[P-EM10] Dynamics of Magnetosphere and Ionosphere

[PEM10-P15] Case studies of STEVE-like emissions observed in the poleward part of the auroral region after the magnetic midnight

*Sota Nanjo¹, Gabriel Arne Hofstra², Satonori Nozawa³, Kazuo Shiokawa³, Tetsuya Kawabata³, Keisuke Hosokawa¹ (1.The University of Electro-Communications, 2.Photographer (Citizen Scientist), 3.ISEE, Nagoya University)

Keywords:STEVE, All-sky digital camera, OMTI, DAPS, Swarm satellite

We present two events showing a pink or purple emission, similar to Strong Thermal Emission Velocity Enhancement (STEVE), which occurred on the poleward of the auroral oval in the post-midnight sector. In the first event, an arc-like emission same as STEVE was observed in Tromsø, Norway, at around 01 UT on 28 December 2021. In the second event, a patchy emission was also observed in Tromsø at around 23 UT on 13 November 2012. Both events had a similar color to STEVE, but the observed locations were different from STEVE. In these regions, however, the north-south electric field commonly produces a strong east-west ion flow during magnetic disturbances. On the evening side, a strong westward (E x B direction) flow, i.e., subauroral ion drift (SAID) generated in the trough region where the local Pedersen conductivity is reduced, produces the thermal emission of STEVE (MacDonald et al., 2018). On the morning side, upward field-aligned currents (FACs) in Region 2 are enhanced, and the Pedersen conductivity increases due to electron precipitation. In Region 1, FACs are also enhanced but without precipitation. This non-uniformity of conductivity makes a southward electric field. This electric field in Region 1 generates an eastward (E x B direction) ion flow, which can be considered the dawn-side counterpart of SAPS/SAID has been reported as Dawnside Auroral Polarization Streams (DAPS; Liu et al., 2020). The emission seen in this study may result from DAPS producing a thermal emission similar to that of SAPS/SAID.
The Swarm satellite and a digital camera in Tromsø simultaneously observed the emission in the first event. The result shows that the emission was located on the boundary between Regions 1 and 2, and a strong eastward ion flow (~2 km/s) was measured. In the second event, the Optical Mesosphere Thermosphere Imagers (OMTI) and the digital camera in Tromsø observed the patchy emission. This emission was observed in all the wavelengths, including background light (572.5 nm), suggesting that this emission has a continuous spectrum same as STEVE. In the presentation, we will discuss the detailed results of the optical and particle observations, the generation mechanism of the emission, and the similarities/differences with STEVE based on the current system in the ionosphere.