Evolution of the diffuse coronal region. Panel a shows a repetition of Fig. 2b displaying the AIA 171 Å image at time t₁, with black and white contours indicating opposite magnetic patches in the photosphere. The red rectangle highlights the location of the diffuse coronal region. Panel b shows four snapshots in this rectangle at times t₁ to t₄, as indicated in panels c and d, (plotted in linear scaling). Panel c shows the variability of the emission averaged within the red rectangle, which does not exceed about 10% over more than five hours. Panel d displays the emission along the slit from A to B, as indicated in panel b. The intensity here is integrated (vertically) between the blue lines. The time axis in panels c and d is set to zero when the IRIS raster starts. The IRIS raster ends at about 210 min. The temporal evolution is available in an online movie. See Sect. 3.2.