wavelength of each line. The wavelength scale was established by fitting the photospheric Ni [i]{} 2799.474Å line over a 3 hour period before the flare, to remove the residual orbital oscillation as described in the IRIS technical note ITN20. We also use the procedure described in IRIS Technical Note ITN24 [^1] to convert the raw DN output to intensity, as done e.g. by @2016ApJ...816...88K and @2017ApJ...836...12K. No subtraction of pre-flare background has been performed.

In Figs. \[fig:spectraA\] and \[fig:spectraB\] we see that all lines show a strong intensity increase at 17:30:26 (second row), with a concomitant, broad satellite component appearing at high red-shifts. The shifted component is weaker at first detection, but in the next two time steps its intensity becomes comparable with the main peak, which remains mostly stationary. While increasing in intensity, the red-shifted component rapidly migrates towards the rest wavelength, so that by 17:30:54 (only 37 s from the “pre-flare” time) the general appearance is that of a single, broad, asymmetric line. It is worth noticing that the red-shifted component is more prominent in the Mg [ii]{} spectra (Fig. \[fig:spectraB\]), and can often be unambiguously detected in them one time-step ahead of the other lines (cf. also 