Examples of Mg II k line spectra: evolution of the Mg II 2796.4 Å spectra at 12:22, 12:28, 12:32, 12:40 UT in slit 1 (left). Examples of 2796.4 Å spectra in the four slits of IRIS located from east to west in the slit jaws (slit 1 at 12:38:29, slit 2 at 12:38:37 UT, slit 3 at 12:38:46 UT, slit 4 at 12:38:55 UT) (right). The spectra are between pixel 8 and pixel 314 along the slit. The wavelength range of the Mg II spectra is 2795.00–2797.78 Å and the length of the slit is 47′′. The dark line at pixel = 136 is an instrumental feature used for the alignment of the different lines in the spectra. The zero is taken as the velocity reference. North is up. A movie of the Mg II k spectra is available, see Fig. 14.