Optical and VLF radio observations of sprites over a frontal storm viewed from O’Brien Observatory of the University of Minnesota

Abstract Video images, and photometric and VLF data, were obtained by the University of Minnesota SKYFLASH system of 38 ‘sprite’ events associated with a strong frontal system located in the upper midwest, U.S.A., on 20–21 June 1996. Besides two image-intensified TV cameras, the SKYFLASH system included telescopic photometers sensitive to Rayleigh scattered lightning flashes by viewing the zenith over the station (O’Brien Observatory, University of Minnesota, about 40 km NE of Minneapolis-St Paul) and also several VLF channels with 300 Hz to 10 kHz bandwidth for recording the electromagnetic ‘sferics’. The sprites covered a wide range of sizes, from small kilometer-size filaments to huge luminous objects 50–60 km in lateral dimension. All the sprites appeared to consist of bundles of filaments, and always followed—within several ms—a ‘trigger’ could-ground discharge which, in 35 of the 38 events, was positive. It is difficult to find physical mechanisms that explain this positive stroke preference. The larger events reached from 80 km almost to cloud tops, but the small events were localized near 60 km altitude, which is the ‘bright’ region of sprite luminosity, a fact also not well explained theoretically. The filamentary structure of sprites also presents challenges to explain. Of about a dozen lightning storms observed with SKYFLASH in the period from 1993 to 1996 in the upper midwest, only two had an appreciable number of sprites.