Spontaneous emergence of space stems ahead of negative leaders in lightning and long sparks

TL;DR

This study explores how space stems form ahead of negative lightning leaders, revealing that they originate from local conductivity depletion and attachment instabilities, which influence leader propagation and explain observed differences between positive and negative leaders.

Contribution

It demonstrates that space stems arise from local conductivity depletion and attachment instabilities, clarifying their role in negative leader stepping and the absence in positive leaders.

Findings

Space stems originate from conductivity depletion in streamer corona.

Attachment instability enhances electric fields, leading to bright regions.

Space stems are prevalent in negative leaders but rare in positive ones.

Abstract

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.