Optical Doppler imaging of atmospheric winds

Doppler imaging is the method of measuring winds in the atmosphere by taking images of airglow emissions through a field widened Michelson interferometer. This is the method used by the Wind Imaging Interferometer (WINDII) on UARS, the Upper Atmosphere Research Satellite. Efforts are under way to develop similar instruments for the infrared in order to extend the technique down to the mesosphere and stratosphere. The basic components of this type of instrument are an imager, such as a CCD camera, that observes the view at the Earth´s limb through a Michelson interferometer and a narrow-band interference filter. The filter ideally isolates a single emission line, and the Michelson modulates the line, producing interference fringes. The path difference is set at a large value, typically ~10⁵ wavelengths from zero, or ~5 cm in the visible. A single fringe is sampled at intervals of λ/4 by moving one of the Michelson´s mirrors piezoelectrically, and the phase, Φ, of the hinge is calculated from these four intensity samples after subtraction of the dark current and background signal. The Doppler shifts corresponding to the winds in the emitting region are detected as phase changes in the interference fringe. In order to accommodate a field of view of sufficient size, the interferometer must be field widened, as the useful field at large path difference is very small. This is accomplished by placing blocks of glass in the interferometer´s arms. By choosing the types of glass and their dimensions carefully, a large central range can be produced, and this can be done over a large range of wavelengths and the interferometer can be thermally compensated as well