Computing twodimensional unambiguous horizontal wavenumber spectra from oh airglow images

There is an inherent 180° ambiguity in the derived wavepropagation direction when using conventional spectralanalysis techniques on OHimager observations. A processing technique for computing the unambiguous twodimensional (2D) horizontal wavenumber spectrum from images of gravitywave perturbations in OH emission intensities is presented. The technique involves computing the (w, k, l) =(w, h, φ) spectrum of OH images collected over a severalhour period, where k is the zonal wavenumber, I is the meridional wavenumber, h (k² + l²)^1/2, φ tan¹(k/l), and w is the temporal frequency. Before computing the spectra, the allsky images are processed by first flat fielding each image, then removing stars by using a median filter, subtracting an estimate of the background continuum, and finally computing the relative OH intensity perturbations. Each pixel is then mapped onto a 256 x 256 rectilinear grid of geographic coordinates by using a 9 x 9 Hammingweighted sineinterpolation function. The interpolated image is restricted to the 600 x 600km2 region centered at zenith so that the resulting horizontal resolution is 2.34 km. Spatial and temporal prewhitening is employed prior to computing spectra to minimize artifacts in the derived unambiguous spectrum. We illustrate the method with a real sequence of images acquired on February 3, 1995, at the Starfire Optical Range (SOR), near Albuquerque, NM. The predominant direction of wave propagation is determined and the k, I, h, and (f> spectra presented.