¹²CO(4–3)/¹²CO(2–1) brightness temperature ratios predicted with RADEX for the plausible range of physical conditions in the ambient cloud of theMusca filament. The lines connect the RADEX models with identical density and temperature for varying ¹²CO column densities. The predicted ratios with RADEX show a huge discrepancy with the observed ¹²CO(4–3)/¹²CO(2–1) ratio towards the Musca cloud (indicated by the dashed line). The standard deviation of the observed values from the average is indicated by the grey area. The same is observed for the ¹²CO(4–3)/¹²CO(3–2) brightness temperature ratio.