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- Sabri, M. H. M., et al.
(författare)
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Initial electric field changes of lightning flashes in tropical thunderstorms and their relationship to the lightning initiation mechanism
- 2019
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Ingår i: Atmospheric research. - : ELSEVIER SCIENCE INC. - 0169-8095 .- 1873-2895. ; 226, s. 138-151
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, the key finding is that all the examined first classic Initial Breakdown (IB) pulses in tropical flashes within the reversal distance were found to be initiated by a clearly detectable Initial E-field Change or IEC (45 -CG, 32 normal IC, and 3 IC initiated by +NBE). The durations of IECs for both -CG and IC flashes in tropical storms were longer than in Florida storms. On the other hand, for the magnitudes of the E-change, the values were smaller compared to Florida storms with averages of 0.30 V/m compared to 1.65 V/m for -CG flashes, and -0.81 V/m compared to -6.30 V/m for IC flashes. The IEC process of lightning flashes in tropical regions took longer to increase the local electric field in order to produce the first IB pulse because of the smaller magnitude of E-change. On the other hand, in Florida storms, the IEC process took a shorter time to increase the local electric field to produce the first IB pulse because of the larger magnitude of E-change. We found that very high frequency (VHF) pulses for tropical thunderstorms started sometime prior to the onset of the IECs. They started between 12.69 and 251.60 mu s before the initiation of the IEC for two normal IC flashes. The first two VHF pulses were detected alone without narrow IB pulses (fast antenna and slow antenna records) or any pulses from the B-field and dE/dt records. Furthermore, the VHF pulses for three IC flashes initiated by + NBEs were also detected before the onset of the IEC. The IEC started immediately after the detection of the + NBE. It is clear that the IEC is initiated by VHF pulses. It can be suggested that lightning is initiated by Fast Positive Breakdowns or FPBs (which emit strong VHF pulses and large + NBEs) and is followed by several negative breakdowns (weak VHF pulses and/or weak NBE-type pulses) before the IEC started. For the case of normal IC flashes, several weaker VHF pulses (mean values of 41.97 mV and 46.4 mV compared to the amplitudes of the VHF pulses of + NBEs of around 800 mV) were detected before the onset of the IEC. As FPBs can occur with a wide range of VHF strengths and E-change amplitudes, it can be suggested these weak VHF pulses accompanied by narrow IB pulses or weak NBE-type pulses detected before the onset of IEC are actually FPBs followed by negative breakdowns or several attempted FPBs.
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- Sabry, R. Z., et al.
(författare)
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Frequency Dependent Permittivity of Soil and Bentonite : For Lightning Protection and High Frequency Earthing Systems
- 2019
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Ingår i: 2019 International Symposium on Lightning Protection (XV SIPDA). - : IEEE. - 9781728118918
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Konferensbidrag (refereegranskat)abstract
- This paper presents the frequency dependent permittivity of a selected type of soil, bentonite and four of their mixes by weight. The permittivity at a low frequencies, vary significantly with moisture content in the cases of both sand and bentonite. For dry sand, bentonite and all bentonite mixes, the permittivity shows a rapid drop with the increment of frequency from near 0 Hz to 0.6 kHz, followed by a moderate decrement up to about 10-20 kHz. All parameters have rather insignificant variation after about 50 kHz and level of around 1MHz. These outcomes provide some key information to analyze the behavior of bentonite based backfill materials that are widely used in electrical earthing networks, especially in the case of high frequency earthing and lightning protection systems.
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