Outer magnetosphere near midnight at quiet and disturbed times

Ogo 5 magnetic-field and energetic-electron (E > 50 kev) data are used to study both the quiet-time, steady-state configuration of the outer magnetosphere or near tail region near midnight and the disturbed time changes of this configuration. The nighttime cusp is found to be a distinct feature within the plasma sheet at quiet times but indistinguishable from the plasma sheet at disturbed times. The sequence of thinning and expansion of the plasma sheet in this region in association with the substorms is studied. The response of the plasma sheet in the near tail at ∼10 RE is found to be similar to that in the more distant tail at >20 RE. Finally, the nature of field-aligned currents flowing on the plasma-sheet boundary is investigated. Assuming infinite current sheets, the sheet current density at Ogo 5 is found to be approximately 10−2 amp m−1. At ionospheric altitudes in the auroral zone, these currents scale to 10−1 amp m−1, in good agreement with low-altitude measurements. These currents in space often appear in double or multiple sheets.