Multi‐Event Analysis of Plasma and Field Variations in Source of Stable Auroral Red (SAR) Arcs in Inner Magnetosphere During Non‐Storm‐Time Substorms

Abstract

Stable auroral red (SAR) arcs are optical events with dominant 630.0-nm emission caused by low-energy electron heat flux into the topside ionosphere from the inner magnetosphere. SAR arcs are observed at subauroral latitudes and often occur during the recovery phase of magnetic storms and substorms. Past studies concluded that these low-energy electrons were generated in the spatial overlap region between the outer plasmasphere and ring-current ions and suggested that Coulomb collisions between plasmaspheric electrons and ring-current ions are more feasible for the SAR-arc generation mechanism rather than Landau damping by electromagnetic ion cyclotron waves or kinetic Alfvén waves. This work studies three separate SAR-arc events with conjunctions, using all-sky imagers and inner magnetospheric satellites (Arase and Radiation Belt Storm Probes [RBSP]) during non-storm-time substorms on December 19, 2012 (event 1), January 17, 2015 (event 2), and November 4, 2019 (event 3). We evaluated for the first time the heat flux via Coulomb collision using full-energy-range ion data obtained by the satellites. The electron heat fluxes due to Coulomb collisions reached ∼10 9 eV/cm 2 /s for events 1 and 2, indicating that Coulomb collisions could have caused the SAR arcs. RBSP-A also observed local enhancements of 7-20-mHz electromagnetic wave power above the SAR arc in event 2. The heat flux for the freshly detached SAR arc in event 3 reached ∼10 8 eV/cm 2 /s, which is insufficient to have caused the SAR arc. In event 3, local flux enhancement of electrons (<200 eV) and various electromagnetic waves were observed, these are likely to have caused the freshly detached SAR arc.Plain Language Summary Stable auroral red (SAR) arcs are aurora with an optical red emission from oxygen atoms at latitudes slightly lower than the auroral oval and often occur during storm-time substorms. The oxygen excitation is caused by low-energy electrons transferred from the inner magnetosphere to the ionosphere. Past studies concluded that these low-energy electrons were generated INABA ET AL.

Isolated Proton Aurora Driven by EMIC Pc1 Wave: PWING, Swarm, and NOAA POES Multi‐Instrument Observations

“…It has been known that other types of detached auroral arcs, such as SAR (stable auroral red) arcs (e.g., Hong et al., 2020; Inaba et al., 2020, 2021) and STEVE (Strong Thermal Emission Velocity Enhancement) (e.g., Archer et al., 2019) are accompanied by ionospheric irregularities with local density enhancements (SAR arc) and depletions (STEVE) in the auroral regions. Our study also showed that an IPA event is accompanied by localized plasma density enhancements in the plasma trough.…”

“…It has been known that other types of detached auroral arcs, such as SAR (stable auroral red) arcs (e.g., Hong et al, 2020;Inaba et al, 2020Inaba et al, , 2021 and STEVE (Strong Thermal Emission Velocity Enhancement)…”

Interaction Between Proton Aurora and Stable Auroral Red Arcs Unveiled by Citizen Scientist Photographs

“…Proton aurora is caused by energetic ions that are scattered by electromagnetic ion cyclotron waves and precipitate to the upper atmosphere. Energetic ions also heat ambient cold plasma (plasmasphere), and heated thermal electrons are believed to cause SAR arcs (Inaba et al., 2021). Proton aurora occurs near the meridian of intensifications in the auroral oval (streamers) (Henderson, 2021; Montbriand, 1971; Nishimura et al., 2014).…”

Statistical Study on Small‐Scale (≤1,000 km) Density Irregularities in the Inner Magnetosphere

“…The detected density fluctuations categorized in the MLT‐dependent plasmasphere near the apogee may be caused by the satellite “skimming” along the plasmapause rather than actual density irregularities (e.g., Figure 8 of Inaba et al. [2021]). To resolve these problems, we used L − L pp to measure the distance between the satellite and the PBL.…”

“…Because of the large and frequent density fluctuations in the PBL, it must be investigated whether the density irregularities in the plasmasphere and plasma trough are associated with the PBL. The detected density fluctuations categorized in the MLT-dependent plasmasphere near the apogee may be caused by the satellite "skimming" along the plasmapause rather than actual density irregularities (e.g., Figure 8 of Inaba et al [2021]). To resolve these problems, we used L − L pp to measure the distance between the satellite and the PBL.…”