Abstract. Using a high-speed digital optical system, we determined the propagation characteristics of two leader/return-stroke sequences in the bottom 400 rn of the channel of two lightning flashes triggered at Camp Blanding, Florida. One sequence involved a dart leader and the other a dart-stepped leader. The time resolution of the measuring system was 100 ns, and the spatial resolution was about 30 m. The leaders exhibit an increasing speed in propagating downward over the bottom some hundreds of meters, while the return strokes show a decreasing speed when propagating upward over the same distance. Twelve dartstepped leader luminosity pulses observed in the bottom 200 m of the channel have been analyzed in detail. The luminosity pulses associated with steps have a 10-90% risetime ranging from 0.3 to 0.8 •s with a mean value of 0.5 •ts and a half-peak width ranging from 0.9 to 1.9 •ts with a mean of 1.3 •ts. The interpulse interval ranges from 1.7 to 7.2 •ts with a mean value of 4.6 •ts. The step luminosity pulses apparently originate in the process of step formation, which is unresolved with our limited spatial resolution of 30 m, and propagate upward over distances from several tens of meters to more than 200 m, beyond which they are undetectable. This finding represents the first experimental evidence that the luminosity pulses associated with the steps of a downward moving leader propagate upward. The upward propagation speeds of the step luminosity pulses range from 1.9x 107 to 1.0x 10 • m/s with a mean value of 6.7x 107 m/s. In particular, the last seven pronounced light pulses immediately prior to the return stroke pulse exhibit more or less similar upward speeds, near 8x 107 m/s, very close to the return-stroke speed over the same portion of the channel. On the basis of this result, we infer that the propagation speed of a pulse traveling along the leaderconditioned channel is primarily determined by the channel characteristics rather than the pulse magnitude. An inspection of four selected step luminosity pulses shows that the pulse peak decreases significantly as the pulse propagates in the upward direction, to about 10% of the original value within the first 50 m. The return-stroke speeds within the bottom 60 rn or so of the channel are 1.3x 108 and 1.5x 108 m/s for the two events analyzed, with a potential error of less than 20%.

[1] Using a high speed optical imaging system operated at a time resolution of either 10 ns or 100 ns, we have documented the initiation process of 14 return strokes in four rockettriggered lightning flashes. Of the 14 strokes, nine occurred following dart leaders and five following dart-stepped leaders. The return strokes are found to initiate at heights ranging from 7.2 ± 1.4 to 21.0 ± 4.6 m above the lightning termination point. Return strokes with larger peak current tend to initiate higher. All the return strokes show initial bidirectional (upward and downward from their initiation height) propagation. We have been able to estimate the initial upward propagation speeds below 60 m for all of the return strokes. The resultant speeds range from 0.4 × 10 8 to 2.5 × 10 8 m/s. For the downward propagation speeds, only six strokes among the 14 strokes allow us to perform a reasonable estimation. Those downward speeds range from 0.6 × 10 8 to 1.9 × 10 8 m/s.

We have analyzed the electric field changes of 14 upward leaders that were initiated from a windmill and its lightning protection tower. We found that these upward leaders can be sub‐classified into two types according to whether they are triggered by nearby lightning discharge activity or they are initiated without any nearby preceding discharge activities. We have also obtained evidence of upward aborted leaders that are initiated from high‐grounded objects in response to nearby lightning. All these results suggest that once the electric field which surrounds the high‐grounded object is large enough, an upward leader can be initiated no matter whether the electric field is built up slowly or created rapidly by a nearby lightning discharge, although the later appears to be more efficient in triggering the leader. In addition, we found that without the assistance of a rising electric field produced by a nearby lightning discharge, compared to a stationary windmill and a tower with a similar height, a rotating windmill tends to have a bigger chance of initiating an upward leader.

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Abstract. In order to study the lighming attachment process, we have obtained highly resolved (about 100 ns time resolution and about 3.6 m spatial resolution) optical images, electric field measurements, and channel-base current recordings for two dart leader/rerumstroke sequences in two lighming flashes triggered using the rocket-and-wire technique at Camp Blanding, Florida. One of these two sequences exhibited an optically discernible upward-propagating discharge that occurred in response to the approaching downwardmoving dart leader and connected to this descending leader. This observation provides the first direct evidence of the occurrence of upward connecting discharges in triggered lighming strokes, these strokes being similar to subsequent strokes in natural lightning. The observed upward connecting discharge had a light intensity one order of magnitude lower than its associated downward dart leader, a length of 7-11 m, and a duration of several hundred nanoseconds. The speed of the upward connecting discharge was estimated to be about 2 x 107 m/s, which is comparable to that of the downward dart leader. In both dart leader/rerum-stroke sequences studied, the rerum stroke was inferred to start at the point of junction between the downward dart leader and the upward connecting discharge and to propagate in both upward and downward directions. This latter inference provides indirect evidence of the occurrence of upward connecting discharges in both dart leader/rerum-stroke sequences even though one of these sequences did not have a discernible optical image of such a discharge. The length of the upward connecting discharges (observed in one case and inferred from the height of the rerum-stroke starting poim in the other case) is greater for the event that is characterized by the larger leader electric field change and the higher rerumstroke peak current. For the two dart leader/rerum-stroke sequences studied, the upward connecting discharge lengths are estimated to be 7-11 m and 4-7 m, with the corresponding rerum-stroke peak currents being 21 kA and 12 kA, and the corresponding leader electric field changes 30 m from the rocket launcher being 56 kV/m and 43 kV/m. Additionally, we note that the downward dart leader light pulse generally exhibits little variation in its 10-90% risetime and peak value over some tens of meters above the rerum-stroke starting point, while the following retum-stroke light pulse shows an appreciable increase in risetime and a decrease in peak value while traversing the same section of the lighming channel. Our findings regarding (1) the initially bidirectional developmere of rerum-stroke process and (2) the relatively strong attenuation of the upward moving return-stroke light (and by inference current) pulse over the first some tens of meters of the channel may have important implications for rerum-stroke modeling.