The Role of Diffuse Electron Precipitation in the Formation of Subauroral Polarization Streams

The role of diffuse electron precipitation in the formation of subauroral polarization streams (SAPS) is investigated with the Multiscale Atmosphere-Geospace Environment (MAGE) model. Diffuse precipitation is derived from the distribution of drifting electrons. SAPS manifest themselves as a separate mesoscale flow channel in the duskside ionosphere, which gradually merges with the primary auroral convection toward dayside as the equatorward auroral boundary approaches the poleward Region-2 field-aligned currents (FACs) boundary. SAPS expand to lower latitudes and toward the nightside during the main phase of a geomagnetic storm, associated with magnetotail earthward plasma flows building up the ring current and intensifying Region-2 FACs and electron precipitation. SAPS shrink poleward and sunward as the interplanetary magnetic field turns northward. When diffuse precipitation is turned off in a controlled MAGE simulation, ring current and duskside Region-2 FACs become weaker, but subauroral zonal ion drifts are still comparable to auroral convection. However, subauroral and auroral convection manifest as a single broad flow channel without showing any mesoscale structure. SAPS overlap with the downward Region-2 FACs equatorward of diffuse precipitation, where poleward electric fields are strong due to a low conductance in the subauroral ionosphere. The Region-2 FACs extend to latitudes lower than the diffuse precipitation because the ring current protons penetrate closer to the Earth than the electrons do. This study reproduces the key physics of SAPS formation and their evolution in the coupled magnetosphere-ionosphere during a geomagnetic storm. Diffuse electron precipitation is demonstrated to play a critical role in determining SAPS location and structure.