SAPS

The Sub-Auroral Polarization Stream (SAPS) as defined by Foster and Burke [EOS, 83(36), 393, 2002] refers to the broad, persistent, poleward-directed electric field which drives sunward plasma convection at sub-auroral latitudes in the evening local time sector. The SAPS often appears in regions of low ionospheric conductivity equatorward of auroral electron precipitation during disturbed geomagnetic conditions. Currents driven into the sub-auroral ionosphere from the disturbed ring current put into play a sequence of magnetosphere-ionosphere coupling and feedback with dramatic consequences for the electric fields and particle populations of the Plasmasphere Boundary Layer. It is important to note that SAPS denotes the electric fields and the mechanisms which drive them, not their effects.

Observations of subauroral plasma convection with the Millstone Hill incoherent scatter radar and the DMSP satellites have been used to characterize SAPS phenomenology at low altitudes (< 1000 km) in the coupled inner-magnetosphere / ionosphere system. A database of 10,000 radar scans across the SAPS region supports a statistical view of the phenomena involved, while case studies reveal details of the mechanisms involved. The statistical investigation of Foster and Vo [J. Geophys. Res., 107, 1475, 2002] found SAPS to be appear at ionospheric heights as a persistent secondary westward (sunward) convection peak which lies equatorward of the auroral two-cell convection and spans the nightside from dusk to the early morning sector for all Kp greater than 4.

The effects of SAPS in the ionosphere and magnetosphere can be spectacular - including Storm Enhanced Density (SED) plumes of greatly enhanced ionospheric total electron content (TEC), erosion of the dusk-sector plasmasphere, and the formation of sunward-reaching plasmasphere drainage plumes. The inward extent of the SAPS electric field overlaps the outer plasmasphere on field lines mapping to the high-density cold plasmas equatorward of the ionospheric trough.

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