The aurora that was missing from the book

Two photographs of the same dunes — Pirjo Koski and Rami Valonen; annotated figure by Minna Palmroth, Maxime Grandin and coauthors (2020)
Processed image. Actual photographs taken at 17:40:59 UT on 7 October 2018: Pirjo Koski in Laitila (top) and Rami Valonen in Ruovesi (bottom). The published figure adds numbered magenta markers used for triangulation and rounds the time to 17:41. This is an annotated photographic pair, not a simulation or a scan of a report page.
CC BY 4.0
· Original image source · Image licence

Finnish skywatchers noticed parallel green bands that did not fit the auroral forms in their new guidebook. Their coordinated photographs revealed a real pattern about 100 kilometres high, and opened a continuing investigation into waves at the edge of space.

World: Earth
Location: Finland
Case date or period: 7 October 2018

A pattern the observers recognised

The unusual feature is the sequence of nearly parallel green fingers extending away from the main auroral arc. Finnish enthusiasts had helped Minna Palmroth prepare a guidebook about auroral forms, yet these bands did not fit its categories. On 7 October 2018, Matti Helin saw the pattern again and alerted the group. Palmroth coordinated an improvised observing campaign, and photographers in several parts of Finland captured the same display. Pirjo Koski named the pattern the dunes. The investigation began with experienced observers noticing a repeatable feature, rather than with a theory looking for an illustration.

The photographs are the measurement

The paired image preserves two actual views by Koski in Laitila and Rami Valonen in Ruovesi, taken within the same second at 17:40:59 UT. Their observing sites were 172 kilometres apart. Maxime Grandin matched six corresponding dune tips, used the background stars and Stellarium to determine viewing angles, and triangulated their altitude. The result placed the pattern near 100 kilometres, in an approximately horizontal layer. The magenta numbers belong to the researchers’ comparison, not to the sky. They show which features were treated as matches across the two photographs.

What the geometry established

The analysis estimated a separation of about 45 kilometres between neighbouring bands. Its precision depended on identifying the same fingertips and matching their positions to stars; limited visible stars and blurred features near the horizon introduced uncertainty. The paper explicitly describes those limitations. The photographs establish a structured luminous field, rather than a solid object or a band of material close to the cameras. They also demonstrate why the two towns should be treated as observing sites: the reconstructed light extended over a much larger region, including eastern Sweden. The map marker here identifies the published Laitila camera region.

Testing an atmospheric explanation

Palmroth’s team proposed that a mesospheric bore, a form of atmospheric wave, could organise variations in oxygen density. Incoming particles would then make those variations visible through green auroral light. This is a physical explanation for the pattern, not a direct photograph of the proposed wave-generating mechanism. In the original study, the absence of suitable satellite measurements meant spatial variations in the incoming particles could not be excluded beyond doubt. The authors therefore treated the bore interpretation as a hypothesis to test, while the measured altitude and photographed bands remained the observational result.

A later test used an earlier night

Grandin and colleagues published a follow-up investigation in 2021 using a separate event from 20 January 2016. Citizen photographs showed dunes from Finland to Scotland, across more than 1,500 kilometres for at least four hours. Spacecraft observations supported particle precipitation and found an atmospheric temperature inversion compatible with the proposed bore mechanism. A Scottish photographic sequence also allowed a propagation-speed estimate. Those additional measurements strengthened the investigation, but they should not be presented as instruments measuring the October 2018 photographs shown here. The two events are distinct, and their dates are retained in the sources and timeline.

A discovery with questions left open

The account is a collaboration between independent observers and professional researchers. The citizen photographers were named coauthors, and their choices of exposure, timing and viewpoint supplied observations outside the routine coverage of many auroral instruments. The photographs and measurements established a previously unclassified auroral form; they did not establish every detail of its cause. One remaining question is whether the energy delivered during geomagnetic activity helps create the atmospheric conditions for the waves, or merely illuminates waves already present. Comparing additional timed observations with independent measurements is how that question can be narrowed.