| 1. |
- Ahmad, Noor Azlinda, et al.
(författare)
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Characteristics of narrow bipolar pulses observed in Malaysia
- 2010
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Ingår i: Journal of Atmospheric and Solar-Terrestrial Physics. - : Elsevier BV. - 1364-6826 .- 1879-1824. ; 72:5-6, s. 534-540
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Tidskriftsartikel (refereegranskat)abstract
- Narrow bipolar pulses (NBPs) are considered as isolated intracloud events with higher peak amplitude and strong high frequency emission compared to the first return strokes and other intracloud discharges. From 182 NBPs recorded in Malaysia in the tropic, 75 were narrow negative bipolar pulses (NNBPs) while 107 were narrow positive bipolar pulses (NPBPs). The mean duration of NNBPs was 24.6 +/- 17.1 mu s, while 30.2 +/- 12.3 mu s was observed for NPBPs. The mean full-width at half-maximum (FVVHM) was 2.2 +/- 0.7 and 2.4 +/- 1.4 mu s for NNBPs and NPBPs, respectively. The mean peak amplitude of NPBPs normalized to 100 km was 22.7 V/m, a factor of 1.3 higher than that of NNBPs which is 17.6 V/m. In contrast to the previous studies, it was observed that the electric field change was characterized by a bipolar pulse with a significant amount of fine structures separated by a few tens of nanoseconds intervals, embedded on it. (C) 2010 Elsevier Ltd. All rights reserved.
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| 2. |
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| 3. |
- Ahmad, Noor Azlinda, 1978-
(författare)
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Broadband and HF Radiation from Cloud Flashes and Narrow Bipolar Pulses
- 2011
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Remote measurement of electric field generated by lightning has played a major role in understanding the lightning phenomenon. Even though other measurements such as photographic and channel base current have contributed to this research field, due to practical reasons remote measurements of electric field is considered as the most useful tool in lightning research. This thesis discusses the remotely measured radiation field component of electric field generated by cloud flashes (ICs) and narrow bipolar pulses (NBPs). The associated HF radiation of these events at 3 MHz and 30 MHz are also discussed. To understand the initiation process of these discharges, a comparative study of the initial pulse of cloud flashes against the initial pulse of cloud to ground flashes was conducted. The result suggests that both discharges might have been initiated by similar physical processes inside the thunderclouds. Comparing the features of initial pulse of cloud and ground flashes with that of pulses that appeared in the later stages of cloud flashes suggests that the initiation process involved in both flashes are not very much different from the initiation of cloud flashes at the later stage. The average spectral amplitudes of electric field of full duration cloud flashes (180 ms) showed f -1 frequency dependence within the interval of 10 kHz to approximately 10 MHz. This is in contrast to the standard f -2 decrement (or even steeper ) at high frequency region for other lightning processes such as return strokes. It was suggested that small pulses which repeatedly appeared at the later stage of cloud flashes might have contributed to enhance the spectral amplitude at higher frequencies. Electric fields generated by Narrow Bipolar Pulses (NBPs), which are considered as one of the strongest sources of HF radiation, were measured in the tropics of Malaysia and Sri Lanka. Their features were also studied and show a good agreement with previously published observations of NBPs from other geographical regions. Thorough analyses and observations of these pulses found previously unreported sharp, fine peaks embedded in the rising and decaying edge of the electric field change of NBPs. Therefore it was suggested that these fine peaks are mostly responsible for the intense HF radiation at 30 MHz.
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| 4. |
- Ahmad, Noor Azlinda
(författare)
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Broadband and HF radiation from cloud flashes and narrow bipolar pulses
- 2010
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Remote measurement of electric field generated by lightning has played a major role in understanding the lightning phenomenon. Even though other measurements such as optical, photographic, channel base current and thunder signatures have contributed in this regards, due to practical reasons remote measurements of electric field is considered as the most useful tool in lightning research. This thesis discussed about the remotely measured radiation field component of electric field generated by cloud flashes (ICs) and narrow bipolar pulses (NBPs). The associated HF radiation of these events at 3 MHz and 30 MHz were also discussed. To understand the initiation process of these discharges, a comparative study of the initial pulse of cloud flashes with the initial pulse of cloud to ground flashes was conducted. The result suggests that both discharges might have been initiated by similar physical process inside the thunderclouds. Comparing the features of initial pulse of cloud and ground flashes with that of pulses appeared in the later stages of cloud flashes suggests that the initiation process involved in both flashes are not very much differ from the initiation of cloud flashes at the later stage. The average spectral amplitudes of electric field for first 120 ms of cloud flashes showed f-1 frequency dependence within the interval of 100 kHz to about 2 MHz and does not follow the standard f-2 decrease (or even more) at high frequency region. It was suggested that small pulses which repeatedly appeared at the later stage of cloud flashes might have contribute to enhance the spectral amplitude at higher frequencies. Electric fields generated by Narrow Bipolar Pulses (NBPs), which are considered as one of the strongest source of HF radiation were measured in Malaysia in the tropic and their features were also studied. Result shows a good agreement with previously published observations of NBPs in other geographical regions. The pulse duration of NBPs is varied within 20 – 30μs with the normalized peak amplitude is of the order of 10 V/m, averagely 2– 3 times larger than the peak amplitude of ordinary return strokes. They were observed to emit intense burst of HF radiation at 30 MHz. Thorough analyses and observation of these pulses found previously unreported sharp, fine peaks embedded at the rising and decaying edge of the electric field change of NBPs. Therefore it was suggested that these fine peaks were most probably to be responsible for the intense HF radiation at 30 MHz.
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| 5. |
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| 6. |
- Ahmad, Noor Azlinda, et al.
(författare)
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Radiation field spectra of long-duration cloud flashes
- 2015
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Ingår i: Atmospheric Science Letters. - : Wiley. - 1530-261X. ; 16:2, s. 91-95
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Tidskriftsartikel (refereegranskat)abstract
- The preliminary results of radiation electric fields produced by long-duration cloud flashes have been Fourier analyzed to determine the frequency spectrum in the range of 10kHz-10MHz. The flashes were recorded within a distance of less than 20km. The spectrum was normalized to 50km distance and it shows a f(-1) dependence within the entire frequency range.
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| 7. |
- Ahmad, Noor Azlinda, 1978-, et al.
(författare)
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Radiation Field Spectra of Long-duration Cloud Flashes
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Ingår i: IEEE transactions on electromagnetic compatibility (Print). - 0018-9375 .- 1558-187X.
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Tidskriftsartikel (refereegranskat)abstract
- The radiation electric fields produced by long-duration cloud flashes have been Fourier analyzed to determined the frequency spectrum in the range of 10 kHz to 10 MHz. The flashes were recorded within a distance of less than 20 km. The spectrum was normalized to 50 km distance and it shows a f-1 dependence within the entire frequency range.
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| 8. |
- Ahmad, Noor Azlinda, 1978-, et al.
(författare)
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Some features of electric field waveform of Narrow Bipolar Pulses
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Ingår i: Atmospheric research. - 0169-8095 .- 1873-2895.
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Tidskriftsartikel (refereegranskat)abstract
- Narrow Bipolar Pulses (NBPs) are generated by intra-cloud discharge processes and they are of interest due to their strong broadband and high frequency (HF) emissions. In this study, we present some features of electric field waveform of NBPs which have not been reported in the literature. The HF emission was observed to begin simultaneously with the onset of NBPs indicating no streamers or stepped-leader process was taking place before the initiation of NBPs. The electric field waveforms of NBPs were characterized by many fine peaks embedded intermittently on the rising and decaying edge of NBPs suggesting that some small scale electrical discharges were involved during the formation of NBPs.
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| 9. |
- Ahmad, Noor Azlinda, et al.
(författare)
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The first electric field pulse of cloud and cloud-to-ground lightning discharges
- 2010
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Ingår i: Journal of Atmospheric and Solar-Terrestrial Physics. - : Elsevier BV. - 1364-6826 .- 1879-1824. ; 72:2-3, s. 143-150
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Tidskriftsartikel (refereegranskat)abstract
- In this study, the first electric field pulse of cloud and cloud-to-ground discharges were analyzed and compared with other pulses of cloud discharges. Thirty eight cloud discharges and 101 cloud-to-ground discharges have been studied in this analysis. Pulses in cloud discharges were classified as [`]small', [`]medium' and [`]large', depending upon the value of their relative amplitude with respect to that of the average amplitude of the five largest pulses in the flash. We found that parameters, such as pulse duration, rise time, zero crossing time and full-width at half-maximum (FWHMs) of the first pulse of cloud and cloud-to-ground discharges are similar to small pulses that appear in the later stage of cloud discharges. Hence, we suggest that the mechanism of the first pulse of cloud and cloud-to-ground discharges and the mechanism of pulses at the later stage of cloud discharges could be the same.
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| 10. |
- Baharudin, Zikri A., et al.
(författare)
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Comparative study on preliminary breakdown pulse trains observed in Johor, Malaysia and Florida, USA
- 2012
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Ingår i: Atmospheric research. - : Elsevier BV. - 0169-8095 .- 1873-2895. ; 117, s. 111-121
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Tidskriftsartikel (refereegranskat)abstract
- In this paper, the preliminary breakdown (PB) pulse train preceding the negative first return stroke (RS) is recorded using a broad band antenna system. These analyses were carried out in Johor Bahru, Malaysia and Florida, United States. This is a novel initiative at examining and identifying the characteristics of the PB pulse trains in the negative cloud-to-ground flashes observed in Malaysia. The arithmetic mean of the total pulse train duration is 12.3 ms and the weighted arithmetic mean of the pulse durations and interpulse intervals are 11 mu s and 152 mu s, respectively. The arithmetic mean ratio between the maximum peak amplitude of the PB pulse and the peak RS electric field was 27.8%, and the corresponding value in Florida was 29.4%. The arithmetic mean of the time duration between the most active part of the pulse train, and the RS was 57.6 ms in Malaysia and 22 ms in Florida. A qualitative comparison of our results with those obtained earlier in Sri Lanka. Sweden and Finland supports the hypothesis that the PBP/RS ratio is higher in the northern regions compared to the tropical regions. (C) 2012 Elsevier B.V. All rights reserved.
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| 11. |
- Baharudin, Zikri Abadi, et al.
(författare)
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Electric field changes generated by preliminary breakdown pulse for positive lightning ground flashes in Sweden
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Annan publikation (övrigt vetenskapligt/konstnärligt)abstract
- This is the new study of the electric field changes generated by the preliminary breakdown for positive cloud-to-ground flashes which concerning on the association of slow field changes in preliminary breakdown process. In this study, a 107 positive cloud-to-ground lightning flashes recorded from the total of 14 thunderstorms generated by the whole flash were examined. The electric fields were measured with nanosecond resolution by using the slow electric field, fast electric field and narrowband radiation field at 3 and 30 MHz signals simultaneously. Our result shows that there is a pre-starting time, i.e. the duration between the first preliminary breakdown pulse and slow field changes starting point, which is found to be after the first preliminary breakdown pulse. The pre-starting time has the arithmetic mean – 3.0 ms and geometric mean – 1.8 ms, ranging from 0.3 to 21.7 ms. This study is consistent with the latest finding for the slow field changes in negative ground flashes where the slow field changes never start before the preliminary breakdown process.
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| 12. |
- Baharudin, Zikri A., et al.
(författare)
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Electric field changes generated by the preliminary breakdown for the negative cloud-to-ground lightning flashes in Malaysia and Sweden
- 2012
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Ingår i: Journal of Atmospheric and Solar-Terrestrial Physics. - : Elsevier BV. - 1364-6826 .- 1879-1824. ; 84-85, s. 15-24
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Tidskriftsartikel (refereegranskat)abstract
- We present the study of the electric field changes generated by the preliminary breakdown for negative cloud-to-ground lightning flashes in Malaysia and Sweden concerning the association of slow field changes associated in preliminary breakdown process. We examined the total of 1685 negative cloud-to-ground lightning flashes from the total of 39 thunderstorms by recording the slow electric field, fast electric field and narrowband radiation field at 3 and 30 MHz signals simultaneously. Our results show that there is a pre-starting time, i.e. the duration between the first preliminary breakdown pulse and slow field changes starting point, which is found to be after the first preliminary breakdown pulse. The pre-starting time has the arithmetic and geometric mean range from 1.4-6.47 and 1-3.36 ms, respectively. The mean values of pre-starting time in Malaysia are greater than the values observed in Sweden by more than a factor of 3. From the two data sets it shows that the slow field changes never start before the preliminary breakdown. Furthermore, the use of single-station electric fields measurement with high resolutions of 12 bits transient recorder with several nanosecond accuracy allow one to distinguish the slow field changes generated by preliminary breakdown, which preceded the negative first return stroke, between tens to hundreds of milliseconds of pre-return stroke duration.
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| 13. |
- Baharudin, Zikri A., et al.
(författare)
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Negative cloud-to-ground lightning flashes in Malaysia
- 2014
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Ingår i: Journal of Atmospheric and Solar-Terrestrial Physics. - : Elsevier BV. - 1364-6826 .- 1879-1824. ; 108, s. 61-67
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Tidskriftsartikel (refereegranskat)abstract
- The characteristics of the negative cloud-to-ground lightning flashes in Malaysia are studied by analyzing the electric fields generated by the whole flash in nanosecond resolution. A total of 405 strokes obtained from 100 successive negative cloud-to-ground lightning flashes were analyzed, which were recorded from seven convective thunderstorms during the southwest monsoon period, i.e. from April to June 2009. It was found that the total number of interstroke intervals has an arithmetic mean value of 86 ms, a geometric mean value of 67 ms and does not depend on the return stroke order. Of the 100 negative ground flashes, 38 flashes (38%) have at least one subsequent return-stroke (SRS) whose electric field peak was greater than that of the first return-stroke (RS). Furthermore, 58 (19%) out of 305 SRS have electric field peak larger than those of the first RS. The arithmetic and geometric mean ratio between the peak electric field of the SRS and the peak electric field of the first RS are 0.7 and 0.6, respectively. The percentage of single-stroke flashes was 16% while the mean number of strokes per flash and maximum number of stroke per flash were 4 and 14, respectively.
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| 14. |
- Baharudin, Zikri Abadi, et al.
(författare)
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On the characteristics of positive lightning ground flashes in Sweden
- 2016
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Ingår i: Journal of Atmospheric and Solar-Terrestrial Physics. - : Elsevier BV. - 1364-6826 .- 1879-1824. ; 138, s. 106-111
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Tidskriftsartikel (refereegranskat)abstract
- In this study the stroke characteristics of positive cloud-to-ground flashes in Sweden were obtained from the electric field records measured from 14 thunderstorms. The electric fields were measured with nanosecond resolution. Together with the fast and the slow electric field records, the narrowband radiation field at 3 and 30 MHz signals were also measured simultaneously. Out of a total of 107 flashes, 30 flashes had two strokes, 7 had three strokes and 3 flashes had four strokes. The arithmetic and geometric means of the interstroke intervals were found to be 116 and 70 ms, respectively. The arithmetic and geometric mean ratio between the peak electric field of the Subsequent Return Stroke (SRS) and the first Return Stroke (RS) were 0.48 and 0.36, respectively. Of the 40 positive multiple-stroke ground flashes, 5% have at least one SRS with field peak higher than the first RS. The percentage of SRS with field peaks greater than the first RS was 6%. In our best of our knowledge, this is the first time a large sample of positive return strokes in Sweden was analysed. It was found to be statistically more significant than the previous studies.
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| 15. |
- Rahman, Mahbubur, et al.
(författare)
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X rays from 80-cm long sparks in air
- 2008
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Ingår i: Geophysical Research Letters. - 0094-8276 .- 1944-8007. ; 35:6, s. L06805-
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Tidskriftsartikel (refereegranskat)
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