Spontaneous Emergence of Space Stems Ahead of Negative Leaders in Lightning and Long Sparks

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.

On the Emergence Mechanism of Carrot Sprites

“…To validate our results against observations, we computed the electron impact excitation of nitrogen molecules to the and electronic states which radiate in the first and second positive systems of (for details, see the supporting information of Malagón‐Romero & Luque, ).…”

“…This glowing structure in the upper part of the channel agrees with observations, as the first frame of Figure shows. As Luque et al () and Malagón‐Romero and Luque () explain, the appearance of this glowing structure is driven by the attachment instability (Douglas‐Hamilton & Mani, , ), which is triggered by relatively high internal electric fields. This is the reason why sprite glows appear in the upper region of the sprite channel (Luque et al, ).…”

The Discharge Preceding the Intense Reillumination in Positive Leader Steps Under the Slow Varying Ambient Electric Field

“…Long spark discharges are driven by the propagation of positive or negative leaders. Negative leaders propagate in discrete steps, with waiting time of tens of microseconds punctuated by sudden jumps (Berger, 1966;C J Biagi et al, 2014;Guimaraes et al, 2017;Hill et al, 2011;Reess et al, 1995;Malagón-Romero & Luque, 2019). The underlying mechanisms leading to the formation of negative leader steps were first discovered in laboratory discharges and only recently confirmed in natural lightning (Bazelyan & Raizer, 2000;Kostinskiy et al, 2016).…”

Radio Emission Reveals Inner Meter-Scale Structure of Negative Lightning Leader Steps

“…Each step appears to be due to luminous structures, generally assumed to be conducting (see Ref. [6] for an alternative interpretation) that form in front of the main conducting channel, called space stems in this work. After their formation, these structures grow backward to connect with the main leader body, resulting in a large current pulse to equalize the electric potential.…”

“…However, the majority of the previous work has been done in the optical regime, which does not directly relate to electrical current (see, e.g., Ref. [6]), or using radio emission below 10 MHz that is only sensitive larger scale electrical currents (see, e.g., Refs. [11,12]).…”