Panel a: map of I₁₃/I₁₂ line ratio. The colours range from 1.6 to 2.3. The map is smoothed to 3 × 3 pixels. Black areas do not have a signal because their counts are too low. The white isocontours indicate the 1.9 values and the black isocontours the 2.1 ratios. Panel b: scatter plot of line ratio as a function of I₁₃, the Si IV 1393.757 Å intensity. The black and blue plus signs represent the selected points with line ratios higher than 2. Black plus signs also indicate selected points with line ratios lower than 1.6