compare the temporal evolution of Fe [i]{} 2814.11Å, Fe [ii]{} 2813.33Å & 2814.45Å, and Mg [ii]{} 2791.6Å spectra, acquired in one pixel near the brightest part of the ribbon (y $\simeq$ 119, cf. e.g. Figures \[fig:irisaia\] and \[fig:difference\]). After a small rise in intensity shortly after 17:30 UT, a strong impulsive burst is observed at the lines’ undisturbed position, accompanied by a noticeable increase of the continuum level around the Fe lines (the variation of the continuum around the Mg [ii]{} 2791.6Å line cannot be clearly appreciated within the small spectral range displayed). At the same time, a separate component appears far in the red wing of all the lines, and rapidly migrates back towards the lines’ centroid over the next 4-5 time steps, i.e. in less than a minute.

The top panel of Figure \[fig:ltspec\] shows the Fe [ii]{} and Fe [i]{} lines, their behaviour essentially identical. The same pattern occurs for the Mg [ii]{} subordinate line (middle panel), and is observed with varying clarity in many other pixels and lines (see Figures \[fig:spectraA\], \[fig:spectraB\], and Appendix \[sec:appendixA\]), so the effect is certainly not due to unidentified lines. Common to all the spectral lines shown in Figure \[fig:ltspec\] is 