A study on Ca II 854.2 nm emission in a sunspot umbra using a thin cloud model

In the present work, we introduce and explain a method of solution of the radiative transfer equation based on a thin cloud model. The efficiency of this method to retrieve dynamical chromospheric parameters from Stokes I profiles of Ca II 854.2 nm line showing spectral emission is investigated. The analyzed data were recorded with the Crisp Imaging Spectro-Polarimeter (CRISP) at Swedish 1-m Solar Telescope on La Palma on 2012 May 5 between 8:11 - 9:00 UT. The target was a large decaying sunspot (NOAA 11471) at heliocentric position W 15◦ S 19◦. This sunspot has a large umbra divided into two umbral cores (UCs). One of these UCs shows steady spectral emission in both Ca II 854.2 nm and H_ lines, where downflows prevail. The other UC shows intermittent spectral emission only in Ca II 854.2 nm, when umbral flashes are propagating. The statistics of the obtained Doppler velocities in both UCs is discussed.