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Rahman, Mahbubur
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Publications (10 of 64) Show all publications
Hettiarachchi, P., Cooray, V., Diendorfer, G., Pichler, H., Dwyer, J. & Rahman, M. (2018). X-ray observations at Gaisberg Tower. Atmosphere, 9(1), Article ID 20.
Open this publication in new window or tab >>X-ray observations at Gaisberg Tower
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2018 (English)In: Atmosphere, ISSN 2073-4433, E-ISSN 2073-4433, Vol. 9, no 1, article id 20Article in journal (Refereed) Published
Abstract [en]

We report the occurrence of X-rays at ground level due to cloud-to-ground flashes of upward initiated lightning from Gaisberg Tower in Austria which is located at a 1300 m altitude.  This is the first time that the X-rays from upward lightning from a tower top located in high altitude is observed. Measurement was carried out using scintillation detectors installed close to the tower top in two phases from 2011 to 2015. X-rays were recorded in three subsequent strokes of three flashes out of the total of 108 flashes recorded in the system during both phases. In contrast to the observations from downward natural or triggered lightning, X-rays were observed only within 10 µs before the subsequent return stroke. This shows that X-rays were emitted when the dart leader is in the vicinity of the tower top and hence during the most intense phase of the dart leader. Both the detected energy and the fluence of X-rays are far lower compared to X-rays from downward natural or rocket-triggered lightning. In addition to above 108 flashes, an interesting observation of X-rays produced by a nearby downward flash is also presented. The shorter dart-leader channels length in Gaisberg is suggested as a possible cause of this apparently weaker X-ray production.

National Category
Meteorology and Atmospheric Sciences Engineering and Technology
Research subject
Engineering Science with specialization in Atmospheric Discharges
Identifiers
urn:nbn:se:uu:diva-338132 (URN)10.3390/atmos9010020 (DOI)000424095100019 ()
Funder
Swedish Research Council, 2015-05026
Available from: 2018-01-08 Created: 2018-01-08 Last updated: 2018-03-19Bibliographically approved
Johari, D., Cooray, V., Rahman, M., Hettiarachchi, P. & Ismail, M. M. (2017). Characteristics of leader pulses in positive ground flashes in Sweden. Electric power systems research, 153, 3-9
Open this publication in new window or tab >>Characteristics of leader pulses in positive ground flashes in Sweden
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2017 (English)In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 153, p. 3-9Article in journal (Refereed) Published
National Category
Meteorology and Atmospheric Sciences Engineering and Technology
Research subject
Engineering Science with specialization in Atmospheric Discharges
Identifiers
urn:nbn:se:uu:diva-293059 (URN)
Available from: 2016-05-11 Created: 2016-05-11 Last updated: 2017-10-20
Johari, D., Cooray, V., Rahman, M., Hettiarachchi, P. & Ismail, M. M. (2017). Characteristics of leader pulses in positive ground flashes in Sweden. Paper presented at 13th International Symposium on Lightning Protection (SIPDA), SEP 28-OCT 02, 2015, Balneario Camboriu, BRAZIL. Electric power systems research, 153(SI), 3-9
Open this publication in new window or tab >>Characteristics of leader pulses in positive ground flashes in Sweden
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2017 (English)In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 153, no SI, p. 3-9Article in journal (Refereed) Published
Abstract [en]

This paper presents the characteristics of the electric field pulses observed during leader propagation in positive ground flashes. We analysed in detail the electric field changes occurring just before the first return stroke in 51 positive ground flashes during 2014 summer thunderstorms in Uppsala, Sweden. Pronounced leader pulses (having the same polarity as the return stroke) were observed in 22% of the cases. They were observed to occur within 1.4ms before the first return stroke. Interpulse duration ranged from 13.3 to 50.3 mu s with a mean value of 24.7 mu s The peak amplitude of the leader pulses relative to the return stroke peak ranged from 2.7 to 17.8%. The presence of these pulses shows that the leaders propagate in a stepped manner. Based on the leader pulses' time of initiation and average speed of the leader, the distance travelled by the leader was also estimated. One case of positive ground flash preceded by opposite polarity leader pulses just before the return stroke is also reported. To the best of our knowledge, this is the first time that such a case in positive ground flashes is reported. We suggest that these opposite polarity leader pulses are due to the negatively-charged leader branch of a bi-directional leader inside the cloud that propagates towards observation point.

Keywords
Positive ground flash, Positive leader, Negative leader, Stepped-leader, Bi-directional leader, Negatively-charged leader branch
National Category
Electrical Engineering, Electronic Engineering, Information Engineering
Identifiers
urn:nbn:se:uu:diva-340678 (URN)10.1016/j.epsr.2016.11.026 (DOI)000413380500002 ()
Conference
13th International Symposium on Lightning Protection (SIPDA), SEP 28-OCT 02, 2015, Balneario Camboriu, BRAZIL
Available from: 2018-02-05 Created: 2018-02-05 Last updated: 2018-02-05Bibliographically approved
Hettiarachchi, P., Cooray, V., Rahman, M. & Dwyer, J. (2017). Energy Distribution of X-rays Produced by Meter-long Negative Discharges in Air. Atmosphere, 8(12), Article ID 244.
Open this publication in new window or tab >>Energy Distribution of X-rays Produced by Meter-long Negative Discharges in Air
2017 (English)In: Atmosphere, ISSN 2073-4433, E-ISSN 2073-4433, Vol. 8, no 12, article id 244Article in journal (Refereed) Published
Abstract [en]

The energy deposited from X-rays generated by 1 m long laboratory sparks in air created by 950 kV negative lightning impulses on scintillated detectors was measured. Assuming the X-ray energy detected in such sparks results from the accumulation of multiple photons at the detector having a certain energy distribution, an experiment was designed in such a way to characterize their distribution parameters. The detector was screened by a copper shield, and eight series of fifteen impulses were applied by stepwise increasing the copper shield thickness. The average deposited energy was calculated in each series and compared with the results from a model consisting of the attenuation of photons along their path and probable photon distributions. The results show that the energy distribution of X-ray bursts can be approximated by a bremsstrahlung spectrum of photons, having a maximum energy of 200 keV to 250 keV and a mean photon energy around 52 keV to 55 keV.

National Category
Meteorology and Atmospheric Sciences Engineering and Technology
Research subject
Engineering Science with specialization in Atmospheric Discharges
Identifiers
urn:nbn:se:uu:diva-335142 (URN)10.3390/atmos8120244 (DOI)000419179200014 ()
Available from: 2017-11-30 Created: 2017-11-30 Last updated: 2018-02-14Bibliographically approved
Johari, D., Cooray, V., Rahman, M., Hettiarachchi, P. & Ismail, M. M. (2017). Features of the First and Subsequent Return Strokes in Positive Ground Flashes based on Electric Field Measurements. Electric power systems research, 150, 55-62
Open this publication in new window or tab >>Features of the First and Subsequent Return Strokes in Positive Ground Flashes based on Electric Field Measurements
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2017 (English)In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 150, p. 55-62Article in journal (Refereed) Published
Abstract [en]

This paper presents the characteristics of the electric fields produced by the first and the subsequent return strokes observed in positive ground flashes in Sweden. Fifty one positive ground flashes containing 60 return strokes recorded during 2014 summer thunderstorms were analyzed. In our analysis, only 12% of the cases were multiple-stroke while 88% were single-stroke. On average, the number of strokes per flash was 1.20 and the highest number of strokes per flash recorded was four. The geometric mean (GM) value of the interstroke interval was 60 ms while the distance between the first and the subsequent strokes ranged between 4.9 and 46.4km. We found that the average duration of the subsequent strokes parameters were smaller than that of the first strokes. For the first strokes, the GM values of the slow front duration, the fast transition 10-to-90% risetime, the zero crossing time, the zero-to-peak risetime and the 10-to-90% risetime were 8.7 mu s, 1.4 mu s, 29 mu s, 11 mu s and 5.7 mu s, respectively while for the subsequent strokes, the values were 4.0 mu s 0.91 mu s, 11 mu s 5.8 mu s and 3.2 mu s, respectively. Possible reasons for the shorter duration of the subsequent return strokes parameters were discussed.

National Category
Meteorology and Atmospheric Sciences Engineering and Technology
Research subject
Engineering Science with specialization in Atmospheric Discharges
Identifiers
urn:nbn:se:uu:diva-317450 (URN)10.1016/j.epsr.2017.04.031 (DOI)000403744100006 ()
Available from: 2017-03-14 Created: 2017-03-14 Last updated: 2017-10-20Bibliographically approved
Ismail, M. M., Rahman, M., Cooray, V., Fernando, M., Hettiarachchi, P. & Johari, D. (2017). On the possible origin of chaotic pulse trains in lightning flashes. Atmosphere, 8(2), Article ID 29.
Open this publication in new window or tab >>On the possible origin of chaotic pulse trains in lightning flashes
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2017 (English)In: Atmosphere, ISSN 2073-4433, E-ISSN 2073-4433, Vol. 8, no 2, article id 29Article in journal (Refereed) Published
Abstract [en]

In this study, electromagnetic field radiation bursts known as chaotic pulse trains (CPTs) and regular pulse trains (RPTs) generated by lightning flashes were analyzed. Through a numerical analysis it was found that a typical CPT could be generated by superimposing several RPTs onto each other. It is suggested that the chaotic pulse trains are created by a superposition of several regular pulse trains. Since regular pulse trains are probably created by dart or dart-stepped leaders or K-changes inside the cloud, chaotic pulse trains are caused by the superposition of electric fields caused by more than one of these leaders or K-changes propagating simultaneously. The hypothesis is supported by the fact that one can find regular pulse trains either in the beginning, middle or later stages of chaotic pulse trains.

Keywords
regular pulse train, chaotic pulse train, numerical superposition, HF radiation
National Category
Meteorology and Atmospheric Sciences Engineering and Technology
Research subject
Engineering Science with specialization in Atmospheric Discharges
Identifiers
urn:nbn:se:uu:diva-313561 (URN)10.3390/atmos8020029 (DOI)000396166200007 ()
Available from: 2017-01-20 Created: 2017-01-20 Last updated: 2017-11-29Bibliographically approved
Hettiarachchi, P., Rahman, M., Cooray, V. & Dwyer, J. (2017). X-rays from negative laboratory sparks in air: Influence of the anode geometry. Journal of Atmospheric and Solar-Terrestrial Physics, 154, 190-194
Open this publication in new window or tab >>X-rays from negative laboratory sparks in air: Influence of the anode geometry
2017 (English)In: Journal of Atmospheric and Solar-Terrestrial Physics, ISSN 1364-6826, E-ISSN 1879-1824, Vol. 154, p. 190-194Article in journal (Refereed) Published
Abstract [en]

In this experimental work, the influence of the grounded anode geometry is studied on the X-ray production from the laboratory sparks in air at atmospheric pressure when a negative impulse voltage is applied to a high voltage rod which served as a cathode. The result shows that the smaller the diameter of the anode, the higher the energy of X-ray bursts. This observation can be explained by the mechanism that the encounter of negative and positive streamer fronts just before the final breakdown is the event that accelerates electrons to X-ray generating energies, but may not be the only mechanism that generates X-rays.

Keywords
X-rays, laboratory sparks, electrodes, runaway electrons
National Category
Meteorology and Atmospheric Sciences Engineering and Technology
Research subject
Engineering Science with specialization in Atmospheric Discharges
Identifiers
urn:nbn:se:uu:diva-268859 (URN)10.1016/j.jastp.2016.07.012 (DOI)000395952000019 ()
Funder
Swedish Research Council, 621-2012-3300
Available from: 2015-12-10 Created: 2015-12-10 Last updated: 2018-01-08Bibliographically approved
Johari, D., Cooray, V., Rahman, M., Hettiarachchi, P. & Ismail, M. (2016). Characteristics of preliminary breakdown pulses in positive ground flashes during summer thunderstorms in Sweden. Atmosphere, 7(3), Article ID 39.
Open this publication in new window or tab >>Characteristics of preliminary breakdown pulses in positive ground flashes during summer thunderstorms in Sweden
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2016 (English)In: Atmosphere, ISSN 2073-4433, E-ISSN 2073-4433, Vol. 7, no 3, article id 39Article in journal (Refereed) Published
Abstract [en]

This paper presents the characteristics of the preliminary breakdown pulses (PBP) in 51 positive ground flashes recorded during 2014 summer thunderstorms in Sweden. Electric field measurements were conducted remotely using a broadband antenna system (up to 100 MHz) for a recording length of 1 s with 200 ms trigger time. In the analysis, PBP trains were observed in 86% of the cases. Based on the number of trains preceding the first return stroke, the PBP were classified into single and multiple train PBP. Characteristics of the first PBP train were determined and based on the initial polarity of the pulses, three types of PBP were identified. Characteristics of the subsequent PBP trains in the multiple train PBP were also analyzed and they were compared with the first PBP train. Based on the conceptual charge cloud configuration, we found that the inverted dipole is consistent with our observation. We also found that PBP in positive ground flashes during summer thunderstorms in Sweden are weak since the average ratios of the PBP peak to the first return stroke peak lie only between 0.21 and 0.26. Possible reasons for no detection of PBP and the different types of PBP observed were also discussed.

National Category
Meteorology and Atmospheric Sciences Engineering and Technology
Research subject
Engineering Science with specialization in Atmospheric Discharges
Identifiers
urn:nbn:se:uu:diva-280866 (URN)10.3390/atmos7030039 (DOI)000373529700002 ()
Available from: 2016-03-15 Created: 2016-03-15 Last updated: 2017-11-30Bibliographically approved
Diaz, O., Hettiarachchi, P., Rahman, M., Cooray, V. & Vayanganie, S. P. (2016). Experimental study of leader tortuosity and velocity in long rod-plane air discharges. IEEE transactions on dielectrics and electrical insulation, 23(2), 806-812
Open this publication in new window or tab >>Experimental study of leader tortuosity and velocity in long rod-plane air discharges
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2016 (English)In: IEEE transactions on dielectrics and electrical insulation, ISSN 1070-9878, E-ISSN 1558-4135, Vol. 23, no 2, p. 806-812Article in journal (Refereed) Published
Abstract [en]

Long air gap electrical discharges are of particular interest among scientists and engineers working on high voltage techniques and lightning research. In the present work we report experimental results obtained while testing a long rod-plane air gap with positive switching-like voltage impulses to study the velocity and tortuous progression of the leader discharge. Voltage and current waveforms were recorded. Two still digital cameras were used to track the leader tortuous path. By using a fast digital camera, the leader temporal evolution was recorded and its propagation velocity was estimated. Three angles were used to describe the leader tortuous progression.

Keywords
Gas discharges, EHV insulation, EHV measurements
National Category
Other Electrical Engineering, Electronic Engineering, Information Engineering
Research subject
Engineering Science with specialization in Atmospheric Discharges
Identifiers
urn:nbn:se:uu:diva-297874 (URN)10.1109/TDEI.2015.005421 (DOI)000377461500024 ()
Available from: 2016-06-28 Created: 2016-06-28 Last updated: 2017-11-28Bibliographically approved
Baharudin, Z. A., Cooray, V., Rahman, M., Hettiarachchi, P. & Ahmad, N. A. (2016). On the characteristics of positive lightning ground flashes in Sweden. Journal of Atmospheric and Solar-Terrestrial Physics, 138, 106-111
Open this publication in new window or tab >>On the characteristics of positive lightning ground flashes in Sweden
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2016 (English)In: Journal of Atmospheric and Solar-Terrestrial Physics, ISSN 1364-6826, E-ISSN 1879-1824, Vol. 138, p. 106-111Article in journal (Refereed) Published
Abstract [en]

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.

Keywords
Lightning flashes, Positive ground flashes, Positive return strokes, Electromagnetic fields
National Category
Meteorology and Atmospheric Sciences Engineering and Technology
Research subject
Engineering Science with specialization in Atmospheric Discharges
Identifiers
urn:nbn:se:uu:diva-274994 (URN)10.1016/j.jastp.2015.12.014 (DOI)000370769900011 ()
Funder
Swedish Research Council, 621-2006-4299
Available from: 2016-01-27 Created: 2016-01-27 Last updated: 2017-11-30Bibliographically approved
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