Dispersion of polarisation vectors (the square root of the second-order structure function) binned to the Nyquist sampled resolution, for two velocity channels of the CO J = 2 − 1 polarised emission around OH 17.7−2.0. The error bars indicate the variance in each bin. The vertical dotted line indicates the size of the beam major axis. The solid line indicates the fit of the structure function analysis described in the text.