Rayleigh Scatter Lidar for Characterizing the Near-Earth Space Environment

The behavior of the neutral atmosphere at the transition between the mesosphere and thermosphere, in the vicinity of 120 km, is significantly affected by space weather. To a great extent, these densities and temperatures form the lower boundary condition for models used to calculate densities throughout the thermosphere. These, in turn, affect reentry predictions for satellites and space debris. Unfortunately, the densities and temperatures at 120 km are extremely hard to measure on a regular basis with high spatio-temporal resolution. In situ sensing on rockets does not give continuous measurements. Satellites orbit at higher altitudes and remote sensing from these platform give global coverage while sacrificing fine spatio-temporal coverage. A much more robust approach would be ground-based observations from a global network of very sensitive Rayleigh-scatter lidars.

Rayleigh-scatter lidar has been used by a number of groups, including the Rayleigh lidar group at Utah State University, for mesospheric observations between about 35 and 90 km. Below 35 km the presence of aerosols induce Mie scattering returns which then have to be separated from the molecular (Rayleigh scatter) signal. Above 90 km, the Rayleigh technique is predominantly limited by the instrumentation (high output power, large receiving aperture, sensitive detectors) required to obtain good signal-to-noise (SNR). The raw Rayleigh lidar signal is proportional to neutral number density and is reduced to obtain absolute neutral temperatures and relative neutral number densities. Recently, these relative densities have been put on an absolute scale (Barton et al., 2016, and Price et al., 2018) by normalizing them to densities from reanalysis models (e.g., MERRA2, ERA20C) at about 45 km. The lidar temperatures have also been compared to those from these reanalysis models (Moser, 2018). Accordingly, the Rayleigh lidar technique can now provide both absolute temperature and absolute density up to 90 km.

The group at USU has also pushed the upper altitude limit of the Rayleigh technique into the lower thermosphere. This was done by increasing the output laser power and the telescope receiver area. So far, measurements have reached 115 km (Wickwar et al., 2016; Sox, 2016). With further optimization and the use of modern detectors, it will be straightforward to obtain absolute neutral temperatures and densities to at least 120 km while maintaining a lower limit below 45 km.

Thus, Rayleigh-scatter lidar has the capability of providing good neutral temperatures and densities at 120 km. This capability can be significantly extended with a global ground-based network of robust, easy-to-use lidars. By being global, this network would minimize problems from clouds and the limitations of nighttime observations. The observations would provide information on the major aspects of space-weather interactions, on both the effects of variations in solar input and geomagnetic activity, and the effects of energy carried from the lower atmosphere by waves and tides. More specifically, they would better define the bottom boundary conditions for thermosphere-ionosphere models, which would improve neutral density forecasts for object (satellites and debris) reentry predictions, collision avoidance, Low Earth Orbit (LEO) access to space, and operational vehicle maneuvering.