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 (~4 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 ≥ 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 ~5 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.