Measurements of magnetic pulse bursts during initial continuous current of negative rocket-triggered lightning

During the SHandong Triggering Lightning Experiment (SHATLE) in summer of 2014 and 2015, and the Guangdong Comprehensive Observation Experiment on Lightning Discharge (GCOELD) campaign in 2018, we have conducted the observations of the magnetic pulse bursts (MPBs) during the initial continuous current (ICC) in negative rocket-triggered lightning.  The MPBs are commonly recorded at the main site of SHATLE (970 m from the rocket launch site), but the synchronous magnetic field (B-field) measurements at close site of SHATLE (78 m from the rocket launch site) show the slow variations with small MPBs superposing on them.  Note that both the charge transfer and the relative brightness increase notably during the appearance of the MPBs.  After shifting up the operation frequency of the magnetic sensor, the MPBs can be observed at close distance (80 m from the rocket launch site) obviously in GCOELD.  Observations show that the radiation sources of MPBs originate from the breakdown in the vicinity of the leader tip, but the sources of the initial magnetic pulses (IMPs) measured at the very initial stage of triggered lightning are from the radiation of the whole steel wire.  The continuous current measured at the channel base during the MPBs can not reflect the characteristics of breakdown current, because the current is attenuated and dispersed when propagating along the high-impedance leader channel.  The average peak current associated with the MPBs is estimated to be on the order of kA.