Amplitudes (in nT/Hz) of the Fourier transform of the magnetic northward component at subauroral NPM sites whose data was shown in Fig. 6. Additionally, data from the federal government observatory 18 km S of ATH at Meanook (MEA) and Inuvik (INU), whose auroral zone position may be seen in Fig. 9, are shown. Gray bands indicate frequencies near 0.0015, 0.003, and 0.0045 Hz, which appear to be harmonically related. A wider spectral peak near 0.0055 Hz is indicated by light gray shading. The same frequencies are detected at all stations, including NPM stations ATH and GPR. Further, MEA is an observatory class instrument 18 km south of the ATH NPM, with an almost identical spectrum. The vertical scale is logarithmic over three decades.