In 2021, a team led by CIRES Fellow Xinzhao Chu discovered layers of sodium and other metals appear with surprising regularity — twice a day at dusk and just before dawn — high in the atmosphere above Boulder, Colorado.

These layers, called Thermosphere-Ionosphere sodium (TINa) layers, contain metals from meteoroids and dust that are vaporized as they enter Earth’s atmosphere. Chu and her colleagues showed the layers co-occur with high-atmospheric tidal winds created by the Sun’s daily cycle. These metal layers are important because they can affect radio communications and GPS navigation.

Now, work led by Yingfei Chen, a member of Chu’s research group and graduate student in CIRES and Aerospace Engineering Sciences at CU Boulder, has identified a transition layer between the TINa layer and a lower, permanent layer of atmospheric metals. Chen and his colleagues are the first to observe this transition layer.

“The transition layer is a bridge connecting two different atmospheric environments,” Chen said. “The atmosphere below this layer is mostly neutral in charge and strongly influenced by waves and tides. Above the transition layer is the thermosphere-ionosphere region, where charged particles become increasingly important. Observing this transition region helps us understand how the neutral atmosphere and the charged ionosphere are coupled.” 

To make the discovery, the team analyzed seven years of data from a powerful lidar system that uses lasers to measure how much of and how high a metal is in the atmosphere. Chu’s group collected the data in Boulder, Colorado, from 2011 to 2017.

The study, published last week in Geophysical Research Letters, identified a previously unknown sodium layer and showed that the layer sinks down through the atmosphere, from 120 to 95 kilometers, each night. They also found that this transition layer descends faster in summer months than in winter months.

Their results suggest tidal winds, driven by daily solar heating of Earth’s atmosphere, help form the sodium transition layer by moving and concentrating sodium ions at specific altitudes. These ions can then be converted into neutral sodium atoms through recombination with electrons, making the layer detectable by lidar.

“The transition layer provides direct observational evidence that the main meteoric sodium layer and the higher TINa layer are connected through a dynamically active region,” Chen said. “This gives us a new tracer for studying coupling across the ionosphere, thermosphere, and mesosphere.”

While the Boulder discovery came from years of lidar observations in Colorado, Chen was analyzing data, writing papers, and working on his PhD from one of the most remote research sites on Earth: McMurdo Station in Antarctica. This was where Chu’s group first discovered the thermosphere metal layers in 2011. 

From late February to the end of October 2024— and through the dark, harsh Antarctic winter — Chen ran two powerful lidar instruments at the Arrival Heights Observatory, measuring sodium and iron in the upper atmosphere. The work was a part of the groundbreaking McMurdo lidar observational campaign, which Chu led for 16 years and was funded by the United States Antarctic Program in collaboration with Antarctica New Zealand.

It was Chen’s third time in Antarctica since 2022 and his longest and most grueling trip yet.

Just before Chen started his winter operation in 2024, Chu led critical refurbishments and upgrades to the lidar systems. Chen supported those efforts, which made his over-winter possible. When Chu and the rest of the team left, they continued to provide support and feedback from Boulder.

“Managing two lidar systems here in Boulder is already very difficult,” Chen said. “But when you are doing a similar thing in Antarctica in this Arrival Heights Observatory, far away from many people, it's a different story.”

Chen excelled in conditions many could not. He battled heavy snow and blustery winds just to make it to the lab, which was a mile away from McMurdo. Once he was there, he managed two complex lidar systems, troubleshooting and fixing issues in between weather windows so that his team could capture high-quality measurements of iron and sodium high in the atmosphere.

“We always treasure the long dark nights in Antarctica — that’s why we were there,” Chen said. “But to prepare for and catch all the clear nights, you need to make sure your laser systems are in very good performance.”

When things broke, he had to figure out how to fix them with what was on hand.

Chen had support from Adam Peattie, an engineer at McMurdo Station who helped Chen repair a laser’s power supply. Their efforts enabled the collection of an impressive amount of sodium lidar data through winter 2024.

In another instance, when a piece on the iron lidar system failed, there wasn’t a replacement part in the lab. Chen sought help from electricians at McMurdo — together, they searched through storage to see if they could find a suitable replacement part. They found a similar part, and although it wasn’t a perfect replacement, Chen and electrician Ryan Woodhouse were able to get the lidar system back up and running in 2 days.

"Yingfei's winter-over lidar work was among the very best,” said Chu, who is also a professor of Aerospace Engineering Sciences. “Not only did his team collect the longest continuous data, but also over 400 hours of data in July 2024 — the highest record of any winter month.”

Working in Antarctica didn’t slow Chen down. During his winter-over, he continued moving his PhD and research forward, analyzing Boulder and McMurdo lidar data, writing manuscripts, responding to reviewers, and advancing the dissertation work that led to this discovery. During his time on the ice, Chen even managed to present a talk via recording for the International Laser Radar Conference in Germany.

Last week, Chen successfully defended his PhD dissertation. He will continue his work as a postdoctoral researcher in Chu’s lab.

“In addition to continuing the science, I’m really excited to be participating in the development and building of brand-new lidar systems,” Chen said.