Return stroke current reflections in rocket-triggered lightning

Abstract

Ten upward propagating return stroke currents in eight triggered lightning flashes have been observed to reflect downward from 140 to 300 m altitude by way of measurements of the channel base current, close electric field and electric field derivative, and high-speed video. The current reflections appear as dips in the measured channel base current and in the electric fields and as bipolar pulses in the electric field derivative waveforms. The current dips occur 2.7 to 13.9 μs after the initiation of the return stroke current at ground. The observed phenomenon results from a portion of the upward propagating return stroke current wave being reflected from a channel impedance discontinuity apparently associated with an isolated section of unexploded triggering wire, as inferred from high-speed video records. A transmission line model is used to model the close electric field and electric field derivative of the postulated initial and reflected current waves, starting with the measured channel base current, and the results are compared favorably with measurements made at 92 to 326 m. From the measured time between the return stroke current initiation at the ground and the time the current reflection reaches the channel base and produces the current dip, along with the reflection height inferred from the video records, we find the average of the upward and downward reflected return stroke current speed for each of the 10 strokes to be between 0.81 and 2.06 × 108 ms−1.