Characteristics of narrow bipolar pulses and negative return strokes of vertical electric fields generated by lightning recorded in Malaysia

Abstract

Lightning that generates electric fields is known to have negative consequences. Therefore, the electric field radiated in microwave and characteristics of different types of lightning activities are studied and analysed to better understand lightning based problems. In this study, two types of lightning electric fields which are Negative Return Strokes (NRS) and Narrow Bipolar Pulses (NBP) are thoroughly analysed. A total of 89 NRS data and 115 NBP data were selected from a set of data recorded during the northeast monsoon period from November to December 2012 in the vicinity of Universiti Teknologi Malaysia (UTM), Johor. For that purpose, flat plate antenna with electronic buffer circuit and a 2.4 GHz monopole antenna were used to measure vertical electric field and microwave radiation. Apart from the electric field characteristics analysis, the measurements of microwave radiation at 2.4 GHz associated with NRS and NBP were also performed and the results are depicted in this study. Nonetheless, the wavelet and the frequency spectrum of both NRS and NBP were examined and the results are discussed inclusively. Furthermore, the number of microwave radiations associated with NBP were notably higher than that of NRS, which indicated that the mechanism of NBP was more distinguished compared to that of NRS. On top of that, the duration of 2.4 GHz emission associated with NBP was discovered to be a factor of a few tens shorter than that of High Frequency (HF) radiation from return strokes. Meanwhile, analysis on the wavelet and frequency spectrum reveals that the energy spectrum for NBP concentrates at high frequency (200 kHz to 500 kHz) with average power spectrum peak of the initial part for NNBP and NPBP to be 76,650 (V/m)2 and 76,309 (V/m)2, respectively. However, the initial part power spectrum peak corresponding to NRS is about 118,931(V/m)2 with lower frequency spectrum (60 kHz). The results suggest that NBP radiates energy at high frequency region compared to NRS. Hence, it can be concluded that electric field pulse for NBP experiences more extensive and rapid ionization process compared to NRS.