The formation of space leaders in streamer coronae of negative leaders

Negative lightning leaders, which are associated with the production of terrestrial gamma-ray flashes, prolonged X- and gamma-ray glows and neutron beams, move in a step-wise manner when the original leader channel connects to the hot, highly conductive space leader forming ahead of the leader tip. However, details about the formation and heating of the space leader, and thus of the leader stepping process, are still unknown. Here, we present a novel mechanism on the origin of space leaders: After streamer coronae have formed ahead of the leader tip, plasma chemistry and heating turn a selection of the corona streamers into a highly-conductive region. Further heating subsequently allows for the inception of secondary streamer coronae at the vertices of the conductive region, which continue to heat the already heated plasma filament, finally translating into the hot and conductive space leader. We simulate the evolution of the electric field and the associated plasma chemistry in single streamer channels and present the temporal evolution of the electron density and the electric field as well as of the temperature increase and the conductivity. We find that one streamer alone cannot be heated sufficiently towards a hot space leader, but that the inception and evolution of branching streamer coronae from this initial streamer are necessary for further heating.