelength range of NASA's InterfaceRegion Imagin Spectrograph (IRIS) small explorer mission. We here study theformation and diagnostic potential of this line by means of nonlocal-thermodynamic-equilibrium modeling, employing both 1D and 3Dradiation-magnetohydrodynamic models. The C I/C II ionization balance isstrongly influenced by photoionization by Ly-alpha emission. The emission inthe C I 135.58 line is dominated by a recombination cascade and the lineforming region is optically thick. The Doppler shift of the line correlatesstrongly with the vertical velocity in its line forming region, which istypically located at 1.5 Mm height. With IRIS the C I 135.58 line is usuallyobserved together with the O I 135.56 line, and from the Doppler shift of bothlines, we obtain the velocity difference between the line forming regions ofthe two lines. From the ratio of the C I/O I line core intensity, we candetermine the distance between the C I and the O I forming layers. Combinedwith the velocity difference, the velocity gradient at mid-chromosphericheights can be derived. The C I/O I total intensity line ratio is correlatedwith the inverse of the electron density in the mid-chromosphere. We concludethat the C I 135.58 line is an excellent probe of the middle chromosphere byitself, and together with the O I 135.56 line the two lines provide even moreinformation, which complements other powerful chromospheric diagnostics of IRISsuch as the Mg II h and k lines and the C II lines around 133.5 nm.

The Formation of IRIS Diagnostics. IX. The Formation of the C I 135.58 Line in the Solar Atmosphere