The geometry of the current region used to calculate the B-field in Figure 8. The current flowing through a 200 km × 200 km region in the X–Y-plane located at a height of 50 km was divided up onto a set of smaller distributed line currents, I, each located in 100 × 100 grids having 2 km × 2 km in cross-sectional size. The width of the current region is 20 km. The B-field at any observation point is then calculated by summing the contributions from each of the smaller line currents I = I (x_s, y_s, z_s).