Skew angle of the field vs. normalized distance along the PIL for the trailing bipole. The skew angle is calculated on day 3 (dotted line), day 10 (dashed line), day 15 (solid line), and day 25 (dash‐dotted line). In each case the skew is calculated from the southernmost point of the bipole to the northernmost point, where the normalized distance along the PIL is used, as on each day the length of the PIL changes.