Temporal evolution of the electron density and the line intensity. Panel a: the electron density in the source region of OIV in the transition region derived from the ratio of the intercombination O IV lines at 1401 and 1400 Å including an error estimate. The black solid curve shows the fit to the density variation. For this fit the time range indicated by the shaded area was ignored because these data points are flawed by the strong blending of O IV 1401 Å by the excess emission in the far blue wing of Si IV 1403 Å. Panels b to d: the normalized intensity of Si IV 1394 Å, O IV 1401 Å, and C II 1335 Å (black solid curves). The black dashed curves in panels b and c show the normalized density squared , while the black dotted curve in panel d shows the normalized density . The black dotted vertical lines indicate the boundary that separates the loop brightening into two phases. The green dotted vertical lines demarcate the time span when the density estimation is not reliable. Just as in Fig. 3, all values are taken at the position in the middle of the loop where it crosses the slit. See Sect. 4.