The initiation and propagation of lightning is a process of charge separation  along positive and negative leaders extending in opposite directions as driven by the thunderstorm generator.  The propagation of positive leader is difficult to observe due to its weak optical emission and very-high frequency (VHF) radiation.  It is the critical problem of lightning physics as how to effectively observe the propagation of positive leader inside clouds and the micro-scale features of its propagation. 

During the artificially triggered lightning experiment in summer of 2013, the lightning research group of Institute of Atmospheric Physics (IAP), Chinese Academy of Sciences deployed the low-frequency magnetic sensor typically used for the remote sensing of lightning and recorded the electromagnetic (EM) pulse train of more than 20 milliseconds accompanying the initial continuous current in triggered lightning.  The timescale of these EM pulses is about several microseconds, and the inter-pulse interval is generally about 25 μs.   The comparison with the two-dimensional (2D) observation result of high-speed camera and the short-baseline VHF lightning imaging system indicates that the EM pulse train was related to the stepping of positive leader.  The analyses of these pulses suggest that the propagation of positive leader bears the similar stepping feature to the negative counterpart during its propagation, whereas the interval between individual stepping is relatively long.  This implies some difference in the physical mechanism of formation and propagation for the positive and negative leaders.  

The electromagnetic pulse train has been commonly observed in the triggered lightning experiment during summer of 2014, indicating that the sensitive magnetic sensor will contribute to further understand the propagation features of positive leader.  At present, the lightning research group of IAP is looking for the electromagnetic pulse trains possibly associated with the stepping feature of positive leader in natural lightning. 

During the initial continuous current stage of artificially triggered lightning, the very-high frequency radiation (indicated by pink points) during the ascending propagation of positive leader became weak, whereas the signal from the low-frequency magnetic sensor (red line) indicates a long sequence of micro-second scale pulses over 20 milliseconds.  The analyses indicate that these pulses are linked to the stepping feature of positive leader propagation. 

Citation: Lu, G., R. Jiang, X. Qie, H. Zhang, Z. Sun, M. Liu, Z. Wang, and K. Liu (2014), Burst of intracloud current pulses during the initial continuous current of a rocket-triggered lightning flash, Geophys. Res. Lett., doi: 10.1002/2014GL062127. 

http://onlinelibrary.wiley.com/doi/10.1002/2014GL062127/abstract?campaign=wolacceptedarticle