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1] 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.

The Electron Drift Instrument (EDI) on the Magnetospheric Multiscale (MMS) mission measures the in-situ electric and magnetic fields using the drift of a weak beam of test electrons that, when emitted in certain directions, return to the spacecraft after one or more gyrations. This drift is related to the electric field and, to a lesser extent, the gradient in the magnetic field. Although these two quantities can be determined separately by use of different electron energies, for MMS regions of interest the magnetic field gradient contribution is negligible. As a by-product of the drift determination, the magnetic field strength and constraints on its direction are also determined. The present paper describes the scientific objectives, the experimental method, and the technical realization of the various elements of the instrument on MMS.

[1] Langmuir wave characteristics in the Earth's foreshock were examined to identify possible nonlinear wave behavior for two case studies with data from the Cluster Wideband Data Plasma Wave Receiver. The occurrence rates of four types of power spectra near the foreshock edge were determined: (1) spectra with power at the local plasma frequency f pe only, (2) spectra with power at f pe and 2f pe , (3) spectra with double peaks near f pe , and (4) spectra with double peaks near f pe and peaks at low frequencies indicative of ion acoustic waves. For electric field waveform amplitudes between 0.1 and 22.0 mV/m, most power spectra fell into the f pe only and double-peaked categories. The maximum Langmuir wave amplitudes and bump-on-tail reduced electron distribution functions from Cluster PEACE data were more consistent with saturation of wave growth by electrostatic decay than modulational instabilities. However, few spectra had the double peaks near f pe and ion acoustic waves indicative of electrostatic decay, suggesting other processes may also be at work. For amplitudes greater than 22.0 mV/m, most power spectra fell into the f pe and 2f pe category, but many of the harmonics were too weak to be clearly distinguished from harmonics caused by instrumental effects.