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Broadband and HF Radiation from Cloud Flashes and Narrow Bipolar Pulses
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. (Lightning Research Group)
2011 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

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 -1 frequency dependence within the interval of 10 kHz to approximately 10 MHz. This is in contrast to the standard -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.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis , 2011. , 64 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 822
Keyword [en]
Lightning, Cloud Flashes, Narrow Bipolar Pulses, HF radiation
National Category
Other Physics Topics
Research subject
Engineering Science with specialization in Atmospheric Discharges
Identifiers
URN: urn:nbn:se:uu:diva-150956ISBN: 978-91-554-8067-7OAI: oai:DiVA.org:uu-150956DiVA: diva2:409468
Public defence
2011-05-25, Å2001, Ångström Laboratory, Lägerhyddsvägen 1, Uppsala, 14:15 (English)
Opponent
Supervisors
Available from: 2011-05-03 Created: 2011-04-08 Last updated: 2011-07-01Bibliographically approved
List of papers
1. The first electric field pulse of cloud and cloud-to-ground lightning discharges
Open this publication in new window or tab >>The first electric field pulse of cloud and cloud-to-ground lightning discharges
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2010 (English)In: Journal of Atmospheric and Solar-Terrestrial Physics, ISSN 1364-6826, E-ISSN 1879-1824, Vol. 72, no 2-3, 143-150 p.Article in journal (Refereed) Published
Abstract [en]

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.

Keyword
Cloud discharges, Electromagnetic field, Lightning, Electric field pulses
National Category
Meteorology and Atmospheric Sciences Engineering and Technology
Research subject
Engineering Science with specialization in Atmospheric Discharges
Identifiers
urn:nbn:se:uu:diva-140337 (URN)10.1016/j.jastp.2009.11.001 (DOI)
Available from: 2011-01-05 Created: 2011-01-05 Last updated: 2016-04-26
2. Radiation Field Spectra of Long-duration Cloud Flashes
Open this publication in new window or tab >>Radiation Field Spectra of Long-duration Cloud Flashes
(English)In: IEEE transactions on electromagnetic compatibility (Print), ISSN 0018-9375Article in journal (Refereed) Submitted
Abstract [en]

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.

Identifiers
urn:nbn:se:uu:diva-150952 (URN)
Available from: 2011-04-08 Created: 2011-04-08 Last updated: 2014-06-30
3. Characteristics of narrow bipolar pulses observed in Malaysia
Open this publication in new window or tab >>Characteristics of narrow bipolar pulses observed in Malaysia
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2010 (English)In: Journal of Atmospheric and Solar-Terrestrial Physics, ISSN 1364-6826, E-ISSN 1879-1824, Vol. 72, no 5-6, 534-540 p.Article in journal (Refereed) Published
Abstract [en]

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.

Keyword
Narrow bipolar pulses, Lightning, Cloud discharges, Electric field
National Category
Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-137067 (URN)10.1016/j.jastp.2010.02.006 (DOI)000276428600020 ()
Available from: 2010-12-15 Created: 2010-12-15 Last updated: 2016-04-18Bibliographically approved
4. Some features of electric field waveform of Narrow Bipolar Pulses
Open this publication in new window or tab >>Some features of electric field waveform of Narrow Bipolar Pulses
(English)In: Atmospheric research, ISSN 0169-8095Article in journal (Refereed) Submitted
Abstract [en]

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.

 

Identifiers
urn:nbn:se:uu:diva-150953 (URN)
Available from: 2011-04-08 Created: 2011-04-08 Last updated: 2014-06-30

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