Average characteristics and activity dependence of the subauroral polarization 
stream

J. C. Foster and H. B. Vo


Data from the Millstone Hill incoherent scatter radar taken over two solar
cycles (1979Ð2000) are examined to determine the average characteristics of the
disturbance convection electric field in the midlatitude ionosphere. Radar
azimuth scans provide a regular database of ionospheric plasma convection
observations spanning auroral and subauroral latitudes, and these scans have
been examined for all local times and activity conditions.We examine the
occurrence and characteristics of a persistent secondary westward convection
peak which lies equatorward of the auroral two-cell convection. Individual scans
and average patterns of plasma flow identify and characterize this latitudinally
broad and persistent subauroral polarization stream (SAPS), which spans the
nightside from dusk to the early morning sector for all Kp greater than 4.
Premidnight, the SAPS westward convection lies equatorward of L = 4 (60 degrees
invariant latitude, V), spans 3-5 degrees of latitude, and has an average peak
amplitude of >900 m/s. In the predawn sector, SAPS is seen as a region of
antisunward convection equatorward of L = 3 (55 degrees V), spanning ~3 degrees
of latitude, with an average peak amplitude of 400 m/s. 

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Journal of Geophysical Research, 107(A12), 1475, doi:10.1029/2002JA009409, 2002