A NASA-funded rocket mission is on its way to measure the global electrical circuit that underlies the Northern Lights. For its second trip into space, the Aurora Current and Electrodynamics Structures II or ACES II instrument will be launched from Andøya Space in Andenes, Norway. The launch window opens on November 16, 2022 at 6:00 p.m. local time.
High above us, electrons pour into our sky from space. As they unwind from Earth’s magnetic field lines, they hit gases in our atmosphere and make them glow. From the ground, observers see sparkling bands of ruby and emerald: the Aurora Borealis and Australis, or Northern and Southern Lights.
But auroras are just part of a much larger system. Like a lightbulb plugged into an outlet, they are powered by a larger circuit that connects our planet to near-Earth space.
“It’s these incident high-energy electrons that create the auroras that we’re familiar with, but there’s also a part of the system that’s invisible,” said Scott Bounds, a physicist at the University of Iowa and principal investigator for the ACES II assignment.
As charged particles flow in, a flow of charged particles flows out of our atmosphere back into space. Together, this inflow and outflow complete a global electrical circuit known as the auroral stream.
One of the biggest mysteries of the Aurora Borealis is what happens at the “turning point,” where the inflow ends and the outflow begins. This turn occurs in the ionosphere, a layer of our atmosphere that begins about 40 miles above us and extends into space, where charged particles and neutral gases coexist and interact.
The ionosphere is like a busy border town where travelers from different countries, unfamiliar with each other’s customs, meet and exchange their goods. Those arriving from above are electrically charged particles from space. Accustomed to the vastness of space, they rarely collide with each other. Their electrical charge keeps them tied to the Earth’s magnetic field lines, around which they swirl as they plummet into our atmosphere or out into space.
Those arriving from lower altitudes are neutral gases from our air. They poke through dense crowds, hopping back and forth hundreds of times per second. With no electrical charge, they move freely across magnetic field lines when carried by the wind.
In the ionosphere, these two populations merge – they collide, combine and separate, and interact in complex ways. It’s a chaotic scene. And yet it is this turbulent mixing in the ionosphere that keeps the auroral stream going.
So far, most studies of the auroral flux have only measured inflow and outflow from high above the ionosphere, making oversimplified assumptions about what’s happening below. ACES II was designed to remedy this by taking a “snapshot” of the total auroral flux at a given point in time. The strategy is to fly two rockets: a “high flyer” that measures particles flowing in and out of our atmosphere, and a “low flyer” that observes the dynamic exchange in the ionosphere at the same time it’s all flowing.
At the Andøya Space Center in Andenes, Norway, the aurora oval — the magnetic “ring” surrounding the Earth’s northern magnetic pole where auroras form — sweeps overhead every night. Bounds and his team will wait for the aurora oval to be overhead – their indication that the aurora stream is flowing overhead.
The team will then launch the high flyer and aim for a peak altitude of approximately 255 miles (410 km). His goal is to see the streams of particles flowing in and out of our atmosphere. About two minutes later, they will launch the low-flying aircraft through the lower parts of the ionosphere, to an altitude of about 159 km. His goal is to capture the exchange of energy that occurs at the turning point, where inflow becomes outflow.
The trajectories of the two rockets are aligned in space and time to ensure they are measuring different parts of the same flow. Like all sounding rockets, both the high-flyer and the low-flyer will carry out their measurements and fall back to earth a few minutes later.
The ACES instrument has flown before, launching in 2009 from the Poker Flat Research Range in Fairbanks, Alaska. There it flew through an active, turbulent aurora. It was like measuring the weather on a particularly stormy day.
“We’ve had great results, but what we want to understand for this flight is the ‘average case,'” Bounds said. Andøya is much closer to Earth’s magnetic north pole, which means milder, more typical auroras that don’t spread as far south are more easily accessible.
If all goes as planned, ACES II will help scientists model the auroral flux as a whole, including its most difficult part: our ionosphere.
“This is just an isolated incident – it doesn’t answer all the questions,” Bounds said. “But it gives us a data point that we need.”
Provided by NASA’s Goddard Space Flight Center
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