Submicrosecond intercomparison of radiation fields and currents in triggered lightning return strokes based on the transmission‐line model

TLDR

The transmission‑line model of return‑stroke radiation, proposed by Uman and McLain (1970), has been tested on rocket‑triggered lightning. Simultaneous measurements of electric radiation field, current at the channel base, and stroke‑propagation velocity were taken, and the best formula for estimating peak currents from remotely measured, truncated peak electric fields at range D is i_p = −3.9×10^−2 D E'_p − 2.7×10^3 (MKS units). Agreement between the model and measurements is generally good during the first few microseconds, but several anomalies were noted, and truncating the sharp initial peak in field waveforms yields an average apparent velocity of (1.51±0.17)×10^8 m/s for 28 strokes, which is in excellent agreement with the average photographically measured two‑dimensional velocity of (1.52±0.17)×10^8 m/s for 18 strokes, although the correlation between apparent and photographic velocities is poor, likely due to uncertainty in the photographic determinations.

Abstract

The “transmission‐line model” of return‐stroke radiation, proposed by Uman and McLain (1970), has been tested on rocket‐triggered lightning. Simultaneous measurements are reported of electric radiation field, current at the channel base, and stroke‐propagation velocity. Agreement between model and measurements is generally good during the first few microseconds, although several anomalies are pointed out. Truncation of the sharp initial peak in many of the field waveforms, motivated by the hypothesis that the return stroke begins a short distance above the ground, leads to an average apparent velocity (derived from the ratio of peak field to peak current) of (1.51±0.17) × 10 8 m/s for 28 strokes, in excellent agreement with the average, photographically measured two‐dimensional velocity of (1.52±0.17) × 10 8 m/s for 18 of these same strokes. Although the correlation between apparent and photographic velocities for these 18 strokes is poor, the uncertainty in the photographic velocity determinations could explain most of the scatter. The best formula for estimating peak currents ( i p ) in subsequent return strokes from remotely measured, truncated peak electric fields ( E ′ p ) at range D appears to be i p = −3.9 × 10 −2 DE ′ p − 2.7 × 10 3 , where all variables are in MKS units and the electric field polarity of return strokes lowering negative charge is considered negative.