A distinct negative leader propagation mode

Olaf Scholten ,B M Hare,T Winchen,S Buitink,P Mitra,N Liu,J P Rachen,K Mulrey,J R Hörandel,H Falcke,T N G Trinh,A Corstanje,A Nelles,Ivana Kolmašová ,C Sterpka,Luděk Uhlíř ,G K Krampah,T Huege,J R Dwyer ,S Ter Veen,Satyendra Thoudam ,O Santolı́k ,Radek Lán ,Hershal Pandya

doi:10.1038/s41598-021-95433-5

Abstract

The common phenomenon of lightning still harbors many secrets such as what are the conditions for lightning initiation and what is driving the discharge to propagate over several tens of kilometers through the atmosphere forming conducting ionized channels called leaders. Since lightning is an electric discharge phenomenon, there are positively and negatively charged leaders. In this work we report on measurements made with the LOFAR radio telescope, an instrument primarily build for radio-astronomy observations. It is observed that a negative leader rather suddenly changes, for a few milliseconds, into a mode where it radiates 100 times more VHF power than typical negative leaders after which it spawns a large number of more typical negative leaders. This mode occurs during the initial stage, soon after initiation, of all lightning flashes we have mapped (about 25). For some flashes this mode occurs also well after initiation and we show one case where it is triggered twice, some 100 ms apart. We postulate that this is indicative of a small (order of 5 km^2) high charge pocket. Lightning thus appears to be initiated exclusively in the vicinity of such a small but dense charge pocket.

Highlights

The common phenomenon of lightning still harbors many secrets such as what are the conditions for lightning initiation and what is driving the discharge to propagate over several tens of kilometers through the atmosphere forming conducting ionized channels called leaders
In this work we show that an Intensely Radiating Negative Leader (IRNL) mode is not confined to the initial stage of a lightning and rather is evidence of a small but dense charge pocket
In this work we have used the infrastructure of the LOFAR radio telescope to provide high-resolution 3D images of two flashes, supplemented with time traces of a broadband antenna

Summary

Introduction

The common phenomenon of lightning still harbors many secrets such as what are the conditions for lightning initiation and what is driving the discharge to propagate over several tens of kilometers through the atmosphere forming conducting ionized channels called leaders. It is observed that a negative leader rather suddenly changes, for a few milliseconds, into a mode where it radiates 100 times more VHF power than typical negative leaders after which it spawns a large number of more typical negative leaders This mode occurs during the initial stage, soon after initiation, of all lightning flashes we have mapped (about 25). The fact that in9 (see[10,11] for an overview of the extensive literature on initial leaders) the transition from the initial leader to a negative stepped leader has been observed with high-speed video and in electric field change data, supports the finding that initial leaders and IRNL’s are the same This is corroborated further by the observation of a correlation between a certain type of broadband radio pulses (called Initial Breakdown, IB) and VHF pulse amplitudes in12,13. From the flashes we have imaged we infer that a large majority (more than 90%) of flashes we have imaged were initiated just below the negative charge layer with a downward negative leader that propagates towards the lower-lying positive charge layers

Methods

Findings

Conclusion