Amplified Eastward SAPS Flows Observed in the Topside Ionosphere Near Magnetic Midnight

In this study, we report the exceptional observations of amplified eastward subauroral polarization streams (SAPS) made in the topside ionosphere near magnetic midnight during 2015-2016 in 17 events. Our results show the eastward SAPS flows streaming sunward after magnetic midnight and antisunward before magnetic midnight: in concert with the duskward-intruding dawn convection cell. These demonstrate that the eastward SAPS flow’s amplification was primarily caused by (1) the extension of equatorward-directed convection electric field (EC) to subauroral latitudes. Further evidence is provided by one set of correlated magnetosphere-ionosphere (M-I) conjugate observations showing (2) the inward (earthward) EC field near the inner-magnetosphere plasmapause (on the tailward side) and the emerging eastward SAPS in the topside ionosphere and thus implying (3) the EC field’s mapping-down and propagation to subauroral latitudes in the coupled M-I system. In the topside ionosphere, the amplified eastward SAPS flow reached ~3000 m/s in magnitude within the deep plasma density trough, where the electron temperature maximized at ~7000 K. There, (4) the underlying positive feedback mechanisms created favorable ionospheric conditions for SAPS growth. Finally, we conclude that the combination of these (1-4) mechanisms played a crucial role in the amplification of the near-midnight eastward SAPS flows observed.