Analysis of the Video Recordings of an Aborted Upward Leader at Daytime from a “Bolt-from-the-Blue” Discharge

In this paper we present the joint analysis of an unconnected upward leader serendipitously recorded by two digital cameras at daytime at a close distance of 7-9 m. Time resolved lightning emissions in the very high frequency (VHF) band located by the New Mexico Tech Lightning Mapping Array (LMA) indicate that this leader was triggered by the negative stepped leader of a “bolt-from-the-blue” lightning flash with one single stroke, which was detected by two independent lightning detection networks with a peak current of 43±6 kA. The recorded upward leader, however, was aborted at a distance of ~140 m from the lightning stroke. The net charge carried by the recorded upward leader, and thus by definition its polarity, is positive. By analyzing two video frames that documented this upward leader at different times and its tortuous structure, we estimate its luminous width to be 2-6 cm, roughly 10 times of the actual width of the longitudinal current region found in the literature. Electric field measurements on the ground and on a balloon-borne instrument indicate that in addition to transferring 6 to 8 C negative charge to ground, this flash also deposited net negative charge over the stroke location, partly through a subsequent negative leader that approached while didn’t cause a