CURRENT AND ELECTRIC-FIELD DERIVATIVES IN TRIGGERED LIGHTNING RETURN STROKES

Forty simultaneous, submicrosecond time-resolved measurements of triggered lightning return-stroke current (I), current derivative (dI/dt), and electric field derivative (dE/dt) were made. Peak currents ranged from about 5 to 50 kA, peak dI/dt amplitudes from 60 to 260 kA/μs in 1985 and from 20 to 140 kA/μs in 1986. The mean peak dI/dt values, 111 kA/μs (1985) and 68 kA/μs (1986), are 2-3 times higher than data from instrumented towers, and peak I and dI/dt appear to be positively correlated. The dE/dt and dI/dt waveform pairs have similar shapes, and the peak amplitudes are linearly proportional. Return-stroke velocity, computed from the ratio of peak dE/dt and dI/dt signal amplitudes using an expression derived from the radiation field term of the transmission line model (TLM) averaged 2.9 × 108 m/s and 3.0 × 10 8 m/s in 1985 and 1986, respectively, which is about 2 times higher than most optical measurements. -from Authors