Fine structure of sprites and proposed global observations

In order to understand sprite processes we have to explain the phenomena from spatial scales of a few meters to the of thunderstorm cells. The intricate small-scale vertical structuring of sprites or the so called beads are particularly difficult to understand. From a two-station triangulation featuring observations from Kitt Peak, Arizona and Socorro, New Mexico, it was possible to make high resolution observations of the sprite structure when the sprite events occurred within the field of view of the narrow field imager. In several cases the lower altitude luminous filamentary structures of columniform sprites (C sprites) consisted of slant directed, nearly vertically aligned columns of intense pinpoint like beads. The distance of the sprites from the observer was measured and the altitude and vertical spacing of the beads were estimated. The distribution of beads showed that the most frequently observed bead spacing is between 0.6 and 1 km. The vertical and horizontal size of the bright luminous beads was about 80 m or less. The bead spacing showed a trend to increase with altitude and the e folding distance or altitude "scale-height" of bead spacing was found to be 20 and in another case 25 km. In order to make systematic observations of the large-scale sprite morphology a satellite based instrument the Imager for Sprites and Upper Atmospheric Lightning (ISUAL) instrument is planned to fly on the Taiwanese satellite, ROCSAT 2. The instrument will consist of an imager and two bore-sighted photometers. The imager will locate the sprites near the earth limb and make global synoptic measurements while the photometers will measure the spectral and temporal properties of sprites and other upper atmospheric luminous phenomena in a number of different wavelength regions uninhibited by atmospheric absorption.