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Photoelectric return‐stroke velocity and peak current estimates in natural and triggered lightning

1989; American Geophysical Union; Volume: 94; Issue: D11 Linguagem: Inglês

10.1029/jd094id11p13237

2156-2202

Douglas M. Mach, W. David Rust,

Ionosphere and magnetosphere dynamics

Using data from a mobile photoelectric device, we present two‐dimensional return‐stroke velocities from 130 strokes that consist of 86 negative natural, 41 negative triggered, one positive triggered, and two positive natural return‐strokes. The velocity measurements are divided into two groups: “short‐channel” values with channel segments starting near the ground and less than 500 m in length and “long‐channel” values that start near the ground and exceed 500 m in length. The average long‐channel velocity is 1.3 ± 0.3 × 10 8 m s −1 for natural return‐strokes and 1.2 ± 0.3 × 10 8 m s −1 for triggered return‐strokes. For the short channels, natural return‐strokes have a statistically higher (at the 98% confidence level) average velocity of 1.9 ± 0.7 × 10 8 m s −1 than triggered strokes with an average velocity of 1.4 ± 0.4 × 10 8 m s −1 . We compare our results with previous measurements of the return‐stroke velocity for comparable channel segment lengths and find that there is no statistical difference between our results and the previous ones. A qualitative analysis indicates that the optical waveform from a return‐stroke generally increases in risetime and decreases in amplitude at greater altitudes. Using the transmission line model (TLM), the shortest segment one‐dimensional return‐stroke velocity, and either the maximum or plateau electric field, we find natural strokes have a peak current distribution that is lognormal with a median value of 16 kA (maximum E ) or 12 kA (plateau E ) while triggered lightning has a median peak current value of 21 kA (maximum E ) or 15 kA (plateau E ). We find substantial differences between TLM and the shunt‐measured peak currents and find no significant correlation between shunt peak currents and triggered return‐stroke velocities. We do find correlations between TLM peak currents and velocities for triggered and natural subsequent return‐strokes but not between TLM peak currents and natural first return‐stroke velocities. In both cases where a correlation is found, the trend is for greater peak currents to be associated with lower return‐stroke velocities. Our peak current calculations are approximately half as large as previously reported. This difference is attributed to the greater return‐stroke velocity measured by our photoelectric device and used in the transmission line model. Hence previous estimates of the return‐stroke peak current seem too high. Two natural single‐stroke positive cloud‐to‐ground flashes had velocities of 1.0 and 1.7 × 10 8 m s −1 and peak currents of 120–150 kA.