Logarithm of the intensity (in DN) for three low-latitude coronal holes and adjacent QS regions in the 193 Å (top panels) and 171 Å (bottom panels) emission: CH1 (2017 April 16; left), CH2 (2017 April 21; middle), CH3 (2017 February 21; right). The boundaries denoted by the black line are specified at 45 DN, 55 DN, and 65 DN of the 193 Å intensity, respectively.