Measured spectra from diffraction orders 174 and 175 for four occultations recorded between L_s 220 and 240° at high northern latitudes. Shown are the individual spectra extracted from a dozen detector rows covering the same diffraction orders (shades of purple). Also shown are the mean spectra with reduced noise, and best-fit lines for the mean spectra. Positions of major HCl and H₂ O lines are indicated with dashed vertical lines in orange and blue, respectively. The first two occultations, panels a and b, do not feature a detectable quantity of HCl or water vapour, while the later occultations, panels c and d, feature 0.5 and 0.9 ppbv HCl and around 60 and 120 ppmv H₂ O, respectively.