Dawnside Auroral Polarization Stream

When the magnetospheric convection increases and pushes the plasma sheet inward, the associated Region-2 field aligned current in the inner edge of plasma sheet (R2 FAC) also increase. The enhanced FAC requires ionospheric currents to enhance to maintain current continuity, which in turn modifies ionospheric electric fields and thus flows. The electric field equatorward of the R2 FAC cancels the original convection electric field, which creates the shielding effect, while the electric field just poleward of the R2 FAC enhances the original convection electric field, and thus flow. In the dusk to pre-midnight sector, such enhancement contributes to the formation of the well-known sub-auroral polarization streams (SAPS), a strong westward flow. Recent simulations show that in the post-midnight to dawn sector within the auroral oval, the enhanced electric field should lead to a strong eastward fast flow. We term this flow the dawnside auroral polarization stream (DAPS).

In this study we present observational evidence for DAPS measured by DMSP and Swarm. The observations show that DAPS is largely limited within the area of Region-1 (R1) FACs just poleward of R2 FACs and has a steep flow gradient near the boundary between R1 and R2 FACs. This steep flow gradient of DAPS likely results from the conductance gradient at the R1/R2 boundary. Within the R2 upward FAC in the post-midnight to dawn sector, the conductance enhances together with the FAC because of enhanced electron precipitation. This high conductance does not allow a high electric field, so equipotentials are pushed to the R1 region to complete the given cross-tail potential drop. The DAPS electric field is thus further enhanced.

The DAPS' strong electric field likely heats the ionosphere, modifies the thermosphere, and thus affects the magnetosphere-ionosphere-thermosphere (M-I-T) coupling. Furthermore, its steep flow gradient may lead to instabilities, such as that responsible for the Omega band (we show an example of coexistence of DAPS and Omega bands). Therefore, DAPS is fundamental to understanding the M-I-T system and should be as important as SAPS.