Intensity formation of the Mg II h (top) and Ca II K (bottom) spectral lines from the 5 keV empty loop model. The subfigures show the heating phase ((a) and (c)) and early post-heating phase ((b) and (d)) for the two lines. The quantities given at the top left corner are shown in greyscale as functions of frequency from line centre (in units of Doppler offset) and height z. The τ_ν = 1 height (purple) and vertical velocity (red dotted) are displayed in all panels. Negative (positive) velocities correspond to upflows (downflows). In addition, the top right panels show the total source function at v_D = 0 (yellow dashed) and Planck function (green dashed) in units of brightness temperature, with high values to the left. The bottom right panels also contain the intensity profile (pink) as brightness temperature. Gamma correction is added to the χ_ν/τ_ν and C_Iν terms to amplify the weaker signals.