Observed one-dimensional return stroke propagation speeds in the bottom 170 m of a rocket-triggered lightning channel

The return-stroke propagation speeds of five strokes from a seven-stroke triggered lightning flash are measured, with a 2 ns sampling interval, using a vertical array of photodiodes. Various methods for determining the reference point to be tracked are explored, and the speed is seen to vary over nearly an order of magnitude depending upon which method is chosen. The speeds are generally in agreement with the values found in the literature. The return-stroke speed appears to increase with height and then decrease with height in four of the five strokes examined.