SPIDER LIGHTNING IN INTRACLOUD AND POSITIVE CLOUD-TO-GROUND FLASHES

The nature of visible, horizontally stratified lightning channels prop agating over large distances near the cloud base during the decaying s tage of a storm (also called ''spider'' lightning) was investigated. T he study was effectuated through the use of the coordinated observatio ns of a VHF interferometer, a high-speed image-intensified video syste m, measurements of electric and magnetic fields, and optical transient s. Spider-lightning events were found to be negative leaders similar t o 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 d uring the inverted (fair weather polarity) phase of the End of the Sto rm Oscillation in the ground electric field. Spider lightning is chara cterized by both the pulsing luminosity at the tips of its branched ch annels and the continuous luminosity (for tens to hundreds of millisec onds) which is maintained by the continuing current flow. The interfer ometer produced mapping of radiation sources closely resembling the sp ider-lightning channels (negative leaders) but only;a weak trace of ra diation sources associated with positive leaders to ground. Both the v ideo images and a few radiation sources of positive leaders were obtai ned within 1 ms of the leader's ground attachment. The interferometer, however, failed to map fast negative leaders that occurred intermitte ntly during the spider-lightning events.