Initiation processes of return strokes in rocket-triggered lightning

Using a high speed optical imaging system operated at a time resolution of either 10ns or 100ns, we have documented the initiation process of 14 return strokes in four rocket-triggered lightning flashes. Of the 14 strokes, nine occurred following dart leaders and five following dart-stepped leaders. The return strokes are found to initiate at heights ranging from 7.21.4 to 21.04.6m above the lightning termination point. Return strokes with larger peak current tend 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 below 60m for all of the return strokes. The resultant speeds range from 0.4x10(8) to 2.5x10(8)m/s. For the downward propagation speeds, only six strokes among the 14 strokes allow us to perform a reasonable estimation. Those downward speeds range from 0.6x10(8) to 1.9x10(8)m/s.