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

TL;DR

This paper characterizes type-II spicules in the solar chromosphere using Ca II K and Mg II k spectral lines, revealing their formation mechanisms and producing synthetic profiles that match observations.

Contribution

First characterization of type-II spicules in Ca II K lines, linking line formation to opacity shifts and producing matching synthetic profiles.

Findings

Line formation dominated by opacity shifts, with K3 best representing velocity.

K2 features are suppressed or enhanced due to Doppler shifts and opacity effects.

Synthetic profiles successfully replicate observed spectral features.

Abstract

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.