Attachment process in rocket‐triggered lightning strokes

Abstract

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.

Observed one‐dimensional return stroke propagation speeds in the bottom 170 m of a rocket‐triggered lightning channel

“…The use of film as a recording medium has some drawbacks, including limited resolution (both temporal and spatial) and non-linear response to incident light intensity. The recent advent of digital sampling and recording methods, along with the development of solid-state photodetectors, has allowed researchers to achieve finer resolution and increased linearity of data collection, typified by the work of Mach and Rust [1997, 1989a, 1989b and by the ALPS system employed by, among others, Wang et al [1999aWang et al [ , 1999bWang et al [ , 2004. The use of rocket-triggered lightning allows researchers to predict and determine more accurately the time and location of the termination of the lightning channel, and to prepare data collection instruments prior to the event.…”

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

“…The stepped-leader/first-return-stroke sequence may be followed by one or more dart-leader/subsequent-return-stroke sequences that also contain an attachment process involving an upward-moving leader that has a propagation speed of the order of 10 7 m/s [17]. However, the length of the upward-connecting leader during the stepped-leader/first-return-stroke sequence is significantly longer (tens of meters to flat ground or small objects [10], [11], [14]) than the length of the upward-connecting leader during the dart-leader/subsequent-return-stroke sequence (a few meters to 20 m [14], [17], [18]). The larger electric potential of the downward-moving stepped leader that travels through virgin air causes a longer upward leader than the downward-moving dart leader that has a lower electric potential (the dart leader travels through a channel conditioned by the first return stroke and possibly by preceding subsequent return strokes and continuing currents).…”

On the simulation of the interception of lightning dart leaders

“…Even though has been evaluated numerically for stepped leaders in [4], an analytical expression for is derived here. The result of simulation is compared with two dart leader interception events in lightning triggered experiment reported by Wang [3].…”

“…In order to validate the estimations of SLIM in response to dart leaders, two events of lightning attachment in rockettriggered lightning strokes reported by Wang [3] are considered. The attachment processes are recorded with optical images via the ALPS (with 100 ns time and 3.6ns spatial resolution) as well as the electric field measurements and channel-base current recording.…”

“…The variation of these properties leads to the different lightning attachment process under dart leaders compared with the generally evaluated case under downward stepped leaders. According to the records of natural lightning and rocket triggered lightning experiments [1][2] [3], upward connecting leader initiation and propagation is observed in the interception of stepped leaders as well as dart leaders. However the upward leader under the descending of dart leader is (several orders of magnitude) faster than its counterpart in stepped leader interception [2] [3].…”