Abstract. Bipolar pulses of ∼25-100 µs in duration have been observed in the wave electric field data obtained by the Wideband plasma wave instrument on the Cluster spacecraft in the dayside magnetosheath. These pulses are similar in almost all respects to those observed on several spacecraft over the last few years. They represent solitary potential structures, and in this case, electron phase space holes. When the time series data containing the bipolar pulses on Cluster are transformed to the frequency domain by a windowed FFT, the pulses appear as typical broad-band features, extending from the low-frequency cutoff of the bandpass filter, ∼1 kHz, up to as great as 20-40 kHz in some cases, with decreasing intensity as the frequency increases. The upper frequency cutoff of the broad band is an indication of the individual pulse durations (1/f). The solitary potential structures are detected when the local magnetic field is contained primarily in the spin plane, indicating that they propagate along the magnetic field. Their frequency extent and intensity seem to increase as the angle between the directions of the magnetic field and the plasma flow decreases from 90 • . Of major significance is the finding that the overall profile of the broad-band features observed simultaneously by two Cluster spacecraft, separated by a distance of over 750 km, are strikingly similar in terms of onset times, frequency extent, intensity, and termination. This implies that the generation region of the solitary potential structures observed in the magnetosheath near the bow shock is very large and may be located at or near the bow shock, or be connected with the bow shock in some way.

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Abstract. To determine the wavelength of waves within a random, isotropic wave field, we introduce the observable of wave coherency measured with plasma wave interferometers. We show generally that within a random direction wave field, wavelengths large compared to the interferometer length produce large coherency (nearly 1), but wavelengths the order of a few times the interferometer length, or smaller, produce small coherency (close to zero). We apply this principle first to examining auroral hiss and lower hybrid waves measured by the Physics of Auroral Zone Electrons (PHAZE) 2 and Topside Probe of the Auroral Zone (TOPAZ) 3 experiments and show that the implied wavelengths are consistent with the expected dispersion relations and with other, different estimates of wavelength for these modes. Next, we apply the principle to broadband extra low frequency (BB-ELF) electric fields observed in both experiments and conclude that the wavelengths are small. In one case we calculate the coherency of BB-ELF electric fields, using an ensemble average of 7889 data samples, and demonstrate that the coherency near the oxygen gyrofrequency is very small (=0.15), corresponding to wavelengths of 10 m and the order of the ion gyroradius. We conclude that because of the short wavelengths, previous satellite measurements of BB-ELF electric fields may have underestimated the electric field amplitudes, unless ion gyroradii are substantially larger than the case for these rocket measurements. Although the wavelengths and frequencies of BB-ELF electric fields are now known, we are unable to assign the wave to a known, normal mode of homogeneous plasmas. This suggests that inhomogeneities may be essential for describing BB-ELF electric fields.

[1] We perform intermittency analyses on the electric field data obtained by the SIERRA sounding rocket in the auroral zone. The electric field fluctuations are broadband, covering the extremely low-frequency range with a power-law relation, similar to the type of fluctuations commonly observed at various altitudes of the auroral region. Our preliminary analyses of the data based on the technique of probability distribution functions indicate that the electric field fluctuations are intermittent in the spacecraft frame. Using the methods of wavelet analyses and local intermittency measures, we determine the degree of intermittency of the fluctuations at various scales. It is found that the electric field fluctuations are more intermittent at smaller scales. Citation: Tam, S. W. Y., T. Chang, P. M.Kintner, and E. Klatt (2005), Intermittency analyses on the SIERRA measurements of the electric field fluctuations in the auroral zone, Geophys. Res. Lett., 32, L05109,

[1] The NASA SIERRA rocket mission was a multiple-payload, auroral plasma physics experiment that flew into a 100 nT auroral substorm at altitudes up to 735 km over the Poker Flat Research Range in Alaska on 14 January 2002. The flight environment was composed of an equatorward region of inverted-V electron precipitation followed by a poleward region of mostly field-aligned suprathermal electron bursts. In the inverted-V region, the measured dE/dB implied a stationary field-aligned current configuration. In the poleward region, the average electric field was much larger, 50 mV/m compared with 20 mV/m, included time-varying components that exceeded 100 mV/m with periods of 0.5-5 s, and dE/dB was Alfvénic. Our multiple-payload observations show that the perpendicular scale size of most of these waves is on the order of a kilometer or less. At smaller scales, broadband, extremely low-frequency (BBELF) fluctuations associated with observed Alfvén waves had spectral ratios (dE/dB) appropriate for inertial Alfvén waves larger and smaller than the electron inertial length (l e = c/w pe ), but the phase angle differences between dE and dB indicate mostly propagating Alfvén waves for spatial scales much larger than l e and a transition to spatial perturbations, embedded in the plasma, for scales near l e . The data are consistent with the model of kilometer-scale oblique Alfvénic arcs that are unstable to the current shear-driven instabilities discussed in previous theoretical investigations, lending support to the conclusion that the observed BBELF fluctuations are likely driven by the wave-associated field-aligned current shear.