Time series observations of the magnetotail lobe magnetic field have been Fourier analyzed to compute the frequency-weighted energy density Pfz in the range 1--30 mHz. Pfz is generally observed in the range 10-4--10-2 &ggr;2--Hz, with a mean value of 1.2¿10-3 &ggr;2--Hz during substorm growth phases and 1.0¿10-3 &ggr;2--Hz in the comparison intervals. No strong correlation of Pfz is found with AE in either set of intervals, but during substorm growth phases Pfz may vary by an order of magnitude over time scales of 30 min, with a tendency for higher power levels to occur later in the growth phase. Increases in Pfz are seen to precede by ~10 min localized expansive phase activity observed in individual magnetograms. The observed levels of Pfz during growth phases are in general accord with the requirements of the thermal catastrophe substorm model. ¿ American Geophysical Union 1990