CIRES PhD Student Uses Powerful Lidar to Drive Discoveries High in the Atmosphere

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.

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“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.”

A new discovery high in the atmosphere

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.

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