Several years ago, amateur astronomers first spotted a rare type of aurora nicknamed “the dunes” because of its luminous, rolling wave patterns. Last year, astronomers proposed a possible underlying mechanism—an increase in the density of oxygen atoms—although the theory was admittedly speculative. Now, a new analysis by researchers at the University of Helsinki provides evidence to confirm that explanation, according to a recent paper published in the journal AGU Advances.

Most people have a passing familiarity with the atmospheric phenomenon known as aurora borealis, aka the northern lights (or the southern lights if they appear in the southern hemisphere). The spectacular kaleidoscopic effects are the result of charged particles from the Sun being dumped into the Earth’s magnetosphere, where they collide with oxygen and nitrogen molecules—an interaction that excites those molecules and makes them glow. Auroras typically present as shimmering ribbons in the sky, with green, purple, blue, and yellow hues. The lights tend to only be visible in polar regions because the particles follow the Earth’s magnetic field lines, which fan out from the vicinity of the poles.

Discoveries of possible new types of auroras are rare. Back in 2016, enthusiasts observed a different kind of aurora that was visible at more southern latitudes. The aurora looked like a ribbon of pink or mauve light, sometimes with “picket fence” columns of green light passing through the ribbon.

The phenomenon earned the moniker STEVE (Strong Thermal Emission Velocity Enhancement), but astronomers have since determined that it is not a true aurora after all. (The name was inspired by the 2006 animated comedy Over the Hedge.) Rather, STEVE is caused by warm atmospheric emissions—charged particles heating up high in the ionosphere. STEVE lacks the distinct spectra associated with regular auroras, which places it in a unique category all its own. The picket fence display, however, is considered to be an unusual type of aurora because it is caused by a precipitation of electrons in the atmosphere; it has also been observed on its own, without STEVE.

Astronomers think the dunes are true aurora. Amateur stargazers in Finland, Norway, and Scotland spotted the dunes in October 2015 and again on January 20, 2016, both at an altitude of about 100 kilometers (62 miles)—that is, in Earth’s upper mesosphere, located above the stratosphere and below the thermosphere. The January light show lasted for nearly four hours across a broad expanse, giving the researchers plenty of time to document the phenomenon with photographs.

Last year, scientists at the Centre of Excellence in Research of Sustainable Space proposed a possible underlying mechanism for the auroral dunes: increased density of atmospheric oxygen atoms, possibly as a result of an atmospheric wave phenomenon known as a mesospheric bore. The bore is similar to the tidal bores that have been observed in some rivers, and it happens when waves of oxygen atoms interact with solar wind, causing the atoms to glow. If a mesospheric bore were traveling horizontally within a waveguide in the upper atmosphere, it would cause an increase in concentrations of atmospheric oxygen and produce the dune-like auroral emissions. That waveguide would form between the boundary of the mesosphere and an inversion layer of air below it, letting waves of certain wavelengths travel for long distances through it.

In other words, the aurora illuminates a wave that is already present in the atmosphere. This latest study, which examines electron precipitation and observational temperature data, supports that hypothesis. For their analysis, the researchers combined observational data from satellites with photographic and video material provided by the hobbyists who observed the dunes in January 2016.

The researchers found that on the night of January 20, 2016—when the dunes were out in full force—there was electron precipitation in the region, making it highly likely that there would be sufficient energetic electrons in the atmosphere to cause auroral emissions. There was also a strong temperature-inversion layer in the mesosphere—ideal conditions for the formation of the necessary waveguide, resulting in an increased density of oxygen atoms. Also, a time-lapse video shot over the course of that night allowed the researchers to estimate a propagation speed of more than 22 m/s for the dunes, which “suggests the presence of strong horizontal winds in the upper atmosphere,” the authors wrote.

However, there is not yet any direct observational data for the mesospheric bore within the waveguide. Getting that data will be the next step in this ongoing research. While we still don’t have a definitive answer about the exact nature of the dunes, this latest study does supply strong observational evidence in support of the working hypothesis. And it’s a testament to the analytical power of combining satellite data with the recorded observations of citizen scientists.

DOI: AGU Advances, 2021. 10.1029/2020AV000338  (About DOIs).

Listing image by Graeme Whipps

https://arstechnica.com/?p=1763383