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

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/ω_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 × B drift of the electrons ∼ 10,000 km/s (0.25 × electron Alfven velocity), which can account for the magnitude of the cross-tail current associated with the thin current sheet.