Characteristics of lightning, sprites, and human-induced emissions observed by nadir-viewing cameras on board the International Space Station

The Lightning and Sprites Observation (LSO) experiment was designed to test a new concept of nadir-viewing sprite measurement on board the International Space Station using spectral differentiation methods for lightning and sprite identification. It was composed of two calibrated cameras: one equipped with a narrowband filter at 763nm to maximize the contrast between sprites and lightning, and the other to monitor lightning. The LSO was operated at night during 15days from 2001 to 2004 during which 197 lightning flashes, several sprites, hundreds of gas flares, and tens of cities were analyzed. The main strength of this experiment was its high spatial resolution of about 400m. The structural details of some lightning are thus observed highlighting complex systems. Some features such as the nonlinear increase of the lightning-illuminated cloud top area with the peak radiance and the radial decrease of the lightning flash radiance were quantified. The median area is 129km(2) with median minor and major axes of 12 and 16km. Two methods of sprite identification are presented and applied to the most intense sprite events observed by LSO. The sprite diameter is 5km and it is shifted of about 22km from the center of the parent lightning. A ratio of 1.7% is deduced for lightning flashes between the radiances measured by both cameras. These observations should be useful for the preparation or the analysis of future space missions dedicated to nadir-viewing observations of sprites.