IRIS Mg II line profiles along slit at x = 46 arcsec crossing the prominence at 16:39:45 UT in 11 selected positions indicated in the panel title and corresponding to pixels along the vertical right line in Fig. 8 (a) and to Table 3 (B1-B11). The unit of the x-axis is Å, and the units for intensity are indicated on the left in 10⁴ erg cm⁻² s⁻¹ sr⁻¹ Å⁻¹ and on the right in 10⁻⁷ erg cm⁻² s⁻¹ sr⁻¹ Hz⁻¹.