The mesosphere and lower thermosphere are parts of the middle atmosphere stretching from around 50 km to well beyond 100 km above the ground. Whilst receiving modest attention in an everyday context, these cold and remote regions actually play an important role on the climate here on Earth. Gravity waves in these regions are largely responsible for the global atmospheric circulation and the subject to significant interest e.g. in connection with climate modelling, and the temperature there is recognised as a marker for global warming and climate change. The study of the mesosphere and lower thermosphere is therefore of great scientific interest, but the altitude makes them difficult to probe with means other than remote sensing instruments; balloons and aircrafts cannot reach the required altitude, most satellites fly too high, and rockets are expensive and offer limited spatial and temporal measurement resolution.

With the initial objective of improving the STAR Na Doppler lidar built for measuring temperature and wind via unbound Sodium in the mesosphere, Smith and Chu first give a thorough description of the receiver architecture of such a resonance-fluorescence lidar. Analytical expressions describing the impact on lidar performance of various detrimental factors owing to the receiver design, including mode-mismatch, pointing error, and aberrations, are presented and explained. This detailed analysis leads the authors to develop a simple but efficient procedure for optimising the alignment of the entire receiver. The procedure involves minimising aberrations using a commercially available camera, and carefully aligning the optics with the detector. This procedure, however, is simple enough to be directly transferred to other similar lidar systems.

The potential of the optimisation procedure is demonstrated on three different lidar systems. The reported improvements for all three systems are impressive, with an increase in received photons per laser shot by a factor of 4-5, bringing them to a level comparable to systems with much higher power-aperture products. The higher photon count per laser shot implies that fewer shots are necessary to obtain a measurement, and thus a higher spatial and temporal resolution than previously seen can be achieved. This will help increasing our knowledge of the mesosphere and lower thermosphere, and perhaps we might even see measurements of dynamic features such as turbulent structures soon.

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