Top row: evolution of Event (a) through time as sampled by the IRIS 1400 Å SJI channel. Second row: Si IV 1394 Å intensity at the line rest wavelength co-spatial and co-temporal to the SJI images. Third row: same but for spectral position corresponding to an apparent Doppler velocity of 58 km s⁻¹ in the red wing of the Si IV 1394 Å line. Fourth row: equivalent plots for the O IV 1401 Å line core. Bottom row: same as above but for 57 km s⁻¹ into the red wing of the O IV 1401 Å line. The downflow can be identified easily as the region of increased brightness in the third and bottom rows. The symbols over-laid on the middle left panel indicates the pixels whose spectra are plotted through time in Fig. 3.