Fig. 2: Community structure snapshots of an example substorm in normalized time, \(t^{\prime}\).

Polar plots are in magnetic coordinates centered at the magnetic pole, where magnetic local time (MLT, h) increases clockwise, with midnight located at the bottom (MLT = 0 h). Maps show the nightside from dusk (MLT = 18 h) to dawn (MLT = 6 h) and 60–90° magnetic latitude. Plotted are the magnetic field perturbation vectors (North and East components, B_N,E, measured in nT) for a substorm on 16/03/1997. The colorbars at the bottom of the figure represent the MLT of the centroid (\(\bar{\theta }(t^{\prime} )\)) of each community, and polar VIS data from left to right, respectively. The vectors are color-coded using the left and match those of c, d in Fig. 1. Each subplot (a–h) represents a snapshot of the community structure in intervals of five normalized minutes from before onset (a, \(t^{\prime} =-5\)) to the time of maximum expansion (h, \(t^{\prime} =30\)), corresponding to the times in Fig. 1. The circles represent ground magnetometers with the line representing the B_N,E vector. Black magnetometers are not part of a community. The dashed lines are the locations of the auroral bulge found from auroral images. The vectors are overplotted on maps provided by superMAG² containing polar VIS data²⁸ in kR, matching the right colorbar.