Volume 631, November 2019
|Number of page(s)||9|
|Section||Letters to the Editor|
|Published online||05 November 2019|
Letter to the Editor
Characterization and formation of on-disk spicules in the Ca II K and Mg II k spectral lines
Institute of Theoretical Astrophysics, University of Oslo, PO Box 1029, Blindern 0315 Oslo, Norway
2 Rosseland Centre for Solar Physics, University of Oslo, PO Box 1029, Blindern 0315 Oslo, Norway
Accepted: 11 October 2019
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.
Key words: Sun: chromosphere / line: profiles / line: formation / radiative transfer / opacity
© ESO 2019
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