Spatial and temporal properties of optical radiation produced by stepped leaders

Abstract

Abstract. The relative light intensities as a function of height and time for two negative downward stepped leaders, A and B, recorded by a high-speed digital 16 x 16 photodiode array photographic system, are studied. For leader A it is found that the light waveform for each segment of the leader channel starts with a series of sharp light pulses followed by several slow-rising and longer-lasting light surges, with both the light pulses and surges superimposed on a continuous luminosity slope that has a long rising front followed by an almost constant light level. Analysis indicates that each light pulse involves a step process; it originates at the leader tip and appears to propagate upward, with the pulse amplitude suffering little degradation within the first several tens of meters to 200 rn from the leader tip up (bright tip length) but with a severe attenuation above. The light surges are observed to be almost constant in amplitude above the bright tip, and for one of them an upward propagation speed of the order of 108 m/s is inferred. From appearances of the light pulses it is determined that the leader A has an overall velocity of 4.5-11.2 x l0 s m/s, a step interval of 5-50/•s, and a step length of 7.9-19.8 m. For leader B the step light pulses are found to propagate from the leader tip back up at a speed of 0.14-1.7 x 108 m/s, and the overall leader velocity, the step interval, and the step length are determined to be about 4.9-5.8 x 10 ø m/s, 18-21 /•s, and 8.5 m, respectively. In addition, on the basis of the light waveforms of the leader A it is inferred that the current of a stepped leader may consist of two parts: an impulsive current within the bright tip and a continuing current above it. After propagating along the bright tip up, because of increasing resistance and capacitance of the leader channel the impulsive current rapidly transforms into part of the continuing current.

Characteristics of the optical pulses associated with a downward branched stepped leader

“…This could be the case because there is no “main channel” until the return stroke occurs. For all optical pulses observed in this event, the GM value of 10–90% risetime is 0.4 μ s, and that of the half‐peak width is 1.1 μ s. The values are greater than those for electric field pulses observed by Krider et al [1977], and less than those observed by Chen et al [1999]. In the bottom 400 m of a rocket‐triggered lightning channel, Wang et al [1999b] observed that the optical pulses associated with dart‐stepped leader steps have a 10–90% risetime ranging from 0.3 to 0.8 μ s with a mean value of 0.5 μ s and a half‐peak width ranging from 0.9 to 1.9 μ s with a mean value of 1.3 μ s, which are quite similar to the present results.…”

“…A high‐speed digital imaging system, like ALPS (Automatic Lightning Progressing Feature Observation System, [ Yokoyama et al , 1990; Wang et al , 1999a]), is very useful for studying stepped leader pulses. To date, several stepped leaders have been documented using ALPS [ Wang et al , 1999b; Chen et al , 1999]. Here we report on a branched stepped leader which was recorded by ALPS with better time and spatial resolution than in previous studies.…”

“…From electromagnetic measurements, it has been reported that the electric field pulses associated with leader steps are characterized by interpulse intervals of 5 to 20 μ s, 10–90% risetimes of 0.2–0.3 μ s, and half‐peak widths of 0.4–0.5 μ s [e.g., Krider and Radda , 1975; Krider et al , 1977; Beasley et al , 1982]. As is known, not only does a stepped leader typically exhibit many branches, but the leader step pulse current also propagates vertically over some distances [e.g., Wang et al , 1999b; Chen et al , 1999]. In order to study the detailed characteristics of leader step pulses, some measurement with fine spatial resolution are also required.…”

Laser Beam Analysis Using Image Processing

“…The TEM mode produces the smallest-diameter and lowestdivergence laser beam. Multimode operation increases the output power but decreases the coherence and increases divergence [5].…”

Experimental Study of Lightning-Induced Currents in a Buried Loop Conductor and a Grounded Vertical Conductor

“…Natural negative cloud-to-ground lightning (e.g., [8]) is the most common lightning discharge between cloud and ground and is initiated by a stepped leader that moves negative charge downward with an average speed of the order of 10 5 m/s [9]- [11]. The interstep time interval for leader steps ranges from 5-100 µs [9], [10], [12]. Krider et al [13] measured the interstep time intervals of 130 leader steps occurring within 200 µs of return stroke initiation and found an average interstep interval of 25 µs.…”