Aurora mysteries unlocked with NASA’s THEMIS mission

A special type of aurora, draped from east to west across the night sky like a shimmering pearl necklace, helps scientists better understand the science of aurora and their powerful pilots in space. Known as auroral beads, these lights often appear just before large auroral displays, which are caused by electrical storms in space called sub-storms. Previously, scientists were unsure whether auroral beads were somehow connected to other auroral displays as a phenomenon in space preceding sub-storms, or whether they were caused by disturbances closer to the atmosphere. earthly.

But powerful new computer models, combined with observations from NASA’s mission on the Temporal History of Events and Macroscopic Interactions during Substorms – THEMIS – provided the first solid evidence of events in space that have led to the appearance of these pearls, and demonstrated the important role they play in our environment close to space.

“We now know for sure that the formation of these beads is part of a process that precedes the onset of a sub-storm in space,” said Vassilis Angelopoulos, THEMIS principal investigator at the University of California at Los Angeles. “It’s an important new piece of the puzzle.”

By providing a larger image than that of the three THEMIS satellites or ground observations alone, the new models have shown that auroral beads are caused by turbulence in plasma – a fourth state of matter, made up of gaseous charged particles and highly conductive. “The surrounding Earth. The results, recently published in journals Geophysical research letters and Journal of Geophysical Research: Space Physics, will ultimately help scientists better understand the full range of swirling structures seen in auroras.

“THEMIS observations have now revealed turbulence in space which causes seen fluxes lighting up the sky like single pearls in the shining auroral necklace,” said Evgeny Panov, lead author of one of the new papers and THEMIS scientist. at the Institute for Space Research of the Austrian Academy of Sciences. “This turbulence in space is initially caused by lighter, more agile electrons, moving with the weight of particles 2000 times heavier, and which theoretically can develop into large-scale auroral subforms.”

Mysteries of Auroral Bead Formation

Auroras are created when charged particles from the Sun are trapped in Earth’s magnetic environment – the magnetosphere – and are channeled into Earth’s upper atmosphere, where collisions cause atoms and hydrogen molecules to glow, d oxygen and nitrogen. By modeling the near-Earth environment at scales ranging from tens of miles to 1.2 million miles, THEMIS scientists were able to show the details of the formation of auroral beads.

As the clouds of plasma scattered by the Sun pass through the Earth, their interaction with the Earth’s magnetic field creates plasma bubbles floating behind the Earth. Like a lava lamp, the buoyancy imbalances between bubbles and the heavier plasma in the magnetosphere create 2,500-mile-wide fingers of plasma that stretch toward Earth. The signatures of these fingers create the distinct pearl-like structure in the aurora.

“We realized that, in summary, these relatively small transient events that occur around the magnetosphere are sort of important,” David Sibeck, THEMIS project scientist at NASA’s Goddard Space Flight Center, told Greenbelt, Maryland. “We only recently got to the point where the computing power is good enough to capture the basic physics in these systems.”

Now that scientists understand that auroral pearls precede sub-storms, they want to understand how, why, and when pearls could trigger a true sub-storm. At least in theory, fingers can entangle magnetic field lines and cause an explosive event known as magnetic reconnection, which is well known for creating large-scale sub-storms and auroras that fill the night sky.

New models open new doors

Since its launch in 2007, THEMIS has carried out detailed measurements during its passage through the magnetosphere in order to understand the causes of sub-storms that lead to auroras. In its first mission, THEMIS was able to show that magnetic reconnection is one of the main drivers of sub-storms. The new findings underscore the importance of smaller-scale structures and phenomena – those hundreds and thousands of kilometers in diameter versus those spanning millions of kilometers.

“In order to understand these features in auroras, you really have to work out the smaller global and local scales. That’s why it was so difficult until now,” said Slava Merkin, co-author of one of the new articles and scientific. at the NASA Center for Geospace Storms, based at the Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland. “It requires very sophisticated algorithms and very large supercomputers.”

The new computer simulations correspond almost perfectly to THEMIS and observations on the ground. After the initial success of the new computer models, THEMIS scientists are eager to apply them to other unexplained auroral phenomena. Particularly in explaining small-scale structures, computer models are essential as they can help interpret what is happening between the spaces where the three THEMIS spacecraft pass.

“There are a lot of very dynamic, very small scale structures that people see in auroras that are difficult to connect to the big picture in space because they happen very quickly and on a very small scale,” said Kareem Sorathia, lead author of one of the new papers and scientist at NASA’s Center for Geospace Storms, headquartered at the Johns Hopkins Applied Physics Laboratory. “Now that we can use global models to characterize and study them, this opens a lot of doors.”