During the night of 9–10 August 1993 more than 150 luminous cloud-ionosphere discharges (CIs) were observed above a thunderstorm complex moving SE across the state of Iowa. Images of the CIs were obtained through clear air by intensified CCD TV cameras at the O'Brien Observatory of the University of Minnesota located about 60 km NE of Minneapolis and 250–500 km from the storm center. The discharges consisted of bright vertical striations extending from 50–80 km altitude, often covering tens of kilometers laterally, with tendrils of decreasing intensity visible for the brighter events down to cloud tops below 20 km altitude. All the more intense CIs were coincident with a VLF sferic in the 300Hz–12kHz range, but small events often did not yield a detectable sferic. There is no unambiguous evidence that CIs were sources of sferics. Some of the CIs were observed to be coincident with a cloud brightening and with a cloud-ground stroke recorded by the National Lightning Detection Network. The duration of the images was generally less than one TV field (< 16.7 ms). Many of these discharges have now been observed by the space shuttle, by aircraft-borne TV cameras and a large number by a ground-based camera observations in Colorado. The present results are compared with these observations and recent theoretical ideas related to the CI events are discussed. It is proposed that CIs arise from intense bursts of cloud electrification and may follow the preexisting paths of cloud-to-ionosphere thunderstorm currents.
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