Tuesday, February 7, 2023

Scott Bounds
Scott Bounds

Assistant Research Scientist Scott Bounds is serving as the principal investigator for the Aurora Current and Electrodynamics Structures II (ACES II) mission, which launched rockets from Andøya Space in Norway on Nov. 20, 2022. The NASA-funded project measured the global electric circuit underlying the Northern Lights. 

High above us, electrons from space stream into our sky. As they wind down Earth’s magnetic field lines, they strike gases in our atmosphere, causing them to glow. From the ground, observers see effervescent ribbons of ruby and emerald: the aurora borealis and australis, or northern and southern lights.

But auroras are just one part of a much larger system. Like a light bulb plugged into an outlet, they are powered by a larger electrical circuit connecting our planet to near-Earth space.

 long-exposure shot of the two ACES II rocket trajectories
A long-exposure shot of the two ACES II rocket trajectories as they launched from Andøya Space in Andenes, Norway on Nov 20, 2022.
Credits: NASA/Lee Wingfield

“It's these incoming high-energy electrons that produce the auroral display we're familiar with, but there's also part of the system that is unseen,” said Bounds.

Just as charged particles flow in, a stream of charged particles flows from our atmosphere back out to space. Together, this inflow and outflow complete a global electrical circuit known as the auroral current.

One of the biggest mysteries about the auroral current is what happens at the “turnaround point,” where the inflow ends and the outflow begins. This turnaround is in the ionosphere, a layer of our atmosphere that begins some 60 miles overhead and extends into space, where charged particles and neutral gases coexist and interact.

The ionosphere is like a bustling border town where travelers from different lands, unfamiliar with each other’s customs, meet and exchange their wares. Those arriving from above are electrically charged particles from space. Accustomed to the wide-open pathways of space, they rarely collide with one another. Their electric charge keeps them tethered to Earth’s magnetic field lines, which they twirl around as they nosedive into our atmosphere or outwards into space.

Those arriving from lower altitudes are neutral gases from our atmosphere. They bump through dense crowds, bouncing back and forth hundreds of times a second. Without an electric charge, they move freely across magnetic field lines as they are carried about by the wind.

In the ionosphere, these two populations merge – colliding, combining with one another and separating again, and interacting in complex ways. It is a chaotic scene. And yet, this turbulent mixing in the ionosphere is what keeps the auroral current churning.

To date, most studies of the auroral current have only measured inflow and outflow from high above the ionosphere, making simplifying assumptions about what is happening below. ACES II was designed to remedy that, taking a “snapshot” of the complete auroral current at one moment in time. The strategy is to fly two rockets: a “high-flyer” that will measure particles flowing in and out of our atmosphere, and a “low-flyer” that, at the same time, will see the dynamic exchange in the ionosphere that keeps it all flowing.

At the Andøya Space Center in Andenes, Norway, the auroral oval – the magnetic “ring” encircling Earth’s northern magnetic pole within which auroras form – passes overhead each night. Bounds and his team waited until the auroral oval was overhead – their clue that the auroral current was flowing above them.

The team launched the high-flyer, aiming for a peak altitude of about 255 miles (410 km). Its goal was to see the streams of particles flowing into and out of our atmosphere. One hundred seconds later, they launched the low-flyer through the lower parts of the ionosphere, peaking at about 99 miles (159 km). Its goal was to capture the energy exchange happening at the turnaround point, where inflow turns into outflow. The trajectories of the two rockets were aligned in space and time, to ensure they were measuring different parts of the same current. Like all sounding rockets, both the high- and low-flyer made their measurements and fell back to Earth a few minutes later.

The experiment team reported that the auroral arc was in a good location for both rockets and good data was obtained from many of the instruments. 

ACES II fact sheet
ACES II fact sheet. Credits: NASA

Two University of New Hampshire undergraduates, two graduate students and a few other scientific staff worked closely with Marc Lessard, University of New Hampshire professor of physics to design and build four instruments — two for each rocket — that would measure the temperature of the electrons in the aurora and measure the currents directly.

Other collaborators on the Aces II mission were from the University of California, Berkeley, the Johns Hopkins University Applied Physics Laboratory, and the University of Calgary.

The ACES instrument has flown once before, launching from the Poker Flat Research Range in Fairbanks, Alaska, in 2009. There, it flew through an active, turbulent aurora. It was like measuring the weather during a particularly stormy day.

“We got great results, but what we want to understand for this flight is the ‘average case,’” Bounds said. Andøya is located much closer to Earth’s magnetic north pole, meaning milder, more typical auroras that don’t spread as far south are more accessible.

ACES II will help scientists model the auroral current, including its trickiest part: our ionosphere.

“This is just a single case – it doesn't answer all questions,” Bounds said. “But it gets us a data point we need.”

This article was adapted from “Rockets to Uncover Electric Circuit That Powers the Northern Lights”  by Miles Hatfield, NASA’s Goddard Space Flight Center, Greenbelt, Md. and “Research Snapshot: Northern Lights” in University of New Hampshire’s UNH Today.