Lightning Properties Inferred from Measurements of Very Close Electric Fields

Lightning properties including dart -leader charge density, return-stroke propagation speed, dart -leader electric potential, and dart-leader propagation speed, inferred from current and close electric field measurements are presented. Although all estimates presented here are based on rather crude models, they are generally in remarkably good agreement with independent measurements and/or theoretical considerations found in the literature. The results are applicable to rocket -triggered lightning strokes and probably to subsequent strokes in natural lightning. Additionally, we present evidence that larger return strokes can leave appreciable unneutralized charge near ground. INTRODUCTION We used close electric field and associated channel-base current measurements for rocket -triggered lightning (i.e., Rakov et al. 1998) to infer various properties of lightning discharges, including (1) dart -leader charge density, (2) return-stroke propagation speed, (3) dart -leader electric potential, and (4) dart-leader propagation speed. Although the analysis is based on simple models, the results are generally similar to those obtained from independent measurements and/or expected from theoretical considerations. Additionally, electric field waveforms due to dart leader/return stroke sequences at 15 and 30 m are examined for the so-called residual electric field which is presumably associated with leader charge left unneutralized near ground by the return-stroke process. The measurements were made at the International Center for Lightning Research and Testing (ICLRT) at Camp Blanding, Florida, in 1999, 2000, and 2001. The instrumentation and experimental set-up used at the ICLRT in these years are described by Crawford et al. (2001), Rakov et al. (2001), Uman et al. (2002), Schoene et al. (2003), and Kodali (2003). Triggered-lightning strokes are similar to subsequent strokes in natural lightning. Therefore, the results presented in this paper are likely to apply to subsequent strokes in natural lightning. LEADER CHARGE DENSITY As shown by Rubinstein et al. (1995), the vertical electric field change ∆ EL due to a uniformly charged leader at a very close distance r can be expressed as ( ) r o L ε π ρ 2 EL = ∆ (1) where ρ L is the line charge density on the leader channel and ε o = 8.85 x 1012 F/m. Further, Crawford et al. (2001) inferred a more or lesss uniform distribution of charge along the bottom kilometer or so of the dart-leader channel in rocket-triggered lightning. We used values of ∆ EL measured at r = 15 m and r = 30 m at the ICLRT in 1999-2001 and Eq. 1 to estimate the corresponding values of ρ L. The results are presented in Figs. 1a and 2a. For all data combined, the mean values of ρ L from the electric field measurements at 15 and 30 m are 98 and 101 μ C/m, respectively. The overall range of variation is from 26 to 210 μ C/m. The values of ρ L are used below for estimating electric potential of the dart-leader channel. RETURN-STROKE PROPAGATION SPEED We obtained estimates of the return-stroke speed as vRS = I/ ρ RS where I is the return-stroke peak current and ρ RS is the line charge density along the return stroke channel. The latter can be found from measured close electric field change ∆ ERS due to the return stroke as RS o RS E r∆ = πε ρ 2 (Thottappillil et al. 1997). Since this equation for speed is valid only when the return-stroke current is a step-function wave propagating along the channel at a constant speed, our speed values are very rough estimates. The results are shown in Figs. 1b and 2b. For all data combined, the mean values estimated using field measurements at 15 and 30 m are 1.9 x 10 and 1.6 x 10 m/s, respectively. The overall range of variation is from about 3 x 10 to 2.7 x 10 m/s. The estimated speed values are consistent with optical measurements (see Rakov et al. 1992 for a review). DART-LEADER ELECTRIC POTENTIAL Electric potential of the lightning leader channel can be estimated as V = ρ L/C where ρ L is the line charge density on the leader channel and C is the capacitance per unit length of the channel. Values of ρ L for the bottom kilometer or so of the lightning channel were computed from measured close leader electric field changes, as discussed above, and C was found as