Unipolar and bipolar pulses emitted during the development of lightning flashes

Abstract

Both unipolar and bipolar magnetic or electric field pulses have been observed during preparatory stages of a lightning flash. We introduce a new simple analytical model to describe both kinds of pulses. We show how the polarity overshoot depends on the parameters of the model, including the propagation velocity of the current pulse, the step length, and the injected current waveshape. We observe that the expression for the radiation part of the magnetic field can be decomposed into two time‐shifted terms with opposite polarities. The time shift of the two terms is determined by the overall propagation time of the current pulse. The model well corresponds not only to observations of the bipolar preliminary breakdown pulses at time scales of tens of microseconds but also to both unipolar and bipolar dart‐stepped leader pulses at submicrosecond time scales.

Subionospheric propagation and peak currents of preliminary breakdown pulses before negative cloud‐to‐ground lightning discharges

“…da Silva and Pasko [] used a generalization of electrostatic and transmission line approximations and showed that bipolar pulses can be a manifestation of a stepwise elongation of the in‐cloud negative leader in the thunderstorm electric field. A new simple analytical model describing both bipolar and unipolar pulses was introduced by Kašpar et al ., []. Radio emissions associated with NBEs and other IC discharges have been detected with radio receiver on board the FORTE satellite [ Light and Jacobson , ; Jacobson and Shao , ; Jacobson and Heavner , ; Jacobson and Light , ].…”

Mathematical constraints on the use of transmission line models to investigate the preliminary breakdown stage of lightning flashes

“…In a recent publication, Kaspar et al [] suggested that an analytical approximate solution for equation can be used to understand the origin of unipolar and bipolar pulses during the preliminary breakdown stage of lightning flashes. More specifically, these authors note that for the TL model and ℓ ≪ D the integrand in the radiation term of equation becomes , and, hence the electric field change at a remote sensor becomes where , h 0 =( h 1 + h 2 )/2, , and τ w = ℓ / v .…”

“…This pulse shape has been used in a number of recent publications [e.g., Watson and Marshall , ; Nag and Rakov , ; Karunarathne et al , ] and has been demonstrated to reproduce well observed morphology of IBPs. Other alternative pulse shapes can also be used, as, for example, a double‐exponential function [see, e.g., Kaspar et al , ]. It should be noted that the general conclusions of our study do not depend on the choice of I 1 ( t ) as long as time scales for current rise and fall can be clearly defined.…”

“…In reality the conductivity discontinuity at h 2 would lead to, at least, a partial reflection of the current pulse downwards. Despite this physical inconsistency, we have decided to keep the TL model in our analysis to compare to conclusions obtained by previous researchers that attempted to use this model to simulate IBPs [e.g., Kaspar et al , ]. Modified transmission line linear (MTLL) [ Rakov and Dulzon , ]. The MTLL model was introduced to qualitatively capture the fact that a current pulse traveling in the return stroke channel undergoes attenuation.…”

“…Throughout the last decade a number of researchers have attempted to use transmission line models, widely employed for return stroke simulations [e.g., Rakov and Uman , , chapter 12 and references therein], to simulate IBPs and NBEs [e.g., Shao and Heavner , ; Watson and Marshall , ; Nag and Rakov , ; Zhu et al , ; Karunarathne et al , ; Kaspar et al , ]. The transmission line (TL) model approach is to infer the source current dynamics to match the measured E‐field change waveform with the purpose of retrieving key information about the source, such as its height, peak current, size, and speed of charge motion.…”