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Abstract First characterization of year‐round Na layers from 75 to 150 km is enabled with 7 years (2011–2017) of high‐detection‐sensitivity lidar observations over Boulder (40.13°N, 105.24°W). Clear annual and semiannual oscillations (AO and SAO) are revealed in the nightly‐mean thermosphere‐ionosphere Na (TINa) (∼105–150 km) number density and volume mixing ratio with the summer maximum but spring equinox (March/April) minimum. Such stark contrast to the summer minimum in the main Na layers (∼75–105 km) supports the theory of TINa formed via TINa⁺ion neutralization. The SAO/AO amplitude ratio profiles (75–150 km) exhibit significant changes (∼0.06–2), linking TINa SAO to thermospheric density SAO and the minimal wave/eddy transport around midlatitude equinoxes which hinders TINa⁺ion production and upward transport via reduced diffusion of the main Na layer. Stronger TINa in autumn than in spring equinox is explained by the maximal (minimal) meteoric influx occurring in September (April). 

Abstract Observational data inherently contain noise which manifests as uncertainties in the measured parameters and creates positive biases or noise floors in second‐order products like variances, fluxes, and spectra. Historical methods estimate and subsequently subtract noise floors, but struggle with accuracy. Gardner and Chu (2020,doi.org/10.1364/AO.400375) proposed an interleaved data processing method, which inherently eliminates biases from variances and fluxes, and suggested that the method could also eliminate noise floors of power spectra. We investigate the interleaved method for spectral analysis of atmospheric waves through theoretical studies, forward modeling, and demonstration with lidar data. Our work shows that calculating the cross‐power spectral density (CPSD) from two interleaved subsamples does reduce the spectral noise floor significantly. However, only the Co‐PSD (the real part of CPSD) eliminates the noise floor completely, while taking the absolute magnitude of CPSD adds a reduced noise floor back to the spectrum when the sample number is finite. This reduced noise floor can be further minimized through averaging over more observations, completely different from traditional spectrum calculations whose noise floor cannot be reduced by incorporating more samples. We demonstrate the first application of the interleaved method to spectral data, successfully eliminating the noise floor using the Co‐PSD in a forward model and in lidar observations of the vertical wavenumber of gravity waves at McMurdo, Antarctica. This high accuracy is gained by sacrificing precision due to photon‐count splitting, requiring additional observations to counter this effect. We provide quantitative assessment of accuracy and precision as well as application recommendations. 

Abstract We have discovered that the peak phase time of predawn thermosphere‐ionosphere Na (TINa) layers (∼110–150 km altitude) undergoes clear annual variations with the earliest occurrence in summer and latest in winter over Boulder (40.13°N, 105.24°W), which are closely correlated to annual phase variations of sunrise and tidal winds. Such discoveries were enabled by the first characterization of 12 monthly composites of TINa layers from January through December using 7 years of lidar observations (2011–2017). Despite their tenuous densities, the predawn TINa layers have nearly 100% occurrence rate (160 out of 164 nights of observations). Monthly composites show downward‐phase‐progression TINa descending at similar phase speeds as Climatological Tidal Model of the Thermosphere tidal winds. These TINa layers occur in ion convergence but neutral divergence regions, modeled using tidal winds. These results support the formation mechanism (neutralization of converged TINa⁺forming TINa) proposed previously and suggest that migrating tidal winds experience annual phase variations. 

Direct measurements of vertical winds in the upper atmosphere present a grand challenge to the lidar research field. However, information of vertical winds is crucial to many science explorations in the upper atmosphere and its important interactions with the plasma space, which play key roles in protecting the life on Earth from the harmful solar winds. In this paper we present a very sophisticated resonance-fluorescence Doppler lidar that has successfully made the direct measurements of vertical winds with high resolutions at McMurdo Station (77.84ºS, 166.67ºE) in Antarctica. Technical considerations on both this Na Doppler lidar transmitter and its receiver are discussed in terms of their influences on the vertical wind measurements and calibrations. Simultaneous measurements of vertical winds, temperatures, and metal species are presented to illustrate new discoveries in vertical flux measurements. Time-mean vertical winds are discussed in terms of unexplained discoveries, suggesting gaps in our understanding of the global meridional circulation. 

McMurdo lidar observations have turned out to be an extraordinary science-discovery journey in Antarctica, owing to advancement in resonance-fluorescence Doppler and Boltzmann lidars. This report presents the high-detection-sensitivity lidars that have enabled the first measurements of heat, Na, and Fe fluxes in Antarctica, allowing the inference of cosmic dust influxes and metal species lifetimes. 

Lidar observations of atmospheric gravity waves (GWs) have been made spanning 14 years above McMurdo Station, Antarctica. Using these extensive observations and interleaved data processing techniques which enable bias‐free/noise‐floor‐free estimation of GW parameters, this study forms seasonal baselines for GW potential energy densities (Epm, Epv), ground‐based frequency (omega) spectrum, and vertical wavenumber (m) spectrum in the stratosphere (30–50 km) and mesosphere (50–70 km). The stratospheric Epm is dominated by an annual oscillation with a winter maximum. Spring/fall Epm profiles show decreased GW dissipation/breaking in 46–56 km. The wintertime Epm profile shows two bending points, where the GW scale height steepens above 39 km and steepens further above 50 km. These bending points are scale dependent, where profiles with  lambda_z= 2–8 km bend only at 39 km, and profiles with lambda_z = 8–30 km bend only at 50 km. GW ‐spectra from 30 to 50 km resembles 50–70 km m‐spectra at high‐m but differs at low‐m, while frequency omega‐spectra grows evenly over all omega. Spectral observations are examined in the context of linear instability, saturated cascade, and diffusive filtering theories, but the exact mechanisms responsible are to be determined. These bending points and spectral trends indicate consistent altitude ranges with enhanced GW dissipation. We propose that this dissipation could potentially serve as a source for the generation of secondary GWs in the middle atmosphere but the lidar data alone cannot confirm this, which deserves future study. 

The Earth’s upper atmosphere separates interplanetary space from the lower atmosphere and biosphere, absorbs harmful solar radiation, dissipates cosmic dust and energetic particles, and regulates gaseous escape and atmospheric waves, therefore protecting living things on Earth. It is difficult to observe the upper atmosphere, posing challenges to studying these processes. Advancement of lidar technologies and observations over the last decades have revolutionized the research field, significantly extended the profiling altitude ranges and capabilities, and created new potential for exploring space-atmosphere interactions. This article summarizes the principles, technologies, and major discoveries of lidar studies of the upper atmosphere and near-space environment. 

We review the mechanism of multi-step vertical coupling (MSVC) via secondary and higher-order gravity waves (GWs), and its relevance for observed GW perturbations and the circulation in the upper mesosphere and thermosphere. Since the momentum deposition following the breaking or dissipation of a GW packet is localized in space and time, it leads to an imbalance in the ambient flow which in turn results in the generation of secondary or higher-order GWs. This local “body force” (LBF) mechanism is essential for MSVC. We argue that small-scale secondary GWs resulting directly from GW instability form a macro-turbulent cascade that leads to the LBF. We present a simple scale analysis supporting this interpretation with respect to observed GW spectra. Several examples of MSVC are reviewed. These include 1) an explanation of the observed persistent GWs and prevailing eastward winds in the winter mesopause region at middle to high latitudes via secondary GWs, 2) evidence that many of the daytime traveling ionospheric disturbances in the F region during winter and low geomagnetic activity are driven by higher-order GWs from MSVC, 3) the dependence of MSVC during wintertime on the strength of the polar vortex, and 4) the secondary GW disturbances in the thermosphere and ionospheric that were triggered by the Tonga volcanic eruption on January 15, 2022. Furthermore, we describe the GW-resolving whole-atmosphere model that was primarily used in corresponding studies of MSVC, and we discuss some open questions.