Geotail observations of low‐frequency waves and high‐speed earthward flows during substorm onsets in the near magnetotail from 10 to 13 R E

Geotail electric and magnetic field data from five substorms were used to examine the relationship between low‐frequency waves and high‐speed earthward flows at radial distances between 10 and 13 R E . Strong compressional fluctuations of the magnetic field in the Pi2 frequency range (0.007–0.03 Hz) were observed during the 26 April 1995 substorm and the other four substorms studied, in association with high‐speed earthward flows and magnetic field dipolarizations. However, the maximum earthward flow was generally observed 30 s to a few minutes after the start of the dipolarizations and magnetic field fluctuations in the Pi2 frequency range. Waves near the ion gyrofrequency (∼0.1–1.0 Hz) and the lower hybrid frequency (∼5–16 Hz) were also observed during all five substorms. The maximum amplitudes at these frequencies were not observed until after the start of the magnetic field dipolarization and earthward flow, which does not appear to be consistent with local substorm initiation by a cross‐field current driven instability. Recent work has shown the importance of high‐speed earthward flow bursts as drivers of substorm activity. However, we found that the earthward kinetic energy flux was much smaller than the Poynting flux or the thermal energy. This is consistent with the idea that currents driven by thermal pressure gradients and magnetic field changes are responsible for a major part of the substorm current wedge.