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We investigate the emergence of space stems ahead of negative leaders. These are luminous spots that appear ahead of an advancing leader mediating the leader's stepped propagation. We show that space stems start as regions of locally depleted conductivity that form in the streamers of the corona around the leader. An attachment instability enhances the electric field leading to strongly inhomogeneous, bright, and locally warmer regions ahead of the leader that explain the existing observations. Since the attachment instability is only triggered by fields above 10 kV/cm and internal electric fields are lower in positive than in negative streamers, our results explain why, although common in negative leaders, space stems, and stepping are hardly observed if not absent in positive leaders. Further work is required to fully explain the streamer to leader transition, which requires an electric current persisting for timescales longer than the typical attachment time of electrons, around 100 ns.

Plain Language Summary Long electrical discharges of negative polarity, such as most cloud-to-ground lightning flashes, propagate in a stepped manner, that is, alternating between standing and jumping suddenly. The underlying mechanism explaining this behavior is not well understood, although we know that space stems are a key element. These are bright and locally warmer segments that appear ahead of a discharge channel and apparently isolated from it. For the first time, we show how these space stems emerge spontaneously in our simulations from regions of locally lower conductivity that latter become bright and warm. Then on one hand, we propose a possible origin of the space stems and, on the other hand, we shed some light on possible mechanisms that grow these stems to longer times, beyond 100 ns.