Characterizing radio frequency magnetic radiation of initial upward leader in triggered lightning with interferometric lightning mapping

The operation bandwidth of magnetic field sensor with high sensitivity was extended to 1.2 MHz during the triggered lightning experiment of Field Experiment Base on Lightning Sciences, China Meteorological Administration (CMA-FEBLS) in Conghua, Guangdong Province during summer of 2019. Measurements with the new magnetic fields reveal the presence of microsecond-scale magnetic pulses during the entire duration of upward positive leaders (UPL), including the quiet stage when few signals can be discerned in previous observations, which indicates that the UPL propagates in a step-wise manner during the initial stage of triggered lightning. Synchronous observations from the broadband VHF interferometer shows that the VHF radiation corresponds to the onset of individual magnetic pulses, indicating that the VHF signals are radiated by the breakdown processes of individual stepping, and these breakdown events launch the meter-scale current pulses as the radiation source of each magnetic pulse.