Interferometric imaging of intensely radiating negative leaders

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.

LOFAR Observations of Lightning Initial Breakdown Pulses

“…This is typical for LOFAR observed flashes, which suggests that thunderstorms in this region typically have an expansive lower positive charge region. The same lightning flash was also analyzed in previous studies (e.g., Scholten et al., 2021a, 2021b; Scholten, Hare, Dwyer, Liu, et al., 2021; Scholten, Hare, Dwyer, Sterpka, et al., 2021), which focused on, for example, flash development and VHF intense stages termed intensely radiating negative leaders.…”

“…Scholten, Hare, Dwyer, Liu, et al. (2021b) found that the structure of the discharge starting at 1 ms or at 3 ms is rather complex. Nearly simultaneous (within 100 μs) VHF sources are distributed in a volume of 1 km 3 , with no easily discernible discharge channels.…”

“…The antenna is sensitive to a frequency range from 5 kHz to 90 MHz, and the sampling frequency is 200 MHz. Details of the system can be found in (Scholten et al., 2021a, 2021b; Scholten, Hare, Dwyer, Liu, et al., 2021; Scholten, Hare, Dwyer, Sterpka, et al., 2021).…”

Implications of Multiple Corona Bursts in Lightning Processes for Radio Frequency Interferometer Observations

“…As a large time integration window is used for this case, it is unnecessary to apply the beamforming technique. Except for the baseline and the number of antennas, Cases B and C represent the configuration of the latest LOFAR studies (Scholten et al., 2021b, 2022). The last case represents a moderate increase in the baseline of the interferometer used by Pu and Cummer (2019).…”

“…Simulations can provide the constraints on the source parameters in order for the sources to be resolved by a particular instrument. In fact, work is currently underway to simulate LOFAR observations to investigate accuracy and imaging artifacts of the LOFAR interferometry analysis (Scholten et al., 2022).…”