Understanding the Radio Spectrum of Thunderstorm Narrow Bipolar Events

Abstract

This paper reports a study to understand the radio spectrum of thunderstorm narrow bipolar events (NBEs) or compact intracloud discharges, which are powerful sources of high-frequency (HF) and very high frequency (VHF) electromagnetic radiation. The radio spectra from 10 kHz to about 100 MHz are obtained for three NBEs, including one caused by fast positive breakdown and two by fast negative breakdown. The results indicate that the two polarities of fast breakdown have similar spectra, with a relatively flat spectrum in the HF and VHF band. The ratio of energy spectral densities in the very low frequency and HF bands is (0.9-5) × 10 5 . We develop a statistical modeling approach to investigate if a system of streamers can explain the main features of fast breakdown. Assuming that the current moment peak and charge moment change of individual streamers vary in the ranges of 5-10 A-m and 5-20 μC-m, respectively, the modeling results indicate that a system of 10 7 -10 8 streamers can reproduce the current moment, charge transfer, and radio spectrum of fast breakdown. The rapid current variation on a time scale of nanoseconds required for fast breakdown to produce strong HF/VHF emissions is provided by exponentially accelerating and expanding streamers. Our study therefore supports the hypothesis that fast breakdown is a system of streamers. Finally, suggestions are given regarding future streamer simulations and NBE measurements in order to further develop our understanding of NBEs and lightning initiation.

Modeling Compact Intracloud Discharge (CID) as a Streamer Burst

“…The current moment of each streamer was assumed to follow a function whose time derivative matches the shape of the NBP. Using these ideas, the authors of [28] managed to obtain a radio spectrum of NBPs which agrees with experimental observations.…”

“…Recently, several attempts were made to model CID's as streamer bursts [25][26][27][28]. In [25,26], the CID is modeled as an interaction between two (or more) bipolar streamer structures formed in a strong large-scale electric field of a thundercloud and the features of the electromagnetic emission resulting from this interaction between streamer structures were examined.…”

“…In [25,26], the CID is modeled as an interaction between two (or more) bipolar streamer structures formed in a strong large-scale electric field of a thundercloud and the features of the electromagnetic emission resulting from this interaction between streamer structures were examined. In the two publications [27,28], the idea of CIDs as a streamer burst was explored to study their physical parameters. In [27], the concept of propagating streamer systems inside the cloud environment as proposed by Griffith and Phelps [29] was utilized.…”

“…Assuming that the streamer channel is of zero conductivity, the authors of [27] showed how the streamer system exhibits an initial exponential growth followed by a quadratic steady state. In [28], the radio spectrum of NBPs was investigated by modelling the CID as a burst of streamers. All the streamers in the burst were assumed to be initiated at the source location.…”

Indirectly Measured Ambient Electric Fields for Lightning Initiation in Fast Breakdown Regions

“…Unfortunately not much is known about the width of fast breakdown regions at the time of this writing, which leads to uncertainty in estimates of the lower bound of the background electric field. Liu et al () suggested that fast breakdown regions could be tens of meters wide, consistent with a small ratio of 0.1. Downward positive streamers in sprites, on the other hand, can fill a region with a ratio of approximately 1.0 although 0.5 is perhaps more typical (Stenbaek‐Nielsen et al, ).…”