Chasing the Eclipse from the Air

Astrophysicist Jenna Samra has been waking up with solar eclipse stress dreams for months. In a particularly vivid one, she is dragging her science instrument down the street as she watches the eclipse pass by in front of her.

“Even I can interpret that,” she jokes.

Samra is the principal investigator of the Airborne Coronal Emission Surveyor (ACES), a National Science Foundation-funded mission run by the Center for Astrophysics, Harvard & Smithsonian. The experiment will fly along the eclipse path, collecting infrared emission from the Sun’s outer atmosphere, called the corona.

With less than two weeks until the eclipse, Samra and her team are now busy testing the instrument and its upcoming flight path. On April 8th, they plan to take to the skies, measuring the temperature and density of the corona — and hoping to observe never-before-seen coronal emissions that reveal details of the corona’s dynamic magnetic field. 

Although the Sun’s corona burns extremely hot — more than 2 million degrees Fahrenheit — the brilliance of the Sun’s visible surface obscures it. Only during a total solar eclipse, when the Moon blocks the Sun’s surface, can scientists briefly study the corona’s emissions and activity right down to the edge of the disk.

Studying the corona helps scientists learn about coronal mass ejections (bursts of particles driven by the Sun’s powerful magnetic field), which have the potential to create space weather near Earth. The Sun is currently near the maximum of its 11-year cycle of activity, so these energetic events are more common now.

But while scientists have extensively studied such particle ejections at visible and ultraviolet wavelengths, their infrared emission has been underexplored. Enter ACES.

Airborne

Quick behind-the-scenes look at scientists and technicians loading and installing the Airborne Coronal Emission Surveyor onto the research aircraft for the upcoming Airborne Coronal Emission Surveyor (ACES) project, which will fly during the total eclipse next month. pic.twitter.com/Q1WZQAlF88

— NSF National Center for Atmospheric Research (@NCAR_Science) March 7, 2024

ACES will observe the eclipse not from the ground but from the sky. Since Earth’s atmosphere absorbs most of the infrared light that the Sun emits, ACES will fly above the densest part of the atmosphere, the troposphere, to achieve clear results. The instrument will be mounted on an airplane operated by the National Center for Atmospheric Research, which will fly at around 14 kilometers, or 46,000 feet — even higher than commercial aircraft.

Flying through the shadow of the solar eclipse has a second key benefit: ACES can stay within the path of totality for longer. On the ground, the longest totality will be about four and a half minutes; in the air, traveling about 500 miles per hour, the instrument will be in darkness for around six minutes. Samra calls ACES’ flight a “mission of opportunity.”

ACES isn’t the only solar science mission capturing the eclipse off the ground. NASA’s Parker Solar Probe and STEREO-A spacecraft as well as the ESA/NASA Solar Orbiter will all be monitoring the Sun and interplanetary space from the same side of the Sun as ACES. Yeimy Rivera, ACES project scientist, says the placement of the four instruments was “one of the best configurations that we could have possibly asked for.”

On eclipse day, ACES’ sensitive spectrograph will capture infrared light from a thin sliver of the corona. The science team is looking for peaks in the resulting spectrum, which indicate particularly bright emission at certain wavelengths due to the presence of specific ions, or atoms with their electrons stripped away. Since the Sun’s intense radiation is what strips those electrons, observing certain ions enables astrophysicists to work backward to determine the temperature and density of the corona itself.

Observing rare ions in the corona will also help scientists decide if they should build dedicated instruments to study them and their role in the magnetic fields that twist through and guide the coronal flow.

“Similar to the way that we want to be able to predict the formation of hurricanes,” Rivera says, “[we] want to do the same thing with coronal mass ejections.” The space weather these ejections cause has the potential to affect not just our communication systems, but GPS navigation, power grids, and internet access.

ACES is not the first instrument in this infrared science mission to fly during an eclipse. Its predecessor, AIR-Spec, flew over Kentucky in the 2017 eclipse and over the southern Pacific Ocean in the 2019 eclipse, recording emissions from iron and sulfur ions that had never been observed before.

Since the 2019 eclipse, the team designed and built Airborne Stabilized Platform for InfraRed Experiments (ASPIRE), a system that stabilizes the telescope’s camera and enables it to capture emission at wavelengths from 1 to 4 microns. However, due to time constraints, a modified camera flying on April 8th will only capture wavelengths from 1 to 1.7 microns; the full-range detector will likely fly on the 2026 eclipse over Europe.

A Moving Target

Aside from testing the instruments in time, the hardest part of the ACES mission is making sure the camera’s small field of view is pointed at the desired location in the corona. The mission plans the alignment about a month in advance, but final adjustments are made in real time on the aircraft.

“You’re trying to catch this moving target in a moving airplane,” explains Samra, who is in charge of pointing the instrument during the eclipse. “You have all of these moving parts that have to go right in order for you to do that.”

Despite the challenges, Samra and her team are excited for their third eclipse and the results it will reveal.

“There’s always a moment as the Sun begins to eclipse just partially, and you know totality is coming,” Samra says. “And there’s just this moment of excitement, and this thought of, ‘How cool is my job?’”

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Find more information and resources for the 2024 solar eclipse.