A satellite-borne sodium lidar will provide key measurements that elucidate the complex relation between the chemistry and dynamics of the Earth’s mesosphere, and thus provide a thorough understanding of the composition and dynamics of this region. The inclusion of a well-characterized mesosphere in global models is essential for weather and climate prediction in the lower atmosphere. It also will help to elucidate the complex vertical coupling processes through which atmospheric weather affects space weather. Furthermore, once the technique is developed, it can be used to study the composition of other planetary atmospheres, which is identified as a key point in the recent Planetary Decadal Survey.
A middle atmospheric lidar is a soft-target lidar. The signal return is defined by the density and backscattering cross-sections of particles and molecules that the transmitted photons encounter in their propagation path. Above 80 km, the atmosphere contains a number of metals of meteoric origin and other atomic species that can resonate with photons that are tuned to the correct wavelength (for sodium it is 589 nm). Although other mesospheric metal of meteoric origin such as K, Fe, Ca, and Ca+ can be observed by lidar, the Na lidar is the most widely utilized, making possible long-term observations by a number of lidar systems that have provided details about seasonal, latitudinal, and diurnal variations.
Two technologies will be key for a future spaceborne sodium (Na) lidar instrument because they will enable daytime Na measurements: (1) burst-mode pulsed laser architectures, and (2) a receiver atomic filter utilizing a Faraday rotation technique. Means to increase the overall laser signal energy are being investigated, including delaying and interleaving multiple laser pulses. For the receiver, the atomic filter has been used in ground-based Na lidar systems to filter the weak sodium signal from the much stronger solar background. The developed instrumentation will serve as the core for planning a spaceborne lidar to measure the mesospheric Na layer.
It is not only atmospheric science that can be addressed with such instruments. Global Na layer models show that the characteristics of the Interplanetary Dust Particle (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.