High Resolution Mapping of the Final Stepped leader Phase from dE/dt TOA Measurements
Abstract
A time of arrival network composed of eight wideband (DC-20 MHz) electric field derivative (dE/dt) antennas and eight co-located NaI(T1) scintillation detectors is deployed at the UF-FIT ICLRT to study the final part of the leader phase in cloud-to-ground lightning. It can resolve lightning processes within several hundred meters of ground with 2-3 m of horizontal uncertainty and typically less than 10 m of altitude uncertainty for sources 50 m or higher above ground. We discuss here the locations of dE/dt stepped-leader and other pulses from a negative first stoke that occurred on 2 June 2006 . A dominant region of stepped-leader activity (dE/dt pulses) was identified 500 us prior to the return-stroke about 350 m above ground. The leader descended toward ground, exhibiting extensive branching. Ninety dE/dt pulses were located during this period. At 70 us before the return stroke, the downward leader had descended to an altitude of roughly 120 m. After this time, the leader descended as four distinct channels to an altitude between 50 and 75 m. Approximately 40 pulses were located . The final dE/dt pulse of the stepped-leader phase corresponded to the final step in the leader branch nearest a tree line. Several us later, a "burst" of four dE/dt pulses, termed here a "leader burst" to distinguish it from the characteristic leader step dE/dt pulses, was observed at all stations, corresponding to the rapid advancement of the downward leader from near the main leader towards the tree line. During the leader burst, the leader channel descended over 30 m in altitude and propagated a horizontal distance of about 60 m in less than a microsecond. The termination of this burst, at an altitude about 10 m, coincided with the start of the initial rising portion of the dE/dt (and E-field) waveform, the so called "slow front" of the return stroke. Two pulses occurring during the slow front were located very near the termination of the leader burst. The locations determined for the slow-front pulses were within 10 m horizontally of a pine tree, about 7 m tall, which was a casualty of this flash. The leader burst and all eleven major leader steps occurring during the 70 us prior to the return stroke were proficient x ray producers. The slow-front pulses and the fast transition were also coincident with x rays although these were much smaller in amplitude (numbers and/or energy) and observed by only a few of the eight stations.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2008
- Bibcode:
- 2008AGUFMAE21A..07H
- Keywords:
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- 3304 Atmospheric electricity;
- 3324 Lightning;
- 7514 Energetic particles (2114);
- 7554 X-rays;
- gamma rays;
- and neutrinos