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Abstract We report on a novel scenario of subauroral arcs within strong subauroral ion drifts (SAID)‐STEVE and Picket Fence. Their explanation requires a local source of low‐energy,ε < 18.75 eV, suprathermal electrons, and N2vibrational and electronic excitation below ∼270 km. We show that the ionospheric feedback instability in strong SAID flows with depleted density troughs generates intense, small‐scale field‐aligned currents and parallel electric fields below the F2peak. With these fields, we employed a rigorous numerical solution of the Boltzmann kinetic equation for the distribution of ionospheric electrons and determined the power going to excitation and ionization of neutral gas (the energy balance). The obtained suprathermal electron population and energy balance at altitudes of ∼130–140 km are just what is necessary for Picket Fence. Concerning STEVE, the kinetic theory predictions are in a good qualitative agreement with its basic features, such as the enhanced continuum emissions. Besides, the theory predicts that subauroral arcs might have the transient phase with typical aurora‐like emissions that fade out afterward.
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The Mysterious Green Streaks Below STEVE
Strong thermal emission velocity enhancement (STEVE) is an optical phenomenon of the subauroral ionosphere arising from extreme ion drift speeds. STEVE consists of two distinct components in true‐color imagery: a mauve or whitish arc extended in the magnetic east–west direction and a region of green emission adjacent to the arc, often structured into quasiperiodic columns aligned with the geomagnetic field (the “picket fence”). This work employs high‐resolution imagery by citizen scientists in a critical examination of fine‐scale features within the green emission region. Of particular interest are narrow “streaks” of emission forming underneath field‐aligned picket fence elements in the 100‐ to 110‐km altitude range. The streaks propagate in curved trajectories with dominant direction toward STEVE from the poleward side. The elongation is along the direction of motion, suggesting a drifting point‐like excitation source, with the apparent elongation due to a combination of motion blur and radiative lifetime effects. The cross‐sectional dimension is <1 km, and the cases observed have a duration of∼20–30 s. The uniform coloration of all STEVE green features in these events suggests a common optical spectrum dominated by the oxygen 557.7‐nm emission line. The source is most likely direct excitation of ambient oxygen by superthermal electrons generated by ionospheric turbulence induced by the extreme electric fields driving STEVE. Some conjectures about causal connections with overlying field‐aligned structures are presented, based on coupling of thermal and gradient‐drift instabilities, with analogues to similar dynamics observed from chemical release and ionospheric heating experiments.
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- Award ID(s):
- 1821135
- PAR ID:
- 10375263
- Publisher / Repository:
- DOI PREFIX: 10.1029
- Date Published:
- Journal Name:
- AGU Advances
- Volume:
- 1
- Issue:
- 4
- ISSN:
- 2576-604X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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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.more » « less
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Abstract Recent studies suggest that, despite its aurora‐like appearance, the picket fence may not be driven by magnetospheric particle precipitation but instead by local electric fields parallel to Earth's magnetic field. Here, we evaluate the parallel electric fields hypothesis by quantitatively comparing picket fence spectra with the emissions generated in a kinetic model driven by local parallel electric fields energizing ambient electrons in a realistic neutral atmosphere. We find that, at a typical picket fence altitude of 110 km, parallel electric fields between 40 and 70 Td (∼80–150 mV/m at 110 km) energize ambient electrons sufficiently so that, when they collide with neutrals, they reproduce the observed ratio of N2first positive to atomic oxygen green line emissions, without producing first negative emissions. These findings establish a quantitative connection between ionospheric electrodynamics and observable picket fence emissions, offering verifiable targets for future models and experiments.more » « less
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