Characteristics of Electromagnetic Signals During the Initial Stage of Negative Rocket‐Triggered Lightning

Abstract

With the measurements in SHandong Triggering Lightning Experiment and GuangdongComprehensive Observation Experiment on Lightning Discharge in China, we examine the electromagnetic signals associated with the upward positive leaders during the initial stage of negative triggered lightning. The magnetic field (B field) signals measured at close range (<100 m) for both sites can be divided into two categories (i.e., impulsive and ripple pulses) according to the discernibility of separation between individual pulses. The impulsive pulses are well simulated by using the transmission line model, which suggests that these pulses are generated by leader current pulses propagating downward along the steel wire. Because the length of extended leader channel ahead of the wire is not negligible during the stage of ripple pulses, the waveform of impulsive current pulses is changed after traveling through the high impedance leader channel. Taking the filtered current pulse as the input variable, the waveform of ripple pulse can be simulated properly, which indicates that ripple pulses are caused by the attenuation of impulsive current along prolonging leader channel. In addition, the paper analyzes the fast electric field (E field) changes measured at 60-m range from the launching site during the initial stage by using the transmission line model and shows that the polarity of E field change at a given range is determined by the inception height of upward leader, namely the surface E field change caused by the individual charge transfer of initial upward leader also involves the problem of reverse distance as present for a vertical dipole. Plain Language SummaryLightning can occur at a predictable time and place by using the technique of rocket-triggered lightning, which launches a small rocket trailing a grounded steel wire toward the charged thundercloud overhead. Therefore, the technique facilitates the research of lightning discharges and their electromagnetic effects. At the beginning of triggered lightning, a sustained train of current pulses can be measured at the base of discharge path, and the synchronous electromagnetic field signals can also be recorded by the electric and magnetic sensors deployed around the rocket launching site. The paper focuses on the characteristics of electromagnetic signals measured at close range (<100 m) during the SHandong Triggering Lightning Experiment and Guangdong Comprehensive Observation Experiment on Lightning Discharge (two experimental sites of triggered lightning in China) campaign. The main contributions of this article include two aspects: first, we clarify the relationship between the electromagnetic signals and current pulses during the initial stage of triggered lightning; second, we demonstrate the influences of the triggering height on the polarity of electric field waveform.

Characterizing Radio Frequency Magnetic Radiation of Initial Upward Leader Stepping in Triggered Lightning With Interferometric Lightning Mapping

“…became wider due to the impact of high impedance of leader channel (Fan et al, 2018;Li et al, 2019). Figure 4a shows that the elevation angles of VHF radiation sources rise first and then fall during the initial stage.…”

“…The broadband interferometer consisted of three broadband VHF discone antennae (25-90 MHz), which were placed at the vertices of an equilateral triangle with side length of 20 m (Zhang et al, 2017). There was another magnetic sensor with 3-dB bandwidth of 6-340 kHz as used by Lu et al (2014) and Fan et al (2018Fan et al ( , 2019 deployed at about 1.9-km range. Figure 1 gives the layout for the triggered lightning experiment in summer of 2019.…”

Measurements of Magnetic Pulse Bursts During Initial Continuous Current of Negative Rocket‐Triggered Lightning

“…Fan et al () have analyzed the interpulse intervals of the IMPs, which are obtained from the five cases recorded at the close site in 2014. Figure a gives the statistical histogram of the interpulse intervals of MPBs, and Figure b shows the corresponding statistical result of IMPs reported by Fan et al (). Note that only the data measured in 2014 are analyzed in Figure a in order to keep the consistency of data as Figure b.…”

“…The IMPs are associated with the sustained upward leaders, and the height of leader tip upon the inception of IMPs is usually below 400 m (Lu et al, ), as these pulses appear at the very initial stage of triggered lightning. Fan et al () have analyzed the interpulse intervals of the IMPs, which are obtained from the five cases recorded at the close site in 2014. Figure a gives the statistical histogram of the interpulse intervals of MPBs, and Figure b shows the corresponding statistical result of IMPs reported by Fan et al ().…”

“…More recently, Biagi et al () and Jiang et al () suggested that the propagating manner of the UPL was stepped during the early stage of triggered lightning from the aspects of high‐speed video images and fast electric field changes, respectively. In addition, Lu et al () and Fan et al () measured the initial magnetic pulses (IMPs) that are related to the sustained UPLs by using the low‐frequency magnetic sensor and also found the stepwise characteristics of these pulses. However, the propagating details of UPLs during the ICC stage of triggered lightning are still not well characterized.…”