Subauroral Green STEVE Arcs: Evidence for Low‐Energy Excitation

Mende, S. B.; Harding, Brian J.; Turner, Charles E.

doi:10.1029/2019gl086145

Cited by 39 publications

(80 citation statements)

References 23 publications

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“…One other interesting result in their theories is that they 10.1029/2020GL088980 predict a parallel electric field on order of a fraction of 1 mV/m, over an altitudinal range of several tens of km between the ionospheric E and lower F region (see, e.g., Figure 6 in Noël et al, 2000). Assuming an average E // of~0.2 mV/m that spans an altitude range of~50 km, the resulting electron acceleration would be sufficient to excite the 557.7 nm green line (~4.19 eV) and N2 1PG emissions (~7.35 eV) (Mende et al, 2019), yet not enough to excite the N 2 + 1 N 427.8 nm emission (~18.75 eV). This is consistent with the absence of blue-line 427.8 nm emissions in the Picket Fence spectra (Gillies et al, 2019;Mende et al, 2019).…”

Section: Summary and Discussionmentioning

confidence: 99%

“…With the TREx RGB camera, it is particularly straight forward to identify the Picket Fence. Besides their characteristic morphological features, the Picket Fence exists predominantly in the green channel and shows only a faint imprint (presumably from N2 1PG; see Mende et al, 2019) in the red channel, yet nothing on the blue channel. Due to their primarily green nature, only the green channel of the RGB image was used for the Picket Fence structures.…”

Section: Apparent Motion Of the Picket Fencementioning

confidence: 99%

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The Apparent Motion of STEVE and the Picket Fence Phenomena

Gillies

Liang

Donovan

et al. 2020

Geophysical Research Letters

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In this study, we present the first data obtained from the new Transition Region Explorer (TREx) RGB Imager and analyze the apparent motion of STEVE and the Picket Fence structures in an event on 31 August 2019. The structures associated with STEVE are found to feature a fast westward motion (~5-10 km/s). This is consistent with the notions that STEVE is colocated with a fast subauroral ion drift (SAID) channel and that SAIDs play a key role in the production of STEVE. The apparent motion of the Picket Fence structures is much slower (~400-600 m/s) and can be both westward and eastward. This implies that either the Picket Fence is located at different magnetic flux tubes from that of STEVE, with much slower plasma convection speed, or that the motion of the Picket Fence does not follow the plasma convection.

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Section: Summary and Discussionmentioning

confidence: 99%

Section: Apparent Motion Of the Picket Fencementioning

confidence: 99%

The Apparent Motion of STEVE and the Picket Fence Phenomena

Gillies

Liang

Donovan

et al. 2020

Geophysical Research Letters

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“…A picket fence was observed at a location magnetically conjugate to a Defense Meteorological Satellite Program observation of detached precipitation and far ultraviolet aurora (Nishimura et al, 2019). However, precipitation should create N + 2 first negative emissions, while there is no evidence for this emission in the picket fence spectrum observed by Gillies et al (2019) (Mende et al, 2019). The horizontal structuring (∼18-km spacing) and magnetic field-aligned structure of the picket fence are suggestive of precipitation and stand in contrast to the purple arc, which appears much less structured.…”

Section: 1029/2020gl087102mentioning

confidence: 95%

A Mechanism for the STEVE Continuum Emission

Harding

Mende

Triplett

et al. 2020

Geophysical Research Letters

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We describe a mechanism to explain the subauroral emission feature called STEVE (Strong Thermal Emission Velocity Enhancement), with a focus on its continuum spectrum. Spacecraft observations show that emissions co-occur with typically invisible plasma flows known as subauroral ion drifts. If these flows are fast enough, nitrogen molecules are vibrationally excited by collisions with ions, overcoming the activation energy of the N 2 + O → NO + N reaction. The resulting NO combines with ambient O, producing NO 2 and spectrally broad light. Importantly, this mechanism also produces N, which reduces the lifetime of NO from hours to seconds and thus explains why the emission is confined to a discrete arc. The predicted emission altitude (≳130 km) and occurrence conditions (≳4-km/s flows) match well with observations. We simulate this mechanism using a simple photochemical model to demonstrate its validity. This mechanism is initiated by fast ion flows and is thus distinct from auroral and airglow processes.Plain Language Summary Citizen scientists and night-sky photographers have been capturing pictures of a peculiar type of polar light for many years but only recently has the scientific community explored its significance. This narrow purple/white arc stretches east-west across the sky and has come to be known as Strong Thermal Emission Velocity Enhancement (STEVE). Although its appearance is suggestive of aurora, it is not caused by fast electrons from the magnetosphere, and it is dominated by a broad spectrum (mostly white light). Most auroral and airglow emissions are caused by electronic transitions of atmospheric constituents initiated by electron or photon impact, producing spectrally discrete light. The physical processes producing the light in STEVEs are unknown, particularly the chemical mechanism that produces light that could appear white. In this work we describe a candidate mechanism where fast-moving ions cause vibrational excitation of nitrogen molecules, which then undergo chemical reactions to produce spectrally broad light. These fast-moving ions are known to co-occur with STEVEs. This hypothesis is supported by a simple chemical simulation, but observational validation is needed.

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“…However, based on the limited evidence thus far obtained, it is also possible that the streaks and the picket fence are not causally connected but, rather, represent two distinct responses to the same free energy source. Mende et al (2019) conjectured that the magnetic field-aligned features could be produced by wave heating induced by the extreme electric fields in the SAID region (Streltsov & Mishin, 2003). Fine-scale green emissions at lower conjugate altitudes could be excited by these same extreme fields.…”

Section: Formation Of Field-aligned Features From Point Sourcesmentioning

confidence: 99%

“…The whitish color is attributed to lower‐altitude events, where the metastable O ( 1 D ) state responsible for the red line component is collisionally quenched (Liang et al, 2019). The companion picket fence region has been found to be predominantly oxygen 557.7‐nm green line, with a trace contribution from N 2 first positive emissions (Mende et al, 2019). The lack of emissions from higher energy

N_{2}^{+}

transitions has argued against precipitating magnetospheric electrons as a source of the green features (Mende et al, 2019), although the role of precipitation in STEVE remains a subject of debate (Nishimura et al, 2019).…”

Section: Introductionmentioning

confidence: 99%

The Mysterious Green Streaks Below STEVE

Semeter

Hunnekuhl²,

MacDonald

et al. 2020

AGU Advances

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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. Plain Language Summary STEVE is a recently identified atmospheric phenomenon caused by supersonic plasma jets flowing at altitudes >100 km. STEVE appears as a whitish pink arc extending in the magnetic east-west direction, which is often accompanied by an adjacent region of green features. In some cases, the green features appear as periodic vertical columns resembling a "picket fence". These features were thought to be caused by the same mechanism as the aurora, but the color is wrong. This article examines these features from a morphological perspective. We use time lapse images recorded by citizen scientists to examine fine-scale structures in STEVE's green emission region. We focus particular attention on a narrow streak of emission commonly observed underneath the picket fence. These streaks have width of just a few hundred meters and propagate horizontally. They have identical coloration to other green objects in the field, suggesting a common source. Although we cannot state conclusively what that source mechanism is, they are most likely excited by turbulent heating related to the extreme plasma flows of STEVE. Our initial analysis suggests a physical connection between these tiny streaks and the extended picket fence features above them. Advancing this physics will benefit greatly from continued involvement of citizen scien...

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Subauroral Green STEVE Arcs: Evidence for Low‐Energy Excitation

Cited by 39 publications

References 23 publications

The Apparent Motion of STEVE and the Picket Fence Phenomena

The Apparent Motion of STEVE and the Picket Fence Phenomena

A Mechanism for the STEVE Continuum Emission

The Mysterious Green Streaks Below STEVE

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