Chu, and 9 co-authors, reported in June, 2021 that the first lidar observations were made of the regular occurrence of mid-latitude thermosphere-ionosphere Na (TINa) layers over Boulder (40.13°N, 105.24°W), Colorado. Detection of tenuous Na layers (∼0.1–1 cm−3 from 150 to 130 km) was enabled by high-sensitivity Na Doppler lidar. TINa layers occur regularly in various months and years, descending from ∼125 km after dusk and from ∼150 km before dawn. The downward-progression phase speeds are ∼3 m/s above 120 km and ∼1 m/s below 115 km, consistent with semidiurnal tidal phase speeds.
One or more layers sometimes occur across local midnight. Elevated volume mixing ratios above the turning point (∼105–110 km) of Na density slope suggest in situ production of the dawn/dusk layers via neutralization of converged Na+ layers. Vertical drift velocity of TI-Na+ calculated with the Ionospheric Connection Explorer Hough Mode Extension tidal winds shows convergent ion flow phases aligned well with TI-Na, supporting this formation hypothesis.
The authors concluded that the STAR Na Doppler lidar with high detection sensitivities has enabled the discovery of mid-latitude TI-Na layers in the E region with fairly regular occurrence over Boulder. A TI-Na layer descends from ∼125 km after dusk and merges with the main layer around 4 UT. Another TI-Na layer occurs broad in time and altitude before dawn with an ascending envelope of Na total density but a descending maximum mixing ratio starting from ∼150 km. The downward-progression phase speeds are ∼3 m/s above 120 km and ∼1 m/s below 115 km, consistent with semidiurnal tidal phase speeds over Boulder as derived from HME tidal winds of the ICON mission.
Occasionally, one or several layers occur across local midnight or between the dusk and dawn layers. Typical number densities of the dawn TI-Na layers are ∼0.1–0.5 cm−3 from 150 to 140 km, ∼1 cm−3 at 130 km, and 3–5 cm−3 at 120 km. The Boulder TI-Na layers exhibit a similar turning point around 110 km as McMurdo, above and below which both the log-scale density and mixing-ratio profiles change to different slopes or trends. The broad peak of mixing ratios above the turning point in Figure 2 rules out pure neutral Na transport from below or from above but provides strong evidence for in situ production of the dawn/dusk neutral TI-Na layers.
The most probable formation mechanism of TI-Na is the neutralization of tidal Na+ layers via recombination with electrons, where TI-Na+ ions are transported and converged by tidal winds and electric fields. The vertical drift velocity of TI-Na+ calculated using ICON-HME tidal winds shows convergent flow phases agreeing with the phases (time and altitude) of observed dusk and dawn TI-Na layers, likely explaining the recurrent occurrence of these dusk and dawn TI Mt layers at semidiurnal tidal phases with enhanced mixing ratios. Numerical simulations and multiple-instrument measurements of neutral and ionized metals as well as electron densities are needed to verify the hypothesis and investigate detailed formation mechanisms. Boulder TI-Na dawn layers occur all year round, including summer months, but the dusk layers occur less frequently. As for the irregular layers occurring between dusk and dawn, some midnight layers even reach ∼170 km. Full characterization of Boulder TI-Na layers and their formation mechanisms are beyond the scope of this paper but will be addressed in future studies.
The TI-Na detection is unequivocal but such low density demands very high detection sensitivities (better than 0.1 cm−3). Most Na lidars operated in the world had/have the detection limits above a few cm−3 and signal levels well below that achieved by the STAR lidar (Smith & Chu, 2015), so the TI-Na signals would be buried in their background noise. Indeed, an earlier version of the STAR lidar, before it was upgraded to high-sensitivity, was not able to detect TI-Na layers above 115 km (Huang et al., 2013). Therefore, it is understandable why many years of lidar observations at midlatitudes did not discover such TI-Na layers until this report. In the future, lidars with high detection sensitivity enabled by large-aperture telescope, high-efficiency receiver architecture, and powerful transmitter will likely lead to the detection of tenuous TI Mt layers globally.
These layers provide a natural laboratory for studying the ion-neutral coupling and act as tracers for profiling the neutral wind and temperature in the E to lower F regions. Observing these metal layers also provides a unique way of monitoring vertical transport in the thermosphere and ionosphere.
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