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Abstract Strong Thermal Emission Velocity Enhancement (STEVE) is a latitudinally narrow, purple‐band emission observed at subauroral latitudes. Stable Auroral Red (SAR) arcs characterized by major red emission, and red/green arcs with both red and green emissions also occur at subauroral latitudes. Characteristics of magnetospheric source plasma and electromagnetic fields of these three types of arcs have not been fully understood because of the limited conjugate observations between magnetosphere and the ground. In this study, we report 11 conjugate observations (2 STEVEs, 7 SAR arcs, and 2 red/green arcs), using all‐sky images obtained at seven ground stations over more than four years from January 2017 to April 2021 and magnetospheric satellites (Arase and Van Allen Probes). We found that, in the inner magnetosphere, the source region of STEVEs and red/green arcs were located outside the plasmasphere, and that of the SAR arc was in the region of spatial overlap between the plasmasphere and ring current region. Electromagnetic waves at frequencies below 1 Hz were observed for STEVEs and red/green arcs. SuperDARN radar data showed a strong westward plasma flow in the ionosphere, especially during STEVE events, whereas the plasma flows associated with SAR arcs and red/green arcs were generally weaker and variable. The STEVE and SAR arc can appear simultaneously at slightly different latitudes and STEVEs and red/green arcs can transform into SAR arcs. These first comprehensive ground‐satellite measurements of three types of subauroral‐latitude auroras increase our understanding on similarlity, differences, and coupling of these auroras in the ionosphere and the magnetosphere.
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Unsolved problems in Strong Thermal Emission Velocity Enhancement (STEVE) and the picket fence
This paper reviews key properties and major unsolved problems about Strong Thermal Emission Velocity Enhancement (STEVE) and the picket fence. We first introduce the basic characteristics of STEVE and historical observations of STEVE-like emissions, particularly the case on 11 September 1891. Then, we discuss major open questions about STEVE: 1) Why does STEVE preferentially occur in equinoxes? 2) How do the solar wind and storm/substorm conditions control STEVE? 3) Why is STEVE rare, despite that STEVE does not seem to require extreme driving conditions? 4) What are the multi-scale structures of STEVE? 5) What mechanisms determine the properties of the picket fence? 6) What are the chemistry and emission mechanisms of STEVE? 7) What are the impacts of STEVE on the ionosphere−thermosphere system? Also, 8) what is the relation between STEVE, stable auroral red (SAR) arcs, and the subauroral proton aurora? These issues largely concern how STEVE is created as a unique mode of response of the subauroral magnetosphere−ionosphere−thermosphere coupling system. STEVE, SAR arcs, and proton auroras, the three major types of subauroral emissions, require energetic particle injections to the pre-midnight inner magnetosphere and interaction with cold plasma. However, it is not understood why they occur at different times and why they can co-exist and transition from one to another. Strong electron injections into the pre-midnight sector are suggested to be important for driving intense subauroral ion drifts (SAID). A system-level understanding of how the magnetosphere creates distinct injection features, drives subauroral flows, and disturbs the thermosphere to create optical emissions is required to address the key questions about STEVE. The ionosphere−thermosphere modeling that considers the extreme velocity and heating should be conducted to answer what chemical and dynamical processes occur and how much the STEVE luminosity can be explained. Citizen scientist photographs and scientific instruments reveal the evolution of fine-scale structures of STEVE and their connection to the picket fence. Photographs also show the undulation of STEVE and the localized picket fence. High-resolution observations are required to resolve fine-scale structures of STEVE and the picket fence, and such observations are important to understand underlying processes in the ionosphere and thermosphere.
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- PAR ID:
- 10430830
- Date Published:
- Journal Name:
- Frontiers in Astronomy and Space Sciences
- Volume:
- 10
- ISSN:
- 2296-987X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.3389/fspas.2023.1087974
