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Abstract
Complete evolution of a lightning discharge, from its initiation at an altitude of about 4 km to its ground attachment, was optically observed for the first time at the Lightning Observatory in Gainesville, Florida. The discharge developed during the late stage of a cloud flash and was initiated in a decayed branch of the latter. The initial channel section was intermittently illuminated for over 100 ms, until a bidirectionally extending channel (leader) was formed. During the bidirectional leader extension, the negative end exhibited optical and radio-frequency electromagnetic features expected for negative cloud-to-ground strokes developing in virgin air, while the positive end most of the time appeared to be inactive or showed intermittent channel luminosity enhancements. The development of positive end involved an abrupt creation of a 1-km long, relatively straight branch with a streamer corona burst at its far end. This 1-km jump appeared to occur in virgin air at a remarkably high effective speed of the order of 106 m/s. The positive end of the bidirectional leader connected to another bidirectional leader to form a larger bidirectional leader, whose negative end attached to the ground and produced a 36-kA return stroke.
Highlights
The latter value is close to 36 kA which was estimated by the National Lightning Detection Network (NLDN) from returnstroke magnetic field peaks measured at multiple stations, separated on average by 300–350 km
The flash studied here was captured by two high-speed cameras (Phantom V310 and Megaspeed HHC-X2; only the Phantom record is shown in this paper)
The field measuring systems were synchronized with Phantom V310 and HHC-X2 cameras with accuracy better than 1.3 μsand 1 ms, respectively[33]
Summary
We presented the first optical observation of a bidirectional leader giving rise to a negative stepped leader/ return-stroke sequence. The transient event could have injected a significant amount of positive charge near the newly-formed positive end (the fanning-out branches at the extremity of the transient-event channel (see inset in Fig. 2h) are probably the most intense streamers of the corona streamer burst) This space charge could have inhibited further extension of the positive end, as was observed in long positive sparks, with the likelihood of overcoming the space charge effect being higher for higher rate-of-rise of voltage across the gap. If the slope of negative charge density is assumed to remain the same until the leader attachment to ground and the increase of positive end length due to connection to another floating channel is neglected, the total charge transfer Q will be 5.6 C This is the charge deposited on the negative part of the bidirectional leader. The latter value is close to 36 kA estimated from radiation magnetic field peaks measured at multiple NLDN stations for the return stroke initiated by the modeled bidirectional leader
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