[1] Measurements from the Cluster spacecraft of electric fields, magnetic fields, and ions are used to study the structure and dynamics of the reconnection region in the tail at distances of $18 R E near 22.4 MLT on 1 October 2001. This paper focuses on measurements of the large amplitude normal component of the electric field observed in the ion decoupling region near the reconnection x-line, the structure of the associated potential drops across the current sheet, and the role of the electrostatic potential structure in the ballistic acceleration of ions across the current sheet. The thinnest current sheet observed during this interval was bifurcated into a pair of current sheets and the measured width of the individual current sheet was 60-100 km (3-5 c/w pe ). Coinciding with the pair of thin current sheets is a large-amplitude (±60 mV/m) bipolar electric field structure directed normal to the current sheets toward the midplane of the plasma sheet. The potential drop between the outer boundary of the thin current sheet and the neutral sheet due to this electric field is 4-6 kV. This electric field structure produces a 4-6 kV electric potential well centered on the separatrix region. Measured H + velocity space distributions obtained inside the current layers provide evidence that the H + fluids from the northern and southern tail lobes are accelerated into the potential well, producing a pair of counterstreaming, monoenergetic H + beams. These beams are directed within 20 degrees of the normal direction with energies of 4-6 keV. The data also suggest there is ballistic acceleration of O + in a similar larger-scale potential well of 10-30 kV spatially coinciding with the larger scale size ($1000-3000 km) portions of current sheet surrounding the thin current sheet. Distribution functions show counterstreaming O + populations with energies of $20 keV accelerated along the average normal direction within this large-scale potential structure. The normal component of the electric field in the thin current sheet layer is large enough to drive an E Â B drift of the electrons $10,000 km/s (0.25 x electron Alfven velocity), which can account for the magnitude of the cross-tail current associated with the thin current sheet.Citation: Wygant, J. R., et al. (2005), Cluster observations of an intense normal component of the electric field at a thin reconnecting current sheet in the tail and its role in the shock-like acceleration of the ion fluid into the separatrix region,

[1] Large-amplitude (up to $50 mV/m) solitary waves, identified as electron holes, have been observed during waveform captures on two of the four Cluster satellites during several plasma sheet encounters that have been identified as the passage of a magnetotail reconnection x line. The electron holes were seen near the outer edge of the plasma sheet, within and on the edge of a density cavity, at distances on the order of a few ion inertial lengths from the center of the current sheet. The electron holes occur during intervals when there were narrow electron beams but not when the distributions were more isotropic or contained beams that were broad in pitch angle. The region containing the narrow beams (and therefore the electron holes) can extend over thousands of kilometers in the x and y directions, but is very narrow in the z direction. The association with electron beams and the density cavity and the location along the separatrices are consistent with simulations shown herein. The velocities and scale sizes of the electron holes are consistent with the predictions of Drake et al. [2003]. Particle simulations of magnetic reconnection reproduce the observed Cluster data only with the addition of a small (0.2 of the reversed field) ambient guide field. The results suggest that electron holes may sometimes be an intrinsic feature of magnetotail reconnection and that in such cases the traditional neglect of the guide field may not be justified. Very large amplitude lower hybrid waves (hundreds of millivolts per meter), as well as waves at frequencies up to the electron plasma frequency, were also observed during this interval.

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[1] Solitary waves have, for the first time, been identified in 3D electric field data at the subsolar, equatorial magnetopause. These nonlinear, bipolar electric field pulses parallel to the magnetic field occur both as individual spikes and as trains of spikes. The solitary waves have amplitudes up to $25 mV/m, and velocities from $150 km/s to >2000 km/s, with scale sizes the order of a kilometer (comparable to the Debye length). Almost all the observed solitary waves are positive potential structures with potentials of $0.1 to 5 Volts. They are often associated with very large amplitude waves in either or both the electric and magnetic fields. Although most of the observed signatures are consistent with an electron hole mode, the events with very low velocities and the few negative potential structures may be indicative of a second type of solitary wave in the magnetopause current layer. The solitary waves may be an important source of dissipation and diffusion at the magnetopause.