New analysis confirms hypothesis for source of mysterious auroral “dunes”

Revisiting the aurora “dunes”: A time-lapse online video recorded by a Scottish aurora borealis hobbyist Graeme Whipps was applied to establish the velocity of the phenomenon at about 200 m/s.

Numerous years in the past, novice astronomers very first noticed a rare sort of aurora nicknamed “the dunes” since of its luminous, rolling wave styles. Final 12 months, astronomers proposed a attainable underlying mechanism—an enhance in the density of oxygen atoms—although the theory was admittedly speculative. Now, a new analysis by scientists at the College of Helsinki gives evidence to verify that rationalization, in accordance to a modern paper published in the journal AGU Innovations.

Most folks have a passing familiarity with the atmospheric phenomenon acknowledged as aurora borealis, aka the northern lights (or the southern lights if they look in the southern hemisphere). The spectacular kaleidoscopic effects are the outcome of billed particles from the Sunlight being dumped into the Earth’s magnetosphere, where they collide with oxygen and nitrogen molecules—an interaction that excites these molecules and makes them glow. Auroras normally present as shimmering ribbons in the sky, with inexperienced, purple, blue, and yellow hues. The lights are likely to only be obvious in polar locations since the particles observe the Earth’s magnetic discipline traces, which enthusiast out from the vicinity of the poles.

Discoveries of achievable new varieties of auroras are exceptional. Back again in 2016, fans observed a various sort of aurora that was obvious at far more southern latitudes. The aurora looked like a ribbon of pink or mauve light-weight, occasionally with “picket fence” columns of eco-friendly gentle passing by the ribbon.

The phenomenon acquired the moniker STEVE (Potent Thermal Emission Velocity Enhancement), but astronomers have considering the fact that identified that it is not a true aurora immediately after all. (The title was encouraged by the 2006 animated comedy Around the Hedge.) Rather, STEVE is triggered by warm atmospheric emissions—charged particles heating up significant in the ionosphere. STEVE lacks the distinctive spectra linked with common auroras, which locations it in a unique group all its have. The picket fence exhibit, having said that, is viewed as to be an abnormal form of aurora due to the fact it is brought on by a precipitation of electrons in the environment it has also been observed on its possess, with no STEVE.

Astronomers assume the dunes are legitimate aurora. Newbie stargazers in Finland, Norway, and Scotland spotted the dunes in October 2015 and again on January 20, 2016, both of those at an altitude of about 100 kilometers (62 miles)—that is, in Earth’s higher mesosphere, found over the stratosphere and below the thermosphere. The January light demonstrate lasted for nearly 4 hours across a wide expanse, providing the researchers a good deal of time to document the phenomenon with pictures.

Last year, researchers at the Centre of Excellence in Analysis of Sustainable Room proposed a doable underlying system for the auroral dunes: improved density of atmospheric oxygen atoms, potentially as a final result of an atmospheric wave phenomenon identified as a mesospheric bore. The bore is very similar to the tidal bores that have been noticed in some rivers, and it occurs when waves of oxygen atoms interact with solar wind, producing the atoms to glow. If a mesospheric bore were touring horizontally in a waveguide in the upper ambiance, it would trigger an enhance in concentrations of atmospheric oxygen and produce the dune-like auroral emissions. That waveguide would sort amongst the boundary of the mesosphere and an inversion layer of air under it, permitting waves of particular wavelengths vacation for lengthy distances via it.

In other words and phrases, the aurora illuminates a wave that is now present in the atmosphere. This hottest research, which examines electron precipitation and observational temperature info, supports that hypothesis. For their assessment, the scientists merged observational data from satellites with photographic and online video material delivered by the hobbyists who observed the dunes in January 2016.

The researchers identified that on the evening of January 20, 2016—when the dunes were being out in complete force—there was electron precipitation in the area, earning it hugely likely that there would be enough energetic electrons in the ambiance to result in auroral emissions. There was also a powerful temperature-inversion layer in the mesosphere—ideal circumstances for the development of the necessary waveguide, resulting in an elevated density of oxygen atoms. Also, a time-lapse video shot about the course of that evening permitted the scientists to estimate a propagation speed of extra than 22 m/s for the dunes, which “indicates the presence of potent horizontal winds in the higher environment,” the authors wrote.

However, there is not nevertheless any immediate observational data for the mesospheric bore within the waveguide. Getting that information will be the up coming move in this ongoing research. Although we however do not have a definitive response about the specific mother nature of the dunes, this most recent study does supply robust observational proof in support of the performing hypothesis. And it is really a testomony to the analytical ability of combining satellite info with the recorded observations of citizen scientists.

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

Listing image by Graeme Whipps

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