Q-band (top panels) and W-band (bottom panels) observations of SO₂ at a total pressure of 0.016 mbar in the chamber. The frequency switching mode has been used with a throw of 37 MHz. The different panels for each band always show the same frequency range but with different intensity scales. The total observing time per W sub-band (WL, WM, and WH) is 4.4 h. Hence, the Q band has been observed for 13.2 h. The negative features seen in the different panels are produced by the folding of the frequency switched data and appear at ±37 MHz of each rotational transition. The data shown in this figure have been smoothed by a two-channel box procedure.