Subauroral polarization streams (SAPS): Intrinsic storm-time response of geospace at mesoscales

Subauroral polarization streams (SAPS) typically refer to a mesoscale westward plasma flow channel in the duskside subauroral ionosphere. SAPS overlap with the low-latitude part of downward Region-2 field-aligned currents (FACs). The relatively low subauroral conductance in this region requires a strong electric field for current closure, which drives the fast sunward plasma flow. Observations have shown dynamic variability of SAPS under various solar wind and interplanetary magnetic field (IMF) conditions, which are related to the variability of FACs and auroral precipitation, as well as their source regions in the ring current and plasmasheet. In this study, we use satellite observations and numerical simulations with the state-of-the-art Multiscale Atmosphere Geospace Environment (MAGE) model to investigate: 1) The role of diffuse electron precipitation in the formation of SAPS; 2) SAPS variability under IMF B_Y; and 3) Dawnside (as opposed to the more conventional duskside) SAPS as a unique feature of major geomagnetic storms. With data-model comparison, we will demonstrate that SAPS result from the different behaviors of ring current ions and plasma sheet electrons, and the corresponding self-consistent response of the ionosphere-thermosphere system via electrodynamic and particle coupling with the magnetosphere. We conclude that SAPS distribution and variability represent a fundamental feature of the geospace response to solar disturbances during storm time.