Density diagnosis for events 1 and 5. (A) and (B) IRIS spectral profiles (black lines), including the O IV 1400/1401 doublet lines used for the density diagnosis. Panel (A) corresponds to event 1 shown in Figs. 1A–D, while (B) corresponds to event 5 shown in Figs. 1E–H. The red dashed lines are the multi-Gaussian fitting result. (C) Theoretically predicted curve associating the O IV intensity ratio with the electron number density. The small black and red squares represent the intensity ratios obtained from (A) and (B), respectively, as well as their corresponding densities. The error bars are also added.