On the nature of transient re-illuminations of decayed branches of negative stepped leaders
Abstract
Using visible-range (50-µs interframe interval) and infrared (1-ms interframe interval) high-speed video cameras at LOG (Lightning Observatory in Gainesville, Florida), we observed luminosity transients that re-illuminated the decayed branches previously created by negative stepped leaders. Stolzenburg et al. (2015) reported similar transients and named them 'sparks'. We will present a detailed analysis of transient re-illuminations of decayed branches of the first two negative stepped leaders in a three-stroke flash. The transients tended to be more frequent as the leader was approaching the ground and becoming more branched.
A total of 26 transients were observed in the visible-range camera records, 22 of which occurred in a single frame and 4 in multiple frames. Three out of four multiple-frame transients (one was recorded in 2 frames, one in 3 frames, and two in 4 frames) clearly showed bidirectional extension with one end connecting to the main channel, and one appeared to extend unidirectionally away from the main channel. Lower-bound average 2D extension speeds were of the order of 106 m/s. For 17 out of the 26 events, we observed luminosity enhancement in the main channel below the branching point. Six transients occurred in the same frame as the main-channel enhancement and eleven in the immediately following frame. This suggests that re-illuminations of decayed branches were caused by the process occurring below the branching point. Many transients were preceded by or occurred in the same frame with other (nearby) transients. One possible triggering mechanism of the observed transients is the step-formation process at the downward-moving leader tip. This process generates an upward propagating wave which carries positive charge/potential upward and can reach the branching point. Interaction between the upward traveling positive charge and the residual negative charge on the decayed branch may serve to initiate a small-scale discharge guided (constrained) by the remnants of the decayed branch. The tendency for transients to become more frequent at the end of the leader stage may be associated with intensification of stepping as the leader approaches the ground. It is also possible that a transient in one decayed branch can trigger (or assist with triggering of) a transient in another decayed branch.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2019
- Bibcode:
- 2019AGUFMAE12A..03D
- Keywords:
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- 3304 Atmospheric electricity;
- ATMOSPHERIC PROCESSES;
- 3324 Lightning;
- ATMOSPHERIC PROCESSES;
- 1704 Atmospheric sciences;
- HISTORY OF GEOPHYSICS