Published: Wednesday, 15 December 2021 00:59
A rare upper-atmosphere phenomenon called Strong Thermal Emission Velocity Enhancement (STEVE) has been well documented by amateur aurora watchers from Canada for decades. STEVE events typically appear as a thin ribbon of pinkish-red or mauve-colored light stretching from east to west, farther south than where aurorae usually appear. Even more strange, STEVE is sometimes joined by green vertical columns of light nicknamed the picket fence. Additional contributions from citizen scientists are now helping researchers identify different types of STEVE emissions and constrain how, and where in Earth’s atmosphere, they’re generated.
“Scientists have known for several years that STEVE is not a regular type of aurora because it appears in lower latitudes and features different colors,” said Dr. Xiangning Chu, a researcher with the Laboratory for Atmospheric and Space Physics at the University of Colorado Boulder.
“But exactly how these light emissions form high in Earth’s atmosphere remains a mystery.”
Because STEVE’s purple and green lights don’t always appear together, Dr. Chu and colleagues wanted to determine whether the two lights share the same driver, but they were not sure how to do this — until a lucky coincidence occurred on July 17, 2018.
That night, two citizen scientists located 400 km (249 miles) apart snapped images of the same STEVE event.
Because the photos were taken simultaneously from two different angles, the researchers were able to perform a simple triangulation, which provided them with the exact location of each point in the images.
They then analyzed this information to try to resolve the location of STEVE’s driver.
“We weren’t sure if we should be looking in part of the ionosphere, 100-400 km (62-249 miles) above the Earth, or in the magnetosphere about 30,000 km (18,641 miles) above the surface,” Dr. Chu said.
The analysis by the team shows that the purple and green emissions are located along the same magnetic field lines, though at different altitudes.
Because they follow the same field lines across an extensive altitude range, there is no known local mechanism in the ionosphere that can account for them.
“Our analysis indicates that both the purple and green STEVE emissions are driven by the same, narrow region, so it must be the magnetosphere,” Dr. Chu said.
“This supports our previous study, which provided evidence that STEVE’s driver region is located at a sharp boundary in the magnetosphere that’s marked by strong waves and particle acceleration.”
The coupling of the ionosphere and the magnetosphere is one of the fundamental topics in space physics, although this process is usually only considered at high latitudes, where aurorae occur.
“But our results indicate that strong coupling might also occur at lower latitudes, transporting vast quantities of energy and momentum between the magnetosphere and ionosphere,” Dr. Chu said.
The scientists will present their findings December 15, 2021 at the American Geophysical Union Fall Meeting 2021 (AGU21).
Xiangning Chu et al. Morphological Characteristics of STEVE emissions. AGU21, abstract # SA34B-09
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