Average thicknesses of the PDR upstream of the TD (|B_N∕B₀|≤ 0.1) (a–b) in MSO coordinates, where , and (c–d) with error bars depending on SZA. The blue and red bars show the mean values of PDR thickness at every 10° of solar zenith angle (SZA) in the (a) northern and (b) southern hemisphere, respectively. The direction of the PDR bar is perpendicular to the mean local TD plane, and the dots are the average location of the inner boundary of the PDR, i.e. TDs as shown in Figs. 9a–c.