Dune auroras shine light on atmospheric dynamics

Auroras have fascinated people for as long as they’ve lived far enough north or south to see the polar light displays. Although we now understand their sources to be errant charged particles rather than gods or spirits, they still yield surprises. Case in point: In January 2020 researchers published the first report of so-called dune auroras , pictured above, which had been photographed by citizen scientists in Finland and Sweden. The dunes’ unusual undulating brightness caused the researchers to speculate about a connection with mesospheric bores, a rare type of atmospheric wave that had never been seen in conjunction with auroras.

The discovery also prompted researchers to question whether the dunes are actually an auroral phenomenon. Auroras arise when charged particles from the solar wind are funneled by Earth’s magnetic field and precipitate into the atmosphere at high altitudes. That process, however, doesn’t underlie all atmospheric light displays. For example, another citizen scientist–observed optical manifestation, STEVE (Strong Thermal Emission Velocity Enhancement), had recently failed that test under scientific scrutiny; in 2018 it was found to be a ribbon of hot gas in the upper atmosphere.

Maxime Grandin and Minna Palmroth, both at the University of Helsinki and authors on the earlier dunes paper, have now extended that study. With their collaborators—including citizen scientists from Finland, Norway, and Scotland—they have scrutinized a dune aurora that appeared on 20 January 2016 to understand its auroral nature and connection to mesospheric bores. The data they compiled are consistent with an atmospheric wave illuminated by the aurora.

Observations by the Special Sensor Ultraviolet Spectrographic Imager, a satellite-based imaging system, confirmed electron precipitation in the area around the time of the dune’s appearance. And the energies it measured, around 10–20 keV, indicate electrons that would be likely to deposit their energy around the established dune altitude of 98–106 km.

Precipitating electrons produce green auroras—similar to the dunes—through their interactions with atomic oxygen, which is abundant above about 100 km but less so at lower altitudes. A wave traveling across the sky around that 100 km altitude could create periodic variations in the concentration of atomic oxygen that would show up as bright and dark stripes. But was the wave a mesospheric bore?

Temperature measurements from the satellite-based Sounding of the Atmosphere using Broadband Emission Radiometry instrument support that explanation. Mesospheric bores travel in thin waveguide-like atmospheric layers that are created by a temperature-inversion layer, and the data indicated the presence of such a layer in the 70–90 km altitude range. Although the evidence falls short of proving the involvement of mesospheric bores, the researchers are hopeful that the dunes will be a useful tool for studying the elusive waves. (M. Grandin et al., AGU Adv. 2, e2020AV000338, 2021 .)