Research on chaotic leader in cloud-to-ground lightning

Based on the data of optical and electromagnetic signals detected during the comprehensive observation experiment on lightning discharge in Guangdong province in 2012 and 2013, the comprehensive characteristics of chaotic leader (CL), including multi-scale entropy of electric field change, optical radiation, wideband radiation (bandwidth 300M) and occurrence regularity, have been analyzed in detail. The results show that two types of CL can be identified. One type exhibits the chaotic pulse connecting to RS, the other type begins with chaotic pulse followed by regular pulse before RS. According to our statistics, near 50% of subsequent return strokes (SRS) are preceded by CL. At the same time, almost all of CLs are accompanied by obvious optical signal, which indicates that CLs tend to occur below the cloud, and near 80% CLs also show a large change of electric field profile. One case shows that CL pulse is a typical discharge signal of obvious attempted leader. The average power spectral density of CL shows substantially higher radiation power in the frequency range of 0-300MHz than that of non-chaotic subsequent leaders. In addition, we found that multi-scale entropy may be used as characteristic parameter to describe CL pulses. There are different entropy values for different leaders such as stepped leader and CL. The value for CL is larger than 1.5 with the scale of over 3(the average value of multi-scale entropy, from 2.0 to 2.1). INTRODUCTION Leader process, as an important discharge process of lightning, has drawn continuous attention. In addition to those conventional leader processes, such as stepped leader and dart leader, there is a somewhat controversial class of events usually known as ‘chaotic leader (CL)’ (as shown in figure1). CL had been presented firstly by Weidman et al. [1982]. He found the irregular pulse train before subsequent RS, and termed these return strokes as ‘Chaotic subsequent strokes’ or subsequent strokes preceded by ‘Chaotic Leaders’. In general, Chaotic leader has been used to refer to a relatively continuous sequence of irregular electric field pulses occurring within about 2 ms preceding a subsequent return stroke in natural lightning [Lan et al., 2011] or triggered lightning [Hill et al., 2012], but no consistent definition has yet emerged. The previous research about CL has focused on the waveform characteristics of electric field, such as pulse width, duration and pulse interval [Weidman et al., 1982; Makela et al., 2007; Gomes et al., 2004]. However, as continuous sequence of irregular electric field pulses, it is difficult for CL pulses to be accurately described only by pulse parameters. At the same time, considering the characteristics of high  Contact information: Yang Zhang, State Key Laboratory of Severe Weather, Chinese Academy of Meteorological Sciences, Email: zhangyang@cams.cma. gov.cn XV International Conference on Atmospheric Electricity, 15-20 June 2014, Norman, Oklahoma, U.S.A. 2 frequency radiation, it is also difficult to give a sufficient information of CL due to the limit frequency range in the previous research. In addition, there is also little report about the detailed process of CL discharge. Up to now, the research on chaotic leader is still not adequately detailed. In the work, the comprehensive characteristics, including multi-scale entropy of electric field change, optical radiation, wideband radiation (bandwidth 300M) and occurrence regularity, have been analyzed in detail by the comprehensive data of optical and electromagnetic signals which has been observed during the comprehensive observation experiment on lightning discharge in Guangdong province in 2012 and 2013. Figure 1 Classical chaotic leader shown by Lan [2011]