Because there seems to be a lot going on with sprites (and associated phenomena) about 100 km up, I think there's a very good chance of a close tie with sporadic E (Es) propagation. This notion seems supported by oservations connecting ionized Es "cloud" formation to clusters and lines of strong thunderstorms. I don't have citations at hand, but I think there is some published research on the subject.
Mark -- WA9ETW _________________ Topband Reflector Archives - http://www.contesting.com/_topband
