DEPLETION AS A SIGNATURE OF INTERCONNECTION

If the magnetosphere is open, the magnetosheath must contain the separatrix surface between magnetic flux tubes connected and not connected to Earth. This paper suggests that the separatrix may form the outer boundary of the depletion layer, which is normally assumed to reside on flux tubes with neither end connected to Earth. Depletion on interconnected flux tubes should be more pronounced owing to their proximity to the magnetopause and to particle precipitation through the cusp to the ionosphere. Observations of magnetospheric ions in the depletion layer and energetic electrons at its outer boundary are consistent with magnetospheric particle outflow on newly opened field lines. Depletion layers as signatures of interconnection should occur for all IMF orientations. For northward IMF, the depletion layer should form part of the overdraped lobe topology that has recently been proposed as a condition for reverse convection. This topology is consistent with reported depletion and boundary layer structure.