Spider lightning in intracloud and positive cloud-to-ground flashes

The nature of visible, horizontally stratified lightning channels propagating over large distances near the cloud base during the decaying stage of a storm (also called "spider" lightning) was investigated. The study was effectuated through the use of the coordinated observations of a VHF interferometer, a high-speed image-intensified video system, measurements of electric and magnetic fields, and optical transients. Spider-lightning events were found to be negative leaders similar to stepped leaders in negative cloud-to-ground flashes, with a similar average speed of propagation horizontally of 2-4 x 10(5) m s(-1) Being slow negative leaders, spider-lightning events are part of intracloud flashes and positive cloud-to-ground flashes occurring prior to and during the inverted (fair weather polarity) phase of the End of the Storm Oscillation in the ground electric field. Spider lightning is characterized by both the pulsing luminosity at the tips of its branched channels and the continuous luminosity (for tens to hundreds of milliseconds) which is maintained by the continuing current flow. The interferometer produced mapping of radiation sources closely resembling the spider-lightning channels (negative leaders) but only;a weak trace of radiation sources associated with positive leaders to ground. Both the video images and a few radiation sources of positive leaders were obtained within 1 ms of the leader's ground attachment. The interferometer, however, failed to map fast negative leaders that occurred intermittently during the spider-lightning events.