Sharp boundary between the inner magnetosphere and active outer plasma sheet

[1] We report observations of a sharp spatial boundary between the outer plasma sheet and the inner magnetosphere. It was successively crossed by the Cluster spacecraft in their pearl-on-string configuration near the perigee (similar to4 Re) at midnight during a substorm expansion phase. Being mapped presumably to 8 - 10 Re in the equatorial tail, this boundary was extremely thin, comparable to a gyroradius of plasma sheet proton. Substantial changes on this spatial scale were observed coherently in ( 1) the fluxes of radiation belt energetic electrons ( exceeding a factor 10 at E greater than or equal to 100 keV), ( 2) the plasma pressure ( by a factor of 2), ( 3) the density and outflow of the cold ionospheric plasma. Strong diverging electric field ( azimuthal shear flow) coincides with this boundary and is accompanied by a strong downward field-aligned current. While this boundary was staying at nearly the same location during the similar to5 min time scale, we also found indications of its dynamical origin. We suppose it could be generated by a sudden braking and azimuthal deflection of localized bursty fast flows produced by the magnetic reconnection which was going on at this time in the near tail.