Multi-event analysis of STEVE, SAR arc, and red/green arc at subauroral latitudes using ground optical and radio instruments and the Arase and Van Allen Probes satellites

Strong Thermal Emission Velocity Enhancement (STEVE) is a latitudinally narrow, and purple band of emission seen at subauroral latitudes, which was discovered in 2016. Well-known Stable Auroral Red (SAR) arcs also occur at subauroral latitudes, and red/green arcs, which are similar to SAR arcs but accompanied by green emission, are found in the same latitude range. However, the characteristics of magnetospheric plasma and electromagnetic field variations as a source of these three types of optical emissions have not fully been studied, because of limited conjugate observations between magnetospheric satellites and ground-based optical and radio instruments. In this study, we report the auroral morphology as seen in all-sky image data obtained at seven ground stations (Athabasca, Gakona, Husafell, Kapuskasing, Magadan, Nyrola and Tromsø) during about four years from January 2017 to April 2021. Using the optical images as well as the ionospheric footprint of magnetospheric satellites (Arase and Van Allen Probes) calculated with the Tsyganenko-Sitnov magnetic field model (TS04), we have identified three cases of STEVE, three cases of SAR arc, and three cases of red/green arcs, in which these satellites were located in the conjugate regions in the magnetosphere of the optical emissions. For all three types of the emissions, satellite data showed that the plasmaspheric and ring-current particles spatially overlapped in the conjugate regions of the magnetosphere. Electromagnetic waves in the mHz to kHz range were observed for STEVE and red/green arc events. SuperDARN radar data showed a strong westward plasma flow in the ionosphere, especially during the STEVE events, while the plasma flows associated with SAR arcs and red/green arcs are relatively weak and variable. Heat fluxes due to Coulomb collisions were larger for STEVE as compared to SAR arc. However, the heat flux may have been underestimated for events that low-energy electrons were not measured. Our analyses have provided the first-ever comparison of magnetospheric particle and electromagnetic field characteristics for the three types of optical emissions in the subauroral region. In the presentation, we will discuss the differences and similarities of the magnetospheric and ionospheric processes responsible for producing these three types of emissions.