Transition region and chromosphere of the active region core and upflow region observed with IRIS on 15 November 2017. The panels show the intensity (top row) and Doppler velocity (bottom row). The column is ordered with the decreasing line formation temperatures from 40 000 K (C II line, left column) to 6300 K (Mg II k2, right column). To distinguish the active region core and upflow region, we added the contour line of the velocity in Fe XII line of 5 km s⁻¹ and −5 km s⁻¹, respectively (defined in Fig. 2).