On the Transition From Precursors to the Initial Upward Positive Leader in Negative Rocket‐Triggered Lightning

Abstract

Precursory current pulses (precursors) and initial upward leader pulses are examined to characterize the charge transfer of precursors and the sustained upward leader during the early stage of rocket‐triggered lightning. According to the analysis of six negative triggered flashes, it is found that both precursors and the initial upward positive leader (iUPL) pulse are led by a small deflection that appears at about 25 μs prior to the major pulses. The characteristics of precursors and iUPL pulses are very similar except that the precursors generally transfer less electric charge (35.07 μC on average) than iUPL pulses do (64.73 μC on average). Some marginal differences in the parameters (e.g., rise time, duration) may be attributed to the modification of the current pulses caused by the different impedances of the leader channel and the steel wire. The ascending rocket causes the enhancement of electric field (E‐field) in the close vicinity of the wire tip, and therefore the breakdown of the iUPL is initiated by a stronger E‐field than that of the precursors. The step lengths of the precursors (defined as the extended length of the wire tip lifted by the rocket between two adjacent isolated precursors) are similar to that of iUPL pulses. Our analyses indicate that the transition from precursors to the iUPL occurs when the E‐field around the wire tip is strong enough, by the accumulation of more positive charge, to launch a meter‐scale breakdown and the ensuing charge transfer to ground.

Quantitative Relationship Between Current Pulses and Associated Low‐Frequency Magnetic Fields During Initial Stage of Rocket‐Triggered Lightning

“…Li et al. (2021) reported their statistical results on the duration of precursor and iUPL current pulses, and the values are 2.27 and 4.2 μs, respectively. Here, we calculated the duration of the current pulse and associated magnetic pulse during the precursory and iUPL stages.…”

“…The LF magnetic sensor (with a 3‐dB bandwidth of 2–800 kHz) deployed at 78 m range was sampled at 10 MS/s (Lu et al., 2016). In addition, optical observations and lightning‐induced electric field changes are also acquired synchronously to provide the ancillary data for our analysis (e.g., Li et al., 2019, 2021).…”

“…In addition, optical observations and lightning-induced electric field changes are also acquired synchronously to provide the ancillary data for our analysis (e.g., Li et al, 2019Li et al, , 2021.…”

“…Fan et al (2018) computed the initial magnetic field near the rocket-triggered lightning channel with a modified standard current waveform, and they suggested that the LF magnetic field at close range (78 m) is dominated by the induction components radiated by the downward flowing current compared with the synchronous VHF signals (Fan et al, 2020). Li et al (2021) conducted the measurement of magnetic fields radiated by the precursory pulses with sensitive magnetic sensors, making it possible to detect weak ripple deflections that are not readily resolved in the typical measurements of channel-base current. This finding shows the feasibility of using the magnetic sensor to get weak current pulses remotely in rocket-triggered lightning with good accuracy.…”

Close Observation of the Evolution Process during Initial Stage of Triggered Lightning Based on Continuous Interferometer

“…After the IPCP, the UPL begins self-sustaining development, and the current waveform is no longer in the form of pulse, but in the form of continuous current, i.e., initial continuous current (ICC). According to the analysis of six negative triggered flashes, Li et al [5] found that both precursor discharges and the initial upward leader begin with a small pulse that leads the subsequent major pulses by about 25 µs.…”