THE INFLUENCE OF CHANNEL GEOMETRY ON THE FINE-SCALE STRUCTURE OF RADIATION FROM LIGHTNING RETURN STROKES

Evidence is presented indicating that channel geometry is a factor in determining the shape of the electric field waveforms radiated by subs equent return strokes in cloud-to-ground lightning. The data consist o f 61 subsequent return strokes in both triggered flashes and natural l ightning recorded at the NASA Kennedy Space Center during the summer o f 1987. In the case of the triggered flashes, the data include the ele ctric field change, the current at the channel base, and video images of the channel. Video images were obtained for about half of the natur al flashes. The fine scale structure of the electric field changes rad iated by these strokes has been compared during the first 10 mu s. In cases where unambiguous identification of the channel could be made fr om the video recordings, this fine scale structure is similar for retu rn strokes in the same channel and differs. significantly for return s trokes in different channels. This is true for both natural and trigge red flashes. On the other hand, the current waveforms measured at the channel base (available for the triggered flashes) are similar in shap e for all strokes, regardless of the channel. Similar results were als o obtained for the natural flashes without video images, in which case strokes from the same flash exhibit a greater similarity than do stro kes from different flashes (and therefore different channels).