Disc-averaged profiles of the Mg II K line (solid lines) seen by plasma at different altitudes when moving with different velocities. Altitudes: 5000 km, 10 000 km, 100 000 km, and 1R_Sun (from left to right). Radial velocities: 20 km s⁻¹, 30 km s⁻¹, 50 km s⁻¹, 70 km s⁻¹, and 100 km s⁻¹ (from top to bottom). The static angle-averaged profile is plotted in each panel using the dashed line. We note that we do not take into account the dilution factor.