Laser and electro-optic technologies are under development to remotely measure sodium (Na) by adapting existing LiDAR technology with spaceflight heritage. The developed instrumentation will serve as the core for planning a heliophysics mission targeted to study the composition and dynamics of Earth's mesosphere based on a spaceborne LiDAR that will measure the mesospheric Na layer.
There is a pressing need in the Ionosphere-Thermosphere-Mesosphere (ITM) community for high-resolution measurements that can characterize small-scale dynamics (i.e. gravity waves with wavelengths smaller than a few hundred km) and their effects in the Mesosphere-Lower-Thermosphere (MLT) on a global basis. This is compelling because they are believed to be the dominant contributors to momentum transport and deposition in the MLT, which largely drive the global circulation and thermal structure, and interactions with the tides and planetary waves in this region.
A spaceborne remote sensing technique will enable acquisition of global Na density, temperature, and wind measurements in the MLT with the spatial and temporal resolution required to resolve issues associated with the structure, chemistry, dynamics, and energetics of this region. A nadir-pointing spaceborne Na Doppler resonance fluorescence LiDAR onboard the International Space Station (ISS) will essentially make high-resolution, in time and space, Na-density, temperature, and vertical wind measurements from 75-115 km (MLT region).
The instrument concept consists of a high-energy laser transmitter at 589 nm and a highly sensitive photon-counting detector that allows for range-resolved atmospheric-sodium-temperature profiles. The atmospheric temperature is deduced from the linewidth of the resonant fluorescence from the atomic sodium vapor D2 line as measured by a tunable laser. A high-power energy laser is under development that will allow for some daytime sodium LiDAR observations with the help of a narrow bandpass filter based on etalon or an atomic sodium Faraday filter with ~5 to 10 pm optical bandwidth. The current baseline detector for the LiDAR instrument is a 16-channel photomultiplier tube with receiver electronics that has been space-qualified for the ICESat-2/ATLAS mission. The technique uses the 16 channels as a photon-number-resolving single detector to provide the required full spectroscopic sodium line-shape waveform for recovering mesospheric temperature profiles.