1Physics Department, Fac ul t y of Science, Imam Khomeini International University, Qazvin 34149-16818, Islamic Republic of Iran
2Astronomical Institute, Academy of Sciences of the Czech Republic (v.v.i.), Fricova 298, CZ25165 Ondˇrejov, Czech Republic;
3Instituto de Astrof´ısica de Andaluc´ıa (CSIC), Apdo. 3004, 18080 Granada, Spain;
4Institute for Solar Physics, Department of Astronomy, Stockholm University, AlbaNova Uni- versity Center, 106 91 Stockholm, Sweden;
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.