Letter to the Editor

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

¹ Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029, Blindern 0315 Oslo, Norway
e-mail: souvik.bose@astro.uio.no
² Rosseland Centre for Solar Physics, University of Oslo, PO Box 1029, Blindern 0315 Oslo, Norway

Received: 2 September 2019
Accepted: 11 October 2019

Abstract

We characterize, for the first time, type-II spicules in Ca II K 3934 Å using the CHROMIS instrument at the Swedish 1 m Solar Telescope. We find that their line formation is dominated by opacity shifts with the K₃ minimum best representing the velocity of the spicules. The K₂ features are either suppressed by the Doppler-shifted K₃ or enhanced via increased contribution from the lower layers, leading to strongly enhanced but unshifted K₂ 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 2796Å observations with very similar properties. Using our interpretation of spicule chromospheric line formation, we produce synthetic profiles that match observations.