Sampling patterns of the ALMA image (panel a), the first (panel b) and the second (panel c) scans of IRIS, after spatial co-alignment. The colours in panels a–c indicate the observing times from the UT midnight. The observing-time difference maps between ALMA and the first and second IRIS scans (Δt₁ and Δt₂) are shown in panelsd and e, respectively. The positive values mean that the ALMA’s sampling leads that of the IRIS. The colour in panelsd and e represents the time-differences. Panel f: a mask where three regions with a maximum time-difference (indicated on top of the panel) are depicted. The mask is the result of combination of the first two IRIS scans (to cover the largest areas with small time-differences with ALMA observations).