  Spacecraft data reveals a nonuniform ambipolar electric field transverse to
the magnetic field in a thin magnetotail current sheet that leads to intense
ExB velocity shear and non-gyrotropic particle distributions. The ExB drift far
exceeds the diamagnetic drift and drives lower hybrid waves localized to the
magnetic field reversal region, which is ideally suited for the anomalous
dissipation necessary for reconnection. It also reveals substructures embedded
in the current density, indicating the formation of a non-ideal current sheet.
