Sub-Auroral Electric Field Effects on the Mid-Latitude Ionosphere

John C. Foster, A. Coster, M. Colerico, W. Rideout, and F. Rich


Magnetospheric electric fields and ionosphere-magnetosphere feedback contribute
strongly to the formation of ionospheric space weather effects over populous
mid-latitude regions during severe geomagnetic disturbances. Electric fields
often appear in regions of low ionospheric conductivity equatorward of auroral
electron precipitation. Currents driven into the sub-auroral ionosphere from the
disturbed ring current put into play a sequence of M-I coupling and feedback
mechanisms with dramatic consequences for the electric fields and particle
populations of the Plasmasphere Boundary Layer (PBL). The Sub-Auroral
Polarization Stream (SAPS) defined by Foster and Burke [EOS, 83(36), 393, 2002]
refers to the broad, persistent, poleward-directed electric fields which drive
sunward plasma convection at sub-auroral latitudes in the evening local time
sector. Observations of subauroral plasma convection with DMSP satellites and
the Millstone Hill incoherent scatter radar, have been used to characterize SAPS
phenomenology at low altitudes (< 1000 km) in the coupled inner-magnetosphere /
ionosphere system.

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 as seen by the IMAGE
EUV instrument. These features constitute significant space weather effects and
an understanding of the global nature of the coupled upper-atmosphere systems
which drive them is needed to model or predict their occurrence and severity.
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. We use ground-based GPS propagation data to produce
two-dimensional maps depicting the evolution of the SED features as they are
carried from their low-latitude source to the cusp ionosphere and across the
polar caps.

Penetrating electric fields and storm positive-phase disturbances combine to
produce greatly elevated TEC in the dusk-sector PBL. This solar-produced
ionization constitutes the source plasma for the SED plumes which are carried to
higher latitudes in the SAPS flow.

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Presented at the URSI National Radio Science Meeting, University of Colorado at 
Boulder, January 4-7, 2006