Magnetic field perturbations in the equatorial plane. The first five plots show the field perturbations due to currents flowing on specific ranges of L-values, as marked and indicated by the dashed lines; the final plot, at the bottom right-hand side, shows the total field perturbation due to currents flowing on magnetic shells in the range L = 6–21, again indicated by the dashed lines. The colour-scale indicates the magnitude of the flux density. The arrows indicate the direction of the perturbation field. The solid line on each plot indicates the orbit of the hypothetical spacecraft discussed in the text.