Samples of spectral profiles from the solar chromosphere and transition region at the site of the UV burst. In panel a, the background image is a Mg ii λ−t plot (in Doppler units) near 2796 Å, obtained from the spatial location marked by a plus in the inset. Here the inset shows a sub-field of view of the raster map at the mark of 37 min (indicated by a small horizontal blue bar). The blue curve is the Mg ii line profile at that instance. Panel b is the same as panel a, but plotted for the Si iv 1403 Å line that traces the solar transition region. See Sect. 2.2.