Observations of 'columniform' sprites

This paper reports observations of a distinctive form of sprites associated with positive CG flashes carrying currents of 23 or less to about 100 kA in mesoscale thunderstorms. The sprites are characterized by long vertical columns about 10 km long, less than 1 km in diameter, and show virtually no variation in brightness along;heir length. Three dimensional triangulation of what we define as a 'columniform' sprite (c-sprite) event on the evening of 19 June 1995 showed that the individual elements had an average terminal altitude of 86.7 km and an average bottom of 76.2 km. Some show faint diffuse 'hair' or tendrils extending above and below the column. The sprite columns are nearly vertical, in video imagery. On some evenings, c-sprites are the dominant fc,rm of sprite activity above thunderstorms but, on other nights with many sprites, they may not be observed at all. Comparison of c-sprite forms vs :National Lightning Detection Network (NLDN) positive cloud-to-ground current, shows a progression from simple thin vertical forms to brighter and more complicated forms. Theoretical explanations which predict the form and vertical structure of the classical sprites do not at present account for these different forms. (C) 1998 Elsevier Science Ltd. All rights reserved.