Rare fragmented auroras and picket fence structures observed together, challenging long-held latitude boundary assumptions

A team of scientists has reported the first simultaneous observation of fragmented aurora-like emissions (FAEs) and picket fence structures within the auroral zone. The finding reshapes how researchers understand where and how these rare lights form.

The event occurred over northern Scandinavia during a magnetic storm on January 1, 2025, when high-sensitivity all-sky cameras recorded faint, fast-changing green patches of light embedded in diffuse red aurora near the poleward edge of the auroral oval.

At the same time, vertically aligned green rays, known as a picket fence, appeared nearby. The findings, led by Sota Nanjo of the National Institute of Polar Research, Japan, were published as a preprint in EGUsphere on October 17, 2025.

Fragmented aurora-like emissions are a newly recognised class of auroral feature. First described from Svalbard in 2021, FAEs appear as small, irregular patches of oxygen-green emission, typically less than 20 km (12 miles) across and lasting under a minute.

They are thought to arise from local electron acceleration in the ionosphere rather than energetic particles streaming down from the magnetosphere. The processes involve short-scale electric-field irregularities and plasma instabilities such as the Farley–Buneman instability that cause gentle heating and produce faint light.

The picket fence has been associated with the subauroral phenomenon STEVE (Strong Thermal Emission Velocity Enhancement). It forms as thin, vertically oriented green columns aligned with Earth’s magnetic field, extending hundreds of kilometres along latitude lines. These structures have usually been observed equatorward of the auroral oval and are linked to subauroral ion drifts (SAIDs), strong plasma flows that generate localized electric fields and plasma heating in regions of low density.

Until now, these two phenomena had been treated as belonging to different latitudinal regimes — with FAEs occurring in the polar cap and picket fences in the subauroral region. The new Scandinavian observations reveal that both can coexist at auroral latitudes, close to the boundary where magnetospheric particle precipitation is normally strongest.

Ground-based optical data show that some FAEs during this event were field-aligned, extending along magnetic lines, which is an uncommon feature. Several appeared simultaneously across multiple longitudes, indicating that their formation was not random but likely governed by the surrounding electric-field structure of the aurora.

Measurements from the European Space Agency’s Swarm satellites detected corresponding changes in ionospheric electron conditions above the region, consistent with an ionospheric rather than magnetospheric origin of the emissions.

The coexistence of FAEs and picket fences suggests that multiple plasma instabilities may act together under disturbed geomagnetic conditions. Depending on local density gradients, collision frequencies and conductivity, the same electrodynamic environment can produce either fragmented, blob-like emissions or vertically aligned rays.

This discovery broadens the known spatial range of both phenomena and points to a continuum of small-scale ionospheric processes connecting polar-cap, auroral and subauroral regions. It adds to the growing understanding that Earth’s upper atmosphere is far more dynamic than once thought, with electric-field structures that can stretch over thousands of kilometres but reorganize in seconds.

Insights from the study help refine models of magnetosphere–ionosphere coupling and improve our understanding of how geomagnetic storms affect high-latitude plasma dynamics.

References:

¹ Observations of Fragmented Aurora-like Emissions and Picket Fence on the Poleward Edge of the Auroral Oval – Sota Nanjo et al. – EGUsphere – October 17, 2025 – https://doi.org/10.5194/egusphere-2025-4560 – OPEN ACCESS

Reet Kaur

I’m a science journalist and researcher at The Watchers, contributing to the Epicenter edition, where I cover peer-reviewed scientific research and emerging discoveries across Earth and space sciences. With a background in astronomy and a passion for environmental science, I’ve worked in shark and coral conservation in Fiji, conducting reef and shark-behavior research, contributing to mangrove restoration, and earning PADI Open Water and Coral Reef Certifications. I bring a blend of scientific rigor and storytelling to illuminate the discoveries shaping our planet and beyond.

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