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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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Unresolved Questions in Subauroral Science: Exploring Key Challenges in Physics and Chemistry
Abstract The subauroral region, located equatorward of the auroral oval, is a highly dynamic and complex interface between the magnetosphere, ionosphere, and thermosphere. While traditionally associated with stable optical structures such as stable auroral red arcs, recent observations have revealed a wide range of transient and extreme phenomena—such as subauroral ion drifts and strong thermal emission velocity enhancement—which highlight the region’s variability and intense coupling. The dynamics of the subauroral ionosphere are not only influenced by processes occurring at higher latitudes within the auroral oval but are also shaped by interactions across multiple regions of geospace, including the inner magnetosphere, ring current, inner plasma sheet, and the lower-altitude thermosphere. This growing body of research has underscored both the scientific richness of the subauroral region and the many outstanding questions regarding its drivers and chemical processes. In this paper, we present a in-depth review of observed subauroral structures, available ground-based and satellite datasets, and current modeling efforts aimed at understanding the region’s dynamics. We also examine the state of knowledge surrounding the subauroral ionospheric/thermospheric chemistry and outline critical gaps that require further investigation. Finally, we discuss the pressing need for targeted experiments and new space missions to advance our understanding of this key geospace region.
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- Award ID(s):
- 2445467
- PAR ID:
- 10674366
- Publisher / Repository:
- Springer
- Date Published:
- Journal Name:
- Surveys in Geophysics
- ISSN:
- 0169-3298
- 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/10.1007/s10712-026-09928-9
