Time-frequency Profile of Discharge Processes Prior the First Return Stroke

Mohd Esa, Mona Riza and Ahmad, Mohd Riduan and Cooray, Vernon (2014) Time-frequency Profile of Discharge Processes Prior the First Return Stroke. In: ICLP, 13-17 October 2014, Shanghai, China. (In Press)

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Abstract

We aim to investigate the time-frequency profile for a sets of lightning processes that occurred prior the first return stroke which include narrow bipolar pulses, preliminary breakdown pulses and stepped leaders. The work is mainly using the wavelet transformation in order to gain the frequency spectrum while not losing the time information of the located pulses. 3 sets of events that include the narrow bipolar pulse (NBP) followed by preliminary breakdown pulses (PBPs), stepped leaders (SLs) and return stroke (RS) are used in this paper. All selected data are recorded during northeastern monsoon season at the end of year 2012 in Malaysia. A total of 3 NBPs, 3 first RS, 67 PBPs and 12 SL pulses undergone wavelet transformation individually and parameters used for characterizing these pulses are the upper and lower frequency spectrum for spectral and spread regions and the power spectrum (PS). The definitions of these parameters are explained in [1]. Fig. 1 shows the wavelet transformation of a NBP and Fig. 2 shows for the first RS of a negative cloud-to-ground (CG) flash. Note that the time frame to transform the RS is only up to 50 μs. We found that in average of 3 events, the upper and lower frequency spectrum for both spectral and spread regions of NBP are more than 10 times higher than the first RS. We found that, RS radiates its highest energy spectrum at much lower frequency spectrum between 2 and 13 kHz, while NBP radiates its highest energy spectrum at much higher frequency spectrum from 121 kHz to 234 kHz. For PBP and SL pulses, we found that PBP gained the higher value in almost all parameters when compared to SL pulses. PBP tends to radiate its highest energy spectrum in the range between 30 kHz and 1004 kHz and SL pulses radiate predominantly from 174 to 347 kHz. The spectral and spread regions for one event are shown in Fig. 3. The vertical axis on the left of Fig. 3B represents the frequency spectrum scale and the vertical axis on the right denotes the magnitude of the power spectrum. Note that the maximum radiated energy is in the middle of the spread region (represented by red thick line). Also note that, there is a single unipolar pulse exists between NBP and PBP, and this pulse does not appear to have spread region. The detail for spectral and spread regions of PBP is expanded in Fig. 4. There are 20 out of 35 pulses having both spectral and spread regions after the transformation and the rest are represents by only their spectral region. It is shown that the power spectrum of each PBP is high in the beginning and becomes lower towards the end of PBP. A detail analysis also been done to SL and the first RS. In this particular event, there were only 3 SLs where only 2 of them have both spectral and spread regions. The power spectrum seems to be high for SL pulses with both spectral and spread regions and low for the last SL pulse (with no spread region).

Item Type: Conference or Workshop Item (Paper)
Subjects: Q Science > Q Science (General)
Divisions: Faculty of Electronics and Computer Engineering > Department of Computer Engineering
Depositing User: En. Mohd Riduan Ahmad
Date Deposited: 19 Jun 2014 03:08
Last Modified: 28 May 2015 04:26
URI: http://eprints.utem.edu.my/id/eprint/12639
Statistic Details: View Download Statistic

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