Underground Measurement of Magnetic Field Pulses During the Early Stage of Rocket‐Triggered Lightning

Abstract

We examined the underground magnetic field near the lightning channel with low‐frequency magnetic sensor based on SHAndong Triggering Lightning Experiment (SHATLE). Two sensors were deployed, one at 1‐m height above ground and another one at 2‐m depth underground at a distance of 78 m from the lightning channel, and the magnetic pulses during the initial stage of triggered lightning were recorded. The experimental results show that the microsecond‐scale magnetic pulses radiated by the upward lightning leader can be detected in the subsurface space and the magnetic signal is modified by the soil medium. Specifically, the amplitude at the depth of 2 m is attenuated typically more than 55%, and the attenuation decreases as the timescale of the magnetic pulse increases; meanwhile, the peak time of the underground magnetic pulse is delayed by about 0.6 μs, and the half‐peak width of the magnetic pulse is increased by 0.2–0.8 μs (namely, by 20% to 32%). The results of Fourier analysis indicate that the component with relatively high frequency is subject to more attenuation than is the component with relatively low frequency. In addition, the simulation of magnetic field with the channel‐base current by using the transmission line model is consistent with the measurement, indicating that the modification on the waveform characteristics of the lightning pulse measured underground could provide valuable information for retrieving the electromagnetic parameters of soil.

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

“…Similar to the magnetic pulses of the quiet stage, the enlarged view shows that, for both the IMPs (Figures 4b and 4d) and MPBs (Figures 4c and 4e), the VHF signals correspond to the onset of individual magnetic pulses. The inception of a sustained UPL was indicated by a very small magnetic field deflection with relatively weak VHF radiation; after the inception, the current pulse measured at the channel base 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.…”

“…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.…”

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

“…For instance, the rise times and durations of iUPL pulses appear to be longer than those for precursors, which merits further study. As shown in Figure 7, the rise times and durations of iUPL pulses exhibit the tendency to increase as the leader channel extends upward (e.g., Li et al., 2019; Lu et al., 2016). The rise times and widths of the first several pulses of the iUPL are the same as for the precursors.…”

“…extension of channel (Li et al, 2019). Another apparent difference is that the charge transferred by the iUPL pulses (mean: 64.73 μC) is significantly greater than that by the precursors (mean: 35.07 μC).…”

“…In the experiments at the two aforementioned sites, coordinated observations were conducted to examine the leader development in triggered lightning (Figure 1). The lightning current was measured at the channel base with a 5 mΩ shunt (with bandwidth of 0–3.2 MHz) by sampling at 20 MS/s over 2 s with a Yokogawa DL750 oscilloscope (e.g., Li et al., 2019). The pre‐triggering part occupies 20% of the sampling duration.…”