Statistical characterization of the American sector subauroral polarization stream using incoherent scatter radar

We examine Millstone Hill incoherent scatter radar data collected over two solar cycles between 1979 and 2001 to determine average characteristics and features of storm time ion velocity and flux transport in the American sector midlatitude ionosphere. We use over 1100 radar azimuth scans identified as containing enhanced westward ion velocities associated with the subauroral polarization stream (SAPS), covering the 12-06 magnetic local time (MLT) sector and 50 degrees-68 degrees invariant latitude for weak to moderate disturbance levels with Dst from 50 to -200 nT. We find the magnetic latitude peak location of the SAPS flow channels decreases linearly with both Dst and MLT with a very good degree of correlation. We also examine for the first time SAPS peak westward ion fluxes, which transport material westward with magnitude between 3 x 10(13) and 3 x 10(14) m(-2) s(-1) in a manner nearly invariant to activity level. This invariance is maintained by an inverse relationship between electron density and ion velocity magnitudes with increasing Dst. Westward log ion flux and ion velocity are maximum in the dusk sector and decrease linearly with increasing MLT, smoothly varying across the dusk terminator. Finally, velocity distributions show that material in the afternoon SAPS flow is swept westward to earlier MLT values, delivering O+ flux to the cusp region. In contrast, SAPS streams in the post terminator sectors are fixed east-west in the Sun-Earth inertial frame, effectively maintaining entrained ion fluxes at the same MLT.