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

Abstract

With the measurements in SHandong Triggering Lightning Experiment and Guangdong Comprehensive 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.