Triangulation of sprites, associated halos and their possible relation to causative lightning and micrometeors

Sprite halos were recently identified as an impulsive but spatially diffuse phenomenon that sometimes occurs just prior to, but distinct from, sprites . The lack of discernible spatial structure and the temporal development se quence in halos differs markedly from the highly structured bodies and tend rils and the complex development sequences of sprites. However, both phenom ena are thought to result from an electric field due to charge moment chang es usually associated with large positive cloud-to-ground (CG) lightning bu t also following negative CG flashes. Three-dimensional triangulations of s prites and sprite halos were made between stations in South Dakota and Wyom ing in August 1999 during the NASA Sprites99 balloon campaign. Halos were f ound to have a Gaussian Ile diameter of similar to 66 km and lie thickness of similar to4 km. Comparison with the location of the underlying lightning strokes, as recorded by the National Lightning Detection Network (NLDN), c onfirms that the horizontal position of sprites may be laterally offset by as much as 50 km from the underlying parent lightning discharge, as has bee n previously reported. The point of maximum apparent brightness for sprite halos occurs at an altitude of similar to 78 km, similar to that of sprites . However, unlike sprites, this point tends to be centered directly above t he underlying parent lightning discharge, 4.6 +/- 2.7 km mean distance from the center of the halo to the NLDN location. This difference in spatial lo cation relative to the underlying lightning suggests that the electrical br eakdown associated with discrete sprites may require a random ionizing even t such as a micrometeor. In contrast, sprite halos do not appear to require such a random component.