Characteristics of Medium-low Frequency Magnetic Fields of Initial Continuous Current in Rocket-triggered Lightning

Abstract

Rocket-triggered lightning experiment conducted in Field Experiment Base on Lightning Sciences, China Meteorological Administration (CMA_FEBLS) provides a good opportunity to study the discharge process and its related electromagnetic effects. During the experiment, two medium-low frequency magnetic antennas are deployed at different distances from the rocket launch site, named close antenna (about 80 m) and far antenna (about 1.9 km), respectively, and magnetic fields are observed with high sensitivity by two antennas. Combined with the synchronous channel-base current and fast electric fields, electromagnetic characteristics of initial continuous current are analyzed. Benefitting from the expansion of the bandwidth of the antenna, magnetic pulse signals can be observed throughout the triggered lightning, including the initial magnetic pulse (IMP), magnetic pulse of the signal quiet period, the magnetic pulse burst (MPB) and the regular magnetic pulse (RMP). IMPs can be divided into two categories (i.e., impulsive and ripple pulses) according to the discernibility of separation between individual pulses. Impulsive pulses are well simulated by the transmission-line model, which suggests that these pulses are generated by leader current pulses propagating downward along the steel wire. Magnetic pulses of the signal quiet period are observed for the first time. The mean pulse width and inter-pulse interval of these pulses are about 1 μs and 14 μs, respectively, which indicates that the propagation of upward leaders during the stage is in the form of small-scale breakdown. The MPB can be observed by both close and far antennas, and the mean inter-pulse interval of the MPB(24.5 μs) is larger than that of the signal quiet period pulse. Furthermore, the channel-base current during the stage of MPB increases to dozens of hundreds of amperes, so it can be concluded that the electric field condition is conductive to the development of the upward leaders. In addition, the magnetic signal recorded at close distance indicates the physical process leading to initial continuous current pulse (ICCP), M-component as well as the direct measurement of current enhancement at the channel base, due to the charge transfer in the ICCP or M-component. Magnetic antennas can also record the regular magnetic pulses (RMPs) that are attributed to the interception of recoil leader with existing lightning channel. Inter-pulse intervals of RMPs are one order smaller than that of MPBs and IMPs, and observations may reflect differences between the positive polarity breakdown and the negative polarity breakdown of leaders.

Keywords

• rocket-triggered lightning
• initial continuous current
• medium-low frequency magnetic fields