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NASA Missions Help Identify What Powers Auroral ‘Space Battery’

Two bright bands of green aurora stretch across the dark night sky from the horizon at the lower right toward the foreground in the upper left, appearing almost like long curtains hanging in the sky. The left band has a series of bright vertical rays along it, like spotlights shining upward. An additional green glow appears just above the horizon in the distance. In the lower right, trees appear silhouetted against the auroras.
Green auroral arcs extend across the sky in this photo captured in northwestern Canada in 2019. The right arc is a typical auroral arc, while the brighter arc on the left is transforming into another type of aurora with bead-like features.
Neil Zeller

Scouring archived observations from NASA missions, scientists may have solved a mystery about what powers a type of aurora called auroral arcs.

The answer, they say, is space waves.

From the ground, auroral arcs look like green, glowing curtains of light sweeping across the night sky. From space, they appear as thin, green lines — or arcs — slicing across the atmosphere.

An arc of green aurora bends over the limb of Earth, which is covered in swirling clouds. A faint red haze appears above the green auroral arc against the black background of space and a couple dozen stars.
A green auroral arc appears above the clouds in this photo taken by an astronaut aboard the International Space Station in 2014.
NASA

Scientists know that auroral arcs form when electrons, accelerated by electric fields in space, slam into atoms in our atmosphere, releasing light. The electric fields work like a “space battery” for the auroral arcs, but scientists weren’t sure what powers that battery.

Searching for clues, a team led by Sheng Tian of the University of California, Los Angeles, found simultaneous observations of an auroral arc in April 2015 made by NASA’s Van Allen Probes, the U.S. military’s Defense Meteorological Satellite Program F19 spacecraft, and ground-based cameras for NASA’s THEMIS (Time History of Events and Macroscale Interactions during Substorms) mission. These combined observations provided different viewpoints over a long enough time to reveal more about the conditions in space that helped create the arc.

The results, published in Nature Communications on Jan. 13, suggest the electric fields are energized by waves in space called Alfvén waves, which travel along Earth’s magnetic field lines.

Similar particle acceleration has been observed by NASA’s Juno spacecraft around Jupiter. Tian’s team thinks Alfvén waves could also power auroral arcs at Jupiter and other worlds where auroras glow.

– Vanessa Thomas