Prominence reconstructed maps based on the IRIS spectra of Mg II k line (raster 8). (a) Integrated intensity of Mg II k line, (b) Doppler velocity, and (c) width (FWHM) of Mg II k line profile obtained by fitting the profiles with a single Gaussian function. The inner contour line (around x = 10 arcsec) corresponds to the solar surface defined as the level 0.6 Å of the FWHM of Mg II k line; the outer contour corresponds to Mask-Hα based on MSDP observations (see Mask-Hα in Fig. 2). This new contour defined by Mask-Hα (pixels on the disk) is used for the statistics of Mg II parameters in and out the Hα prominence (see Sect. 4.7). The blue dotted lines in panel a are the two selected slit positions (A, B) used to analyse the profiles (Figs. A.3 and A.5) and for plasma parameter diagnostics (Table 3).