Dawnside Auroral Polarization Streams

Abstract

Although the postmidnight‐to‐dawn sector of the auroral ionosphere contains interesting dynamic phenomena that may significantly impact the magnetosphere‐ionosphere‐thermosphere system, it has been much less studied than the dusk‐to‐premidnight sector. We discuss a dynamic phenomenon in the postmidnight‐to‐dawn sector of the auroral oval, eastward fast flows mainly within the expanse of the Region 1 current (its part equatorward of the polar cap). These flows peak and have a steep speed gradient (increase from low to high latitude) near the interface between the Region 1 and Region 2 currents. Because such flows correspond to an electric field that most likely comes from enhanced Region 2 currents and an associated spatial conductivity gradient, their generation mechanism is analogous to that of a subauroral polarization stream. Therefore, we refer to such an eastward flow as a dawnside auroral polarization stream (DAPS). We show several examples of the presence and absence of DAPS under different geomagnetic activity levels. A DAPS' electric field can heat the ionosphere (and thus the thermosphere), change the convection pattern of the magnetosphere‐ionosphere system, and modify the drift path of magnetospheric particles. Because a DAPS' flow peak maps to a major site of magnetic‐kinetic energy conversion in the magnetosphere (the transition region between dipole and stretched field), it may be important for the conversion. A DAPS' steep flow gradient is also potentially important; it may lead to instabilities, such as that responsible for auroral Omega bands. Given its potential importance, knowledge of DAPS is fundamental for understanding the magnetosphere‐ionosphere‐thermosphere system.

Embedded Regions 1 and 2 Field‐Aligned Currents: Newly Recognized From Low‐Altitude Spacecraft Observations

“…When appearing in the postmidnight-to-dawn sector, the embedded R2 current coincides with a DAPS. This is also so for the events in Liu et al (2018Liu et al ( , 2020. This coincidence further supports the idea that embedded FACs result from a current generator, because a DAPS results from a current generator (Liu et al, 2020).…”

“…This relationship is also present in the embedded FACs we can identify from previous studies such as those in Figures 2 and 3 of Liu et al. (2020) (the embedded FACs can be identified from the magnetic field variations in these figures; their Figure 3 does not show a clean embedded R1 FAC) and Figure 1 of Liu et al. (2018).…”

“…10.1029/2021JA029207 10 of 15 studies such as those in Figures 2 and 3 of Liu et al (2020) (the embedded FACs can be identified from the magnetic field variations in these figures; their Figure 3 does not show a clean embedded R1 FAC) and Figure 1 of Liu et al (2018). Thus, an upward embedded FAC is so strong that field-aligned potential drops develop (Evans, 1974;Lyons, 1981) to support it.…”

“…(2018, 2020). This coincidence further supports the idea that embedded FACs result from a current generator, because a DAPS results from a current generator (Liu et al., 2020).…”

Generation and Evolution of Two Opposite Types of Mesoscale Plasma Sheet Bubbles

“…On the other side of the bubble at 0.5 MLT, eastward flows are enhanced within the bubble's b1‐d FAC with a peak at ~70° (72°) MLAT in the type‐1 (type‐2) bubble (indicated by the black vertical dotted line). This enhanced eastward flow is termed dawnside auroral polarization stream (DAPS) (Liu et al, 2020). The magnitudes of the b1‐d FAC are similar in both types, but the smaller conductance produced by the electrons within the type‐2 bubble (see Figures 10g–10i) results in stronger peak DAPS flow speed, as compared to the type‐1 bubble.…”