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A space stem is a luminous plasma segment that has been observed to form ahead of the leader tip during the negative leader stepping process (Biagi et al., 2009(Biagi et al., , 2010Gorin et al., 1976). A space stem may eventually thermalize, undergo a sharp increase in conductivity, and develop into a space leader. Due to the intensification of electric field at its extremities, a space leader can start extending toward the pre-existing leader channel (PELC), eventually connecting with the PELC, leading to its extension and the completion of a new leader step. Characterizing the details of the formation and progression of space leaders is, therefore, essential for understanding the leader stepping process as well as the extent to which electrical characteristics of the PELC influence the formation and extension of space leaders. Gorin et al. (1976) first observed in laboratory spark experiments, space stems which were incepted in the vicinity of the negative leader tip and later evolved into space leaders that merged with the negative primary leader channel, leading to its extension. Biagi et al. (2009Biagi et al. ( , 2010 were the first to report separated luminous segments that they believed were space stems and space leaders in dart-stepped leaders of triggered lightning flashes using high-speed video cameras; they observed 1-4 m long luminous segments 1-10 m ahead of the leaders. Biagi et al. (2014) observed eight luminous segments 1-6 m in length and 3-8 m ahead of the downward stepped leader preceding the first stroke of an altitude-triggered flash. Hill et al. (2011) and

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