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Cooray, Vernon
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Publications (10 of 331) Show all publications
Cooray, V. & Cooray, G. (2019). A Novel Interpretation of the Electromagnetic Fields of Lightning Return Strokes. Atmosphere, 10(1), Article ID 22.
Open this publication in new window or tab >>A Novel Interpretation of the Electromagnetic Fields of Lightning Return Strokes
2019 (English)In: Atmosphere, ISSN 2073-4433, E-ISSN 2073-4433, Vol. 10, no 1, article id 22Article in journal (Refereed) Published
Abstract [en]

Electric and/or magnetic fields are generated by stationary charges, uniformly moving charges and accelerating charges. These field components are described in the literature as static fields, velocity fields (or generalized Coulomb field) and radiation fields (or acceleration fields), respectively. In the literature, the electromagnetic fields generated by lightning return strokes are presented using the field components associated with short dipoles, and in this description the one-to-one association of the electromagnetic field terms with the physical process that gives rise to them is lost. In this paper, we have derived expressions for the electromagnetic fields using field equations associated with accelerating (and moving) charges and separated the resulting fields into static, velocity and radiation fields. The results illustrate how the radiation fields emanating from the lightning channel give rise to field terms varying as <mml:semantics>1/r</mml:semantics> and <mml:semantics>1/r2</mml:semantics>, the velocity fields generating field terms varying as <mml:semantics>1/r2</mml:semantics>, and the static fields generating field components varying as <mml:semantics>1/r2</mml:semantics> and <mml:semantics>1/r3</mml:semantics>. These field components depend explicitly on the speed of propagation of the current pulse. However, the total field does not depend explicitly on the speed of propagation of the current pulse. It is shown that these field components can be combined to generate the field components pertinent to the dipole technique. However, in this conversion process the connection of the field components to the physical processes taking place at the source that generate these fields (i.e., static charges, uniformly moving charges and accelerating charges) is lost.

Place, publisher, year, edition, pages
MDPI, 2019
Keywords
Electromagnetic fields, return strokes, dipole fields, accelerating charges, radiation fields, static fields, velocity fields
National Category
Mathematical Analysis
Identifiers
urn:nbn:se:uu:diva-378742 (URN)10.3390/atmos10010022 (DOI)000459133000022 ()
Funder
Swedish Research Council, VR-2015-05026
Available from: 2019-03-11 Created: 2019-03-11 Last updated: 2019-03-11Bibliographically approved
Sabri, M. H., Ahmad, M. R., Esa, M. R., Periannan, D., York, S. B., Mohammad, S. A., . . . Yahaya, M. P. (2019). Environmental Analysis of Quasi-Static Electric Field Changes of Tropical Lightning Flashes. Ekoloji, 28(107), 373-378, Article ID UNSP e107013.
Open this publication in new window or tab >>Environmental Analysis of Quasi-Static Electric Field Changes of Tropical Lightning Flashes
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2019 (English)In: Ekoloji, ISSN 1300-1361, Vol. 28, no 107, p. 373-378, article id UNSP e107013Article in journal (Refereed) Published
Abstract [en]

The environmental conditions leading to the bouncing-wave discharge and the subsequent electron beam remain to be investigated in more detailed future studies. The analysis of quasi-static initial electric field changes (IECs) were found at the beginning of all 24 lightning flashes detected within reversal distance (22 Negative Cloud-to-Ground (–CG) and 2 normal Intra-Cloud (IC) flashes) in a tropical storm on June 15th, 2017 close to our station in Malacca, Malaysia (2.314077° N, 102.318282° E). The IECs durations averaged 4.28 ms for –CG flashes (range 1.48 to 9.45 ms) and averaged 11.30 ms for normal ICs flashes (range 7.24 to 15.35 ms). In comparison to Florida storms, the duration of IECs for –CG and IC flashes were 0.18 ms (range 0.08 to 0.33 ms) and 1.53 ms (range 0.18 to 5.70 ms), respectively. Moreover, the magnitudes of E-change for tropical thunderstorm were 0.13 V/m (range 0.03 to 0.44 V/m) for –CG flashes and -0.20 V/m (range -0.13 to -0.27 V/m) for IC flashes. The E-change magnitudes of tropical flashes are significantly larger than Florida flashes.

Keywords
breakdown pulse, cloud to ground flash, intra-cloud flash, lightning initiation, quasi-static electric field, environmental factors
National Category
Meteorology and Atmospheric Sciences
Identifiers
urn:nbn:se:uu:diva-381389 (URN)000461678300045 ()
Available from: 2019-04-09 Created: 2019-04-09 Last updated: 2019-04-09Bibliographically approved
Nanayakkara, S., Fernando, M. & Cooray, V. (2019). Features of K-Changes Observed in Sri Lanka in the Tropics. Atmosphere, 10(3), Article ID 141.
Open this publication in new window or tab >>Features of K-Changes Observed in Sri Lanka in the Tropics
2019 (English)In: Atmosphere, ISSN 2073-4433, E-ISSN 2073-4433, Vol. 10, no 3, article id 141Article in journal (Refereed) Published
Abstract [en]

General characteristics of K-changes, including their duration and probability of occurrence associated with ground flashes in Sri Lanka in the tropics, together with their fine structure, are presented. In 98 ground flashes where the small step changes associated with K-changes are clearly visible, there were about two K-changes per flash on average. The mean K-change time duration observed in this study is 0.38 ms. In 53 of the ground flashes, there were 120 consecutive K-changes. In these cases, the geometric mean of the time interval between K-changes was 12 ms. Analysis of the fine structure of the K-changes reveals the K-changes are always associated with either a chaotic pulse train or a combination of chaotic and regular pulse trains. The results suggest that the small step-like static electric fields identified in the literature as K-changes are the step-like static fields associated with the processes that generate chaotic or a combination of chaotic and regular pulse trains. Thus, at larger distances where the static fields are negligible, K-changes may appear as a chaotic pulse train or a combination of chaotic and regular pulse trains.

Keywords
lightning flash, return stroke, K-change, chaotic pulse train, regular pulse train
National Category
Computer Sciences
Identifiers
urn:nbn:se:uu:diva-382660 (URN)10.3390/atmos10030141 (DOI)000464157700001 ()
Funder
Swedish Research Council, 2015-05026
Available from: 2019-05-07 Created: 2019-05-07 Last updated: 2019-05-07Bibliographically approved
Sabri, M. H., Ahmad, M. R., Esa, M. R., Periannan, D., Lu, G., Zhang, H., . . . Kadir, M. Z. (2019). Initial electric field changes of lightning flashes in tropical thunderstorms and their relationship to the lightning initiation mechanism. Atmospheric research, 226, 138-151
Open this publication in new window or tab >>Initial electric field changes of lightning flashes in tropical thunderstorms and their relationship to the lightning initiation mechanism
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2019 (English)In: Atmospheric research, ISSN 0169-8095, E-ISSN 1873-2895, Vol. 226, p. 138-151Article in journal (Refereed) Published
Abstract [en]

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.

Place, publisher, year, edition, pages
ELSEVIER SCIENCE INC, 2019
Keywords
Fast positive breakdown, Initial breakdown pulse, Initial electric field changes, Narrow bipolar event, Negative breakdown
National Category
Meteorology and Atmospheric Sciences
Identifiers
urn:nbn:se:uu:diva-387710 (URN)10.1016/j.atmosres.2019.04.013 (DOI)000469904100011 ()
Available from: 2019-06-26 Created: 2019-06-26 Last updated: 2019-06-26Bibliographically approved
Rahman, M., Hettiarachchi, P., Cooray, V., Dwyer, J., Rakov, V. & Rassoul, H. (2019). Observations of X-rays from Laboratory Sparks in Air at Atmospheric Pressure under Negative Switching Impulse Voltages. Atmosphere, 10(4), Article ID 169.
Open this publication in new window or tab >>Observations of X-rays from Laboratory Sparks in Air at Atmospheric Pressure under Negative Switching Impulse Voltages
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2019 (English)In: Atmosphere, ISSN 2073-4433, E-ISSN 2073-4433, Vol. 10, no 4, article id 169Article in journal (Refereed) Published
Abstract [en]

We present observations of X-rays from laboratory sparks created in the air at atmospheric pressure by applying an impulse voltage with long (250 µs) rise-time. X-ray production in 35 and 46 cm gaps for three different electrode configurations was studied. The results demonstrate, for the first time, the production of X-rays in gaps subjected to switching impulses. The low rate of rise of the voltage in switching impulses does not significantly reduce the production of X-rays. Additionally, the timing of the X-ray occurrence suggests the possibility that the mechanism of X-ray production by sparks is related to the collision of streamers of opposite polarity.

Keywords
X-rays, high energy radiation, laboratory sparks, switching impulse
National Category
Meteorology and Atmospheric Sciences
Research subject
Engineering Science with specialization in Atmospheric Discharges
Identifiers
urn:nbn:se:uu:diva-338135 (URN)10.3390/atmos10040169 (DOI)000467313400008 ()
Funder
The Swedish Foundation for International Cooperation in Research and Higher Education (STINT), IG2004-2031Swedish Research Council, 621-2009-2697Swedish Research Council, 621-2006-4299Swedish Research Council, 621-2012-3300
Available from: 2018-01-08 Created: 2018-01-08 Last updated: 2019-06-14Bibliographically approved
Guo, J., Rubinstein, M., Cooray, V. & Rachidi, F. (2019). On the Modeling of Non-Vertical Risers in the Interaction of Electromagnetic Fields With Overhead Lines. IEEE transactions on electromagnetic compatibility (Print), 61(3), 631-636
Open this publication in new window or tab >>On the Modeling of Non-Vertical Risers in the Interaction of Electromagnetic Fields With Overhead Lines
2019 (English)In: IEEE transactions on electromagnetic compatibility (Print), ISSN 0018-9375, E-ISSN 1558-187X, Vol. 61, no 3, p. 631-636Article in journal (Refereed) Published
Abstract [en]

This paper proposes a simple method to take into account non-vertical risers through an equivalent partial inductance. The proposed approach was validated considering several examples and taking as reference full-wave results obtained using a numerical electromagnetics code numerical electromagnetics code (NEC)-4.

Place, publisher, year, edition, pages
IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC, 2019
Keywords
Induced current, non-vertical riser, total voltage, transient electromagnetic field, transmission line model
National Category
Telecommunications
Identifiers
urn:nbn:se:uu:diva-390035 (URN)10.1109/TEMC.2019.2903335 (DOI)000471683500005 ()
Available from: 2019-08-07 Created: 2019-08-07 Last updated: 2019-08-07Bibliographically approved
He, L., Azadifar, M., Rachidi, F., Rubinstein, M., Rakov, V. A., Cooray, V., . . . Xing, H. (2018). An Analysis of Current and Electric Field Pulses Associated With Upward Negative Lightning Flashes Initiated from the Santis Tower. Journal of Geophysical Research - Atmospheres, 123(8), 4045-4059
Open this publication in new window or tab >>An Analysis of Current and Electric Field Pulses Associated With Upward Negative Lightning Flashes Initiated from the Santis Tower
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2018 (English)In: Journal of Geophysical Research - Atmospheres, ISSN 2169-897X, E-ISSN 2169-8996, Vol. 123, no 8, p. 4045-4059Article in journal (Refereed) Published
Abstract [en]

We present a study on the characteristics of current and electric field pulses associated with upward lightning flashes initiated from the instrumented Santis Tower in Switzerland. The electric field was measured 15km from the tower. Upward flashes always begin with the initial stage composed of the upward-leader phase and the initial-continuous-current (ICC) phase. Four types of current pulses are identified and analyzed in the paper: (1) return-stroke pulses, which occur after the extinction of the ICC and are preceded by essentially no-current time intervals; (2) mixed-mode ICC pulses, defined as fast pulses superimposed on the ICC, which have characteristics very similar to those of return strokes and are believed to be associated with the reactivation of a decayed branch or the connection of a newly created channel to the ICC-carrying channel at relatively small junction heights; (3) classical M-component pulses superimposed on the continuing current following some return strokes; and (4) M-component-type ICC pulses, presumably associated with the reactivation of a decayed branch or the connection of a newly created channel to the ICC-carrying channel at relatively large junction heights. We consider a data set consisting of 9 return-stroke pulses, 70 mixed-mode ICC pulses, 11 classical M-component pulses, and 19 M-component-type ICC pulses (a total of 109 pulses). The salient characteristics of the current and field waveforms are analyzed. A new criterion is proposed to distinguish between mixed-mode and M-component-type pulses, which is based on the current waveform features. The characteristics of M-component-type pulses during the initial stage are found to be similar to those of classical M-component pulses occurring during the continuing current after some return strokes. It is also found that about 41% of mixed-mode ICC pulses were preceded by microsecond-scale pulses occurring in electric field records some hundreds of microseconds prior to the onset of the current, very similar to microsecond-scale electric field pulses observed for M-component-type ICC pulses and which can be attributed to the junction of an in-cloud leader channel to the current-carrying channel to ground. Classical M-component pulses and M-component-type ICC pulses tend to have larger risetimes ranging from 6.3 to 430s. On the other hand, return-stroke pulses and mixed-mode ICC pulses have current risetimes ranging from 0.5 to 28s. Finally, our data suggest that the 8-s criterion for the current risetime proposed by Flache et al. is a reasonable tool to distinguish between return strokes and classical M-components. However, mixed-mode ICC pulses superimposed on the ICC can sometimes have considerably longer risetimes, up to about 28s, as observed in this study.

Keywords
lightning, upward lightning, return stroke, tall structures, charge transfer mode, lightning electric field
National Category
Meteorology and Atmospheric Sciences Engineering and Technology
Identifiers
urn:nbn:se:uu:diva-357584 (URN)10.1029/2018JD028295 (DOI)000433071200013 ()
Funder
EU, Horizon 2020, 737033-LLR
Available from: 2018-08-17 Created: 2018-08-17 Last updated: 2018-08-20Bibliographically approved
Perera, C., Fernando, M. & Cooray, V. (2018). Analysis of Channel Tortuosity using 8 m Long Laboratory Sparks. In: : . Paper presented at 34th International Conference on Lightning Protection, September 2 - September 7, 2018,Rzeszow, Poland.
Open this publication in new window or tab >>Analysis of Channel Tortuosity using 8 m Long Laboratory Sparks
2018 (English)Conference paper, Published paper (Refereed)
National Category
Meteorology and Atmospheric Sciences
Identifiers
urn:nbn:se:uu:diva-368795 (URN)
Conference
34th International Conference on Lightning Protection, September 2 - September 7, 2018,Rzeszow, Poland
Available from: 2018-12-07 Created: 2018-12-07 Last updated: 2019-03-06Bibliographically approved
Abegunawardana, S., Bodhika, J. A., Nanayakkara, S., Sonnadara, U., Fernando, M. & Cooray, V. (2018). Audible Frequency Analysis of Ground Flashes. In: : . Paper presented at 34th International Conference on Lightning Protection, September 2 - Friday, September 7, 2018, Rzeszow, Poland.
Open this publication in new window or tab >>Audible Frequency Analysis of Ground Flashes
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2018 (English)Conference paper, Published paper (Refereed)
Abstract [en]

Thunder signatures categorized into three types based on peak pressure and variation in fundamental frequency, have been studied by using acoustic spectrum of thunder. S-transformation has been used to estimate the dominant frequency variation around the peak pressure. The mean fundamental frequencies of type 3 ground and cloud flashes are 160 Hz and 98 Hz respectively. The mean frequencies of type 2 ground and cloud flashes are 108 Hz and 82 Hz respectively.

National Category
Meteorology and Atmospheric Sciences
Identifiers
urn:nbn:se:uu:diva-368747 (URN)
Conference
34th International Conference on Lightning Protection, September 2 - Friday, September 7, 2018, Rzeszow, Poland
Available from: 2018-12-07 Created: 2018-12-07 Last updated: 2019-03-06Bibliographically approved
Bodhika, J. A., Dharmarathna, W., Fernando, M. & Cooray, V. (2018). Characteristics of thunder pertinent to tropical lightning. In: : . Paper presented at 34th International Conference on Lightning Protection, 2-7 September, 2018, Rzeszow, Poland.
Open this publication in new window or tab >>Characteristics of thunder pertinent to tropical lightning
2018 (English)Conference paper, Published paper (Refereed)
National Category
Meteorology and Atmospheric Sciences
Identifiers
urn:nbn:se:uu:diva-368792 (URN)
Conference
34th International Conference on Lightning Protection, 2-7 September, 2018, Rzeszow, Poland
Available from: 2018-12-07 Created: 2018-12-07 Last updated: 2019-03-06Bibliographically approved
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