Measuring the winds on earth using a satellite-borne laser
Willem van de Water
Eindhoven University of Technology
Abstract:
To measure the winds on earth, the European Space Agency plans to launch a satellite with an on-board ultraviolet laser that is directed downwards at the atmosphere. The velocity of the wind will be inferred form the change in frequency of the backscattered light. For this satellite to work, very precise knowledge of the line shape of the scattered radiation is needed.
Not only will this light be scattered off individual molecules, but also off spontaneous thermal sound. This 'hypersound' has wavelength comparable to that of light, and comparable to the mean free path between collisions of the air molecules. The line shapes were measured in two very precise experiments, one measured scattering off spontaneous sound, the other one scattered light off coherent sound created in the standing optical field of two crossing laser beams.
Understanding the effect of sound on light at these extremely small scales and extremely high frequencies opened Pandora's box of a highly controversial quantity of hydrodynamics: the bulk viscosity.