The metal layers at mesospheric altitudes are excellent tracers of neutral atmosphere dynamics, and have been used since the 1960s to study the chemistry and dynamics of the mesosphere. Ablation from meteors is believed to be the chief source of metals such as Na, Mg, K, Fe, and Ca in the middle atmosphere. Due to its relative abundance, large backscatter cross-section, and visible atomic transition, sodium (Na) has been used extensively for lidar studies of the mesosphere.
A spaceborne technique is being developed to remotely measure Na by adapting existing lidar technology with spaceflight heritage. Layers of neutral metal atoms such as Fe, Mg, Ca, K, and Na — which peak between 85 and 95 km, and are ≈20 km in width — are produced by the daily ablation of billions of interplanetary dust particles (IDPs). As these metallic species are ionized during ablation by sunlight's ultraviolet photons, or by charge exchange with existing atmospheric ions, meteoroids affect the structure, chemistry, dynamics, and energetics of the ionosphere. Once the meteoric metals are injected into the atmosphere, they are responsible for a diverse range of phenomena. The strong optical signals that some of these metal layers produce (in particular, the Na layer) make them an optimal tracer of atmospheric circulation and dynamics. Measuring and quantifying the metal budget and distribution of Na in the upper atmosphere will enable scientists to address several compelling questions related to the Earth's upper atmosphere and the geospace environment related to the composition, chemistry, and dynamics of planetary atmospheres.
Satellite-borne Na lidar will provide key measurements that elucidate the complex relationship between the chemistry and dynamics of the Earth's mesosphere, and thus provide a thorough understanding of the composition of this region. The inclusion of a well-characterized mesosphere in global models is essential for weather and climate prediction in the lower atmosphere. The key technologies are the laser transmitter, optical filter, and the photon-counting detectors.
Global Na layer models show that the characteristics of the IDP input required to model the observed atomic Na layer correlate roughly linearly with the poorly understood parameterization of vertical transport. Since advanced lidars are also able to measure parameters of the background atmosphere in the MLT (i.e., wind and temperature), they provide a crucial set of measurements that will constrain the IDP's input and consequently zodiacal cloud models (ZCM).
Modeling the climatology and global distribution of the Na mesospheric layer requires the utilization of a complex combination of ZCM, chemistry of the meteoroid ablation, and global circulation models (GCM). The measurements provided by spaceborne lidar would enable not only the constraint of ZCM, but also the utilization of the layer as a tracer for global circulation, thus validating and improving GCMs as needed.