Is the Microsecond-Scale Pulse a Necessary Feature of Lightning K Process?

Simultaneous VLF/LF magnetic fields and VHF emissions associated with K changes from the later portion of 16 cloud-to-ground lightning flashes and 12 intra-cloud lightning flashes were examined to clarify whether the microsecond-scale pulse activity is a necessary feature of K process of lightning discharges.Our results indicate that the microseconds-scale pulse is a necessary feature of K process:(1) the overwhelming majority of K changes have their VLF/LF magnetic fields characterized by a small-amplitude train of dense narrow pulses occurring regularly or irregularly,with sometime one or more large pulses randomly superimposing on it.The width of individual narrow pulse ranges a few microseconds and the average pulse separation is in the order of ten microseconds.(2) A small fraction of K changes were associated with VLF/LF magnetic fields dominated by one or more large pulses which were similar to those occurring during the initial breakdown of cloud-to-ground or/and intra-cloud flashes.K processes were also accompanied by VHF emissions of Q-noise type or separated pulse type.These VHF emissions tend to correlate with VLF/LF magnetic fields in time and occur at the beginning of K changes.The incidence azimuth of VLF/LF magnetic pulses were calculated to change regularly during K changes,possibly implying that the microsecond-scale VLF/LF pulses of K process were produced when the K streamer extended the in-cloud lightning channel.