Dependence of the vertically emergent intensity in Mg II k on how the electron density is computed. Panel a: spatially averaged relative change in the intensity, which is computed using either non-LTE or LTE electron densities. Panels b,c: line intensities expressed as brightness temperature T_b and computed using LTE (blue) or non-LTE (red) electron density at two structures indicated in Fig. 7. Panel b: dark fibril, red dot in Fig. 7. Panel c: flare ribbon, blue dot in Fig. 7.