Ultra‐low‐frequency wave power in the magnetotail lobes 1. Relation to substorm Onsets and the auroral electrojet index

Abstract

Time series observations of the magnetotail lobe magnetic field have been Fourier analyzed to compute the frequency‐weighted energy density P fz in the range 1–30 mHz. P fz is generally observed in the range 10 −4 – 10 −2 γ 2 – Hz, with a mean value of 1.2 × 10 −3 γ 2 – Hz during substorm phases and 1.0 × 10 −3 γ 2 – Hz in the comparison intervals. No strong correlation of P fz is found with AE in set of intervals, but during substorm growth phases P fz 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 P fz are seen to precede by ∼ 10 min localized expansive phase activity observed in individual magnetograms. The observed levels of P fz during growth phase are in general accord with the requirements of the thermal catastrophe substorm model.