Daily sodium layers appear in Boulder’s sky

At about 100 km above Earth’s surface, a permanent layer of neutral sodium pulled from passing meteors rests near the top of the mesosphere. Above that, scientists have observed temporary Na layers that form and then descend back to the permanent layer. Called thermosphere–ionosphere Na (TINa) layers, the temporary sheets are mainly observed sporadically at latitudes near the poles or the equator. But some hypotheses predict regular layers at midrange latitudes. Observing the dynamics of such layers at those latitudes could help scientists better understand the transportation of ions in the ionosphere and study neutral molecules in a part of the upper atmosphere where few other measurements detect them.

Now, Xinzhao Chu, her coworkers from the University of Colorado Boulder, and their collaborators have discovered regular TINa layers over Boulder, Colorado, a city at a midrange latitude. Using data from the Student Training and Atmospheric Research (STAR) Na Doppler lidar at Table Mountain Observatory, the researchers observed bands that form daily before dawn and again after dusk. Their work introduced the first lidar observation of regular layers at a midrange latitude.

The STAR lidar measured atmospheric Na density using a narrowband laser focused to the Na absorption line at 589 nm. Chu and her colleagues collected data during various months over more than two years and found that the TINa layers appeared regularly. Whenever the researchers started their observations at dusk, the first Na blanket was already descending to the permanent layer. The second layer would form by dawn.

One explanation for the appearance of TINa layers posits that the neutral Na sputters off from meteors when they pass through the atmosphere. But meteors do not regularly arrive over the same spot every day for multiple years, so Chu and her colleagues have expressed doubt that the theory explains their observations.

Instead, the researchers suggest the Na in the temporary layer comes from the permanent layer. The Na atoms can become ionized when they interact with photons and molecular ions. The charged particles begin to interact with Earth’s electric field and thermospheric winds, so they move to higher altitudes. There, they become neutralized through recombination with electrons. Because the forces required for the mechanism are ever present, it could explain the regular TINa layers seen in Boulder.

The ionosphere is not completely understood, and research on TINa layers could reveal some of the mechanisms that govern it. But detecting the tenuous layers requires a sensitive instrument, and most of today’s lidars would be unable to separate the Na sheet from the background. Researchers will need improved lidars to see whether TINa layers appear at midrange latitudes worldwide. (X. Chu et al., Geophys. Res. Lett. 48, e2021GL093729, 2021 .)