Reduced χ² of the residuals between each 100-day sub-profile and the main template as a function of date. Two sub-profiles are in the time interval when imperfect polarisation calibration was performed (‘UnperfectPolCal’, see main text). The inset shows the residuals obtained for one of these sub-profiles, while others do not show discernible structure (i.e. noise). The two horizontal lines delimit the two-standard-deviation interval around the mean of the reduced χ² distribution corresponding to σ_red.χ² ≃ 0.043 (imperfect polarisation region excluded).