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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.

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[1] We performed a statistical study of the locations of chorus emissions observed by the Polar spacecraft's Plasma Wave Instrument (PWI) from March 1996 to September 1997, near the minimum of solar cycles 22/23. We examined how the occurrence of chorus emissions in the Polar PWI data set depends upon magnetic local time, magnetic latitude, L shell, and L*. The Polar PWI observed chorus most often over a range of magnetic local times extending from about 2100 MLT around to the dawn flank and into the dayside magnetosphere near 1500 MLT. Chorus was least likely to be observed near the dusk flank. On the dayside, near noon, the region in which Polar observed chorus extended to larger radial distances and higher latitudes than at other local times. Away from noon, the regions in which chorus occurred were more restricted in both radial and latitudinal extent. We found that for high-latitude chorus near local noon, L* provides a more reasonable mapping to the equatorial plane than the standard L shell. Chorus was observed slightly more often when the magnitude of the solar wind magnetic field B SW was greater than 5 nT than it was for smaller interplanetary magnetic field strengths. We also found that near solar minimum, chorus is twice as likely to be observed when the solar wind speed is greater than 450 km/s than it is when the solar wind speed is less than 450 km/s.

[1] We present the first results of Langmuir wave observations in the foreshock from the Cluster WBD Plasma Wave Receiver. When the data were binned by distance to the foreshock boundary, the Langmuir wave amplitude probability distributions followed the lognormal statistics predicted by stochastic growth theory for all regions of the foreshock. The Cluster data show for the first time that the centers of the probability distributions shift to lower amplitudes with increasing distance to the boundary, and that a spatially averaged power law distribution results from summing these distributions.

[1] Ion beams flowing downward, into the ionosphere, along the Earth's magnetic field have frequently been observed by the FAST satellite in the auroral zone. These discrete downward moving ion beams (DFI) have been characterized by Klumpar et al. [1999] who interpreted the horseshoe-shaped distributions as being consistent with acceleration in a parallel potential drop above the satellite, followed by motion into a region of increased magnetic field strength. The down-flowing ion beams are associated with an intense narrowband electrostatic emission at the lower hybrid frequency, polarized perpendicular to the geomagnetic field. Hydrogen cyclotron harmonics both above and below the lower hybrid frequency are also very common. These are the first observations of down-flowing ions and associated waves outside of the cusp, and the physical mechanism producing the ions is very different from the one associated with cusp ion injections. The DFI events that had a monotonic increase in energy were associated with a clear field-aligned current signature. The DFI densities were usually $5-10/cc, whereas the background plasma had densities up to 100/cc. The wave and DFI observations are consistent with linear dispersion relation calculations and simulations with ion ring distributions that show that the instability is due to coupling of the ion Bernstein waves to the lower hybrid wave. In addition, for a few events, electrostatic ion cyclotron waves were observed. Such waves are usually associated with up-going ion beams and have not previously been seen with DFI, which have a very different shape in the distribution.