Fig. 4

A minimum curvature fit for the onset times in the (left) j = 5 or 48–192 s, (centre) j = 6 or 24–96 s, and (right) j = 7 or 12–48 s period bands as derived at each magnetometer station. Coloured, annotated contours represent the onset times, and the contours of each panel are separated by 32 s to show consistent onset contours across wave bands. The maximum error in this fit is of the order of seconds and hence represents the actual onset time at each station within stated errors. Stations that did not record an onset in the right-hand panel at lower latitudes are not used in this analysis