Increase of the spread of EVPA differences between 353 and 217 GHz as a function of offset angle between HI structures from integration in LVC and IVC ranges (‘Implementation 1’). All sky pixels with N_c > 1.5 are binned according to their Δ(θ_IVC, θ_LVC) values and the statistic is computed for each subsample with observational uncertainties propagated. The error bars in each bin represent the 1σ value of from a bootstrap resampling of the data 10³ times per bin.