An experimental study of positive leaders initiating rocket-triggered lightning

Abstract

Simultaneous, co-located measurements of ambient, electrostatic-field profiles and rocket-triggered lightning phenomenology beneath Florida thunderstorms are reported. Ambient-field conditions that are sufficient to initiate and sustain the propagation of positive lightning leaders are identified. It is found that lightning can be triggered with grounded triggering wires approximately 400 m long when the ambient fields aloft are as small as 13 kV/m (foul-weather polarity). Ambient potential differences between the height of the triggering wires and ground were as small as −3.6 MV (negative wrt. earth) when lightning occurred. `Precursors,' the first measurable current pulses from the triggering wires, were initiated at similar fields aloft but at wire heights only about half as large, where ambient potentials were as small as −1.3 MV. The mean speed of one `failed leader' is estimated at 1.9×10⁴ m/s over 35 m of propagation. The lengths of `leader' extension and positive-streamer `fan' during individual impulses of multiple-pulse precursors are estimated to be 0.7 and 2.0 m, respectively.

Introduction

`Classical' rocket-triggered lightning (St. Privat D'Allier Group, 1985) is initiated by a small rocket towing a grounded wire aloft under a thunderstorm. The rocket-and-wire technique of triggering lightning was pioneered by Newman (1958)and Newman et al. (1967). The key to its success is likely an observation by Brook et al. (1961)that the sufficiently rapid introduction of a grounded conductor into a high-field region might actually initiate the discharge. It is now well-established that this type of lightning normally begins with a positively charged `leader' propagating upward from the tip of the triggering wire toward the electrified cloud. In this paper, the term, `leader,' denotes a highly ionized, conducting, filamentary channel extending into virgin air. The term, `positive streamer,' in contrast, will always refer to the poorly conducting `corona' space-charge waves that have been studied by Dawson and Winn (1965), Phelps and Griffiths (1976), Allen and Ghaffar (1995)and others.

Apparently identical positive leaders have been shown to initiate `altitude'-triggered lightning (Laroche et al., 1989b), which is produced by a similar rocket towing an ungrounded wire aloft. The same phenomenon has also been inferred to begin most strikes to instrumented aircraft (Boulay et al., 1988; Mazur, 1989b) and most `upward-initiated' discharges to towers (Uman, 1987, Chap. 12), and it is surely important in natural lightning as well (Mazur, 1989a). Thus, the classical rocket-and-wire technique offers a unique opportunity to study the physics of a key process in both the artificial initiation of, and the continued development of most forms of, lightning.

Considerable information is available on the phenomenology of rocket-triggered positive leaders, including currents measured at the base of the triggering wires and electric-field changes at the ground produced by these currents (e.g., Laroche et al., 1988; Lalande et al., 1998), and propagation velocities and other interesting optical characteristics (Idone and Orville, 1988; Idone, 1992). From first principles (e.g., Smythe, 1968, Sections 2.19 and 3.11), it is evident that the energy that drives all lightning discharges is extracted from the ambient electrostatic field. Therefore, one would like to know the ambient-field intensity, and its spatial distribution, associated with both unsuccessful and successful triggering attempts. Unfortunately, it is well-known that surface-based measurements can be screened from more intense fields aloft by a layer of corona-produced space charge (e.g., Standler and Winn, 1979). There are few in situ measurements aloft from which to determine the necessary or sufficient conditions for propagation of positive leaders or with which to explain variations in their behavior. These previous measurements are discussed in Section 2.

This paper describes in detail a major field experiment conducted in Florida during the summer of 1996. The objective was, in effect, to extend experimental work on long laboratory sparks to kilometer length scales. We present an overview of the results, including sample measurements from the various instruments, and offer preliminary conclusions about the triggering conditions. The long-term goal of this work is to gain insight into the conditions for triggering lightning and to determine the response of lightning-scale positive leaders to their ambient energy source.