High speed optical observation on initiation process of lightning return strokes

Using a high speed optical imaging system operated at a time resolution better than 100 ns, we have documented the initiation process of 14 subsequent return strokes contained in 4 rocket-triggered lightning flashes. Of the 14 strokes, 9 occurred following dart leaders and 5 following dart-stepped leaders. The return strokes are found being initiated at a height ranging from 2.3 m to 26.0 m above the lightning termination point. A return stroke with a larger peak electric current tends to initiate higher. All the return strokes show initial bidirectional (upward and downward from their initiation height) propagation. We have been able to estimate the initial upward propagation speeds for all the return strokes. The resultant speeds range from 0.4×10⁸ m/s to 2.5×10⁸ m/s. For the downward propagation speeds, only two strokes among the 14 strokes allow us to perform a reasonable estimation. The resultant speeds are 0.6×10⁸ m/s, 0.5×10⁸ m/s, respectively. A negative correlation has been noted between their initial upward propagation speeds and their downward leaders´ speeds.