Characterization and formation of on-disk spicules in the Ca II K and Mg II k spectral lines

We characterize, for the first time, type-II spicules in Ca II K 3934\AA
using the CHROMIS instrument at the Swedish 1-m Solar Telescope. We find that their line formation is dominated by opacity shifts with the K${}_3$ minimum best representing the velocity of the spicules. The K${}_2$ features are either suppressed by the Doppler-shifted K${}_3$ or enhanced via an increased contribution from the lower layers, leading to strongly enhanced but un-shifted K${}_2$ peaks, with widening towards the line-core as consistent with upper-layer opacity removal via Doppler-shift. We identify spicule spectra in concurrent IRIS Mg II k 2796\AA\ observations with very similar properties. Using our interpretation of spicule chromospheric line-formation, we produce synthetic profiles that match observations.