The space stem/leader described above seems not the luminous crown residual25, but there may be some relations with the space-leader-like segment reported by previous literature26. The results reported here powerfully proves the existence of space stem/leader in positive leader steps, ending a long-standing argument27. The intense leader corona streamers burst occurring after the space leader merges with the primary positive leader injects a strong current pulse into the leader channel. Current and luminosity waves then travel backward the leader channel, plausible with the lightning observations that the luminosity surge propagates back through the leader channel after a step10,28, which is difficult to understand in the past. The current pulse of positive leader steps led by the space stems/leaders exhibits a sharp front. Current pulses of upward positive leader steps with the same feature were also registered in triggered-lightning observations23. These facts indicate that the space stem/leader reported here may also exist in positive lightning leader steps. Due to the short lifetime of space stems/leaders in positive lightning leader steps, it was ignored by the previous photography systems.
VHF radiation from the positive lightning leaders is still not well understood. The needle-like discharge that may occur after positive leader development pausing is identified as a critical source. It is rational to infer that the collision of streamers with opposite polarity in the positive lighting leader steps led by space stems/leaders may be another important source.
The ambient absolute humidity during our experiments varied from 18 g/m3 to 22 g/m3, closing to or less than the absolute humidity during the summer thunderstorms. In the hundreds of positive lightning-like discharges produced in a dry environment of which the absolute humidity is less than 7 g/m3, the step current pulse with a sharp front was not registered. That indicates the absolute humidity may play an important role in the emergence of the space stem/leader in positive leader steps. The emergence of space stems is favored in the streamers launched from the primary leader tip with the strong internal electric field29. The internal electric field in negative streamers is more powerful than that in positive streamers under dry conditions30. Therefore, it is not easy for space stems to emerge ahead of the propagating positive leader tips. However, water vapor molecules in the air have strong electronegativity, which can accelerate the electron attachment process in positive streamer channels. As the humidity increases, the internal electric field in a positive streamer channel is enhanced. That may create a condition necessary for the emergence of space stems. On the other hand, the temperature is critical to the transition from the stem to the leader31–33. When electrons flow along the stem, only a fraction of collisional energy loss is immediately transferred to the thermal energy of the gas molecules and contributes to the temperature increase. A larger fraction goes into the vibrational excitation of molecules. The vibrational energy relaxes into the thermal energy with a relaxation time constant, causing a delayed increase in temperature. The relaxation time is depended on the air humidity. As the humidity increases, the time constant decreases, and the space stem develops more easily into the space leader8. These two facts may explain why space leaders could emerge ahead of propagating positive leaders at a certain humidity.