Interferometric imaging of Intensely Radiating Negative Leaders

TL;DR

This study uses advanced interferometric imaging with LOFAR to reveal that intensely radiating negative lightning leaders involve a large ionization front and corona bursts, indicating a dense charge pocket and a new propagation mode.

Contribution

Refined TRI-D imaging technique incorporating antenna function to visualize the large ionization front in IRNLs using LOFAR data.

Findings

IRNLs have a >500m ionization front with corona bursts

Normal negative leaders show corona at the tip (~10m)

IRNL mode suggests presence of dense charge pockets

Abstract

The common phenomenon of lightning still harbors many secrets and only recently a new propagation mode was observed for negative leaders. While propagating in this `Intensely Radiating Negative Leader' (IRNL) mode a negative leader emits 100 times more very-high frequency (VHF) and broadband radiation than a more normal negative leader. We have reported that this mode occurs soon after initiation of all lightning flashes we have mapped as well as sometimes long thereafter. Because of the profuse emission of VHF the leader structure is very difficult to image. In this work we report on measurements made with the LOFAR radio telescope, an instrument primarily built for radio-astronomy observations. For this reason, as part of the present work, we have refined our time resolved interferometric 3-Dimensional (TRI-D) imaging to take into account the antenna function. The images from the TRI-D imager show that during an IRNL there is an ionization front with a diameter in excess of 500~m where strong corona bursts occur. This is very different from what is seen for a normal negative leader where the corona bursts happen at the tip, an area of typically 10~m in diameter. The observed massive ionization wave supports the idea that this mode is indicative of a dense charge pocket.