[1] We examine the characteristics of the initial stage (IS) in object-initiated lightning derived from current measurements on the Gaisberg tower (100 m, Austria), the Peissenberg tower (160 m, Germany), and the Fukui chimney (200 m, Japan) and their counterparts in rocket-triggered lightning in Florida. All lightning events analyzed here effectively transported negative charge to ground. For rocket-triggered lightning the geometric mean (GM) values of the three overall characteristics of the initial stage, duration, charge transfer, and average current, are similar to their counterparts for the Gaisberg tower flashes and the Peissenberg tower flashes, while the Fukui chimney flashes are characterized by a shorter GM IS duration and a larger average current. The GM IS charge transfer for the Fukui chimney flashes is similar to that in the other three data sets.The GM values of the action integral differ considerably among the four data sets, with the Fukui action integral being the largest. The observed differences in the IS duration between the Fukui data set and all other data considered here are probably related to the differences in the lower current limits, while the differences in the action integral cannot be explained by the instrumental effects only. There appear to be two types of initial stage in upward lightning. The first type exhibits pulsations (ringing) during the initial portion of the IS, and the second type does not. The occurrence of these types of IS appears to depend on geographical location. The characteristics of pulses superimposed on the initial continuous current (ICC pulses) in object-initiated (Gaisberg, Peissenberg, and Fukui) lightning are similar within a factor of 2 but differ more significantly from their counterparts in rocket-triggered lightning. Specifically, the ICC pulses in object-initiated lightning exhibit larger peaks, shorter risetimes, and shorter half-peak widths than do the ICC pulses in rocket-triggered lightning.

[1] Electric field waveforms at horizontal distances from the triggered lightning channel attachment point ranging from 0.1 to 1.6 m have been measured with Pockels sensors at the International Center for Lightning Research and Testing at Camp Blanding, Florida. The measuring system had a dynamic range from 20 kV/m to 5 MV/m and a bandwidth from 50 Hz to 1 MHz. The corresponding currents at the channel base and electric fields at 5, 15, and 30 m from the lightning channel were also measured using a current viewing resistor and flat-plate antennas, respectively. Very close vertical electric fields for 36 strokes in nine triggered lightning flashes were obtained using Pockels sensors. For 8 out of the 36 strokes, horizontal electric fields were also measured using Pockels sensors. Electric field waveforms appear as pulses, with the leading edge of the pulse being due to the leader and the trailing edge due to the return stroke. Of the 36 vertical electric field waveforms, six were more or less V-shaped, while 30 exhibited a considerably slower variation during the return-stroke stage than during the leader stage. Vertical electric field pulse peaks are in the range from 176 kV/m to 1.5 MV/m (the median is 577 kV/m), and horizontal electric field pulse peaks are in the range from 495 kV/m to 1.2 MV/m (the median is 821 kV/m). On-site calibration results show that these electric fields measured using Pockels sensors may be underestimated by 40% or so due to the insufficient upper frequency response of 1 MHz of the measuring system. Additionally, vertical electric fields due to M components were measured and compared to electric fields produced by leader/return stroke sequences. For 8 out of 10 M components having channel-base peak currents greater than 500 A, vertical electric fields at 0.1 to 1.6 m were below 20 kV/m, the lower measurement limit. For the remaining 2 of 10 M components, whose current peaks were between 2.3 and 3.2 kA, vertical electric field peaks were about 100 and 48 kV/m at a distance of 0.1 m from the attachment point, apparently unaffected by the upper frequency response of the measuring system. The vertical electric field measured very close to the lightning channel tends to increase with an increase in the previous no-current interval, that is, in the time elapsed from the cessation of current of the preceding stroke (or of the initial-stage current).

[1] A stepped leader that preceded a natural first return stroke was recorded using a high-speed optical imaging system with a time resolution of 0.1 ms and a spatial resolution of about 40 m. Within the view of the imaging system, 440 m  480 m just above ground level, the leader produced one ground termination (main channel for the return stroke) and six ungrounded branches. A total of 153 optical pulses associated with the leader were identified for analysis, with 61 originating from the main channel and 92 from the six branches. The pulses originating from the main channel and from the branches are almost identical in terms of geometric mean (GM) values of 10-90% risetime and half-peak width, near 0.4 ms and 1.1 ms, respectively. The pulses from the main channel show smaller interpulse intervals than those from the branches. When the interpulse intervals for pulses from both the main channel and all branches are combined, of a total of 152 intervals, five are essentially 0 and 58 are between 0 and 1 ms. Thus a significant percentage of optical pulses occurred almost simultaneously in different channel sections separated by distances up to several hundred meters. The light intensity just prior to each pulse exhibited a tendency to increase as the leader approached ground, while the other parameters, such as pulse peak, risetime, half-peak width, and interpulse intervals, showed no systematic changes with time.

[1] Electric fields in the immediate vicinity (within 0.1 to 1.6 m) of the triggered-lightning channel were measured with Pockels sensors at the International Center for Lightning Research and Testing at Camp Blanding, Florida. These fields and the associated currents measured at the base of a 2-m strike object were used to compute the input power and energy, each per unit channel length and as a function of time, associated with return strokes in rockettriggered lightning. In doing so, we assumed that the vertical component of the electric field at horizontal distances of 0.1 to 1.6 m from the lightning attachment point is not much different from the longitudinal electric field inside the channel (Borovsky, 1995). The estimated mean input energy over the first 50 ms or so is between 10 3 and 10 4 J/m, consistent with predictions of gas dynamic models, but one to two orders of magnitude smaller than Krider et al.'s (1968) estimate for a naturallightning first stroke, based on the conversion of measured optical energy to total energy using energy ratios observed in laboratory long-spark experiments. The mean channel radius and resistance per unit channel length at the instance of peak power are estimated to be 0.32 cm and 7.5 W/m, respectively. Citation: Jayakumar, V