Digitala Vetenskapliga Arkivet

Change search
Refine search result
1 - 10 of 10
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Rows per page
  • 5
  • 10
  • 20
  • 50
  • 100
  • 250
Sort
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
  • Standard (Relevance)
  • Author A-Ö
  • Author Ö-A
  • Title A-Ö
  • Title Ö-A
  • Publication type A-Ö
  • Publication type Ö-A
  • Issued (Oldest first)
  • Issued (Newest first)
  • Created (Oldest first)
  • Created (Newest first)
  • Last updated (Oldest first)
  • Last updated (Newest first)
  • Disputation date (earliest first)
  • Disputation date (latest first)
Select
The maximal number of hits you can export is 250. When you want to export more records please use the Create feeds function.
  • 1.
    Ahmad, Mohd Riduan
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Mohd Esa, Mona Riza
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Johari, Dalina
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Ismail, Mohd Muzafar
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Cooray, Vernon
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Chaotic Pulse Train in Cloud-to-Ground and Cloud Flashes of Tropical Thunderstorms2014Conference paper (Refereed)
    Abstract [en]

    In this paper, we report for the first time the observation of chaotic pulse train (CPT) preceding natural subsequent negative return strokes and also CPT occurrence in IC flashes from tropical thunderstorms in South Malaysia. In CG flashes, all CPTs were occurred in between return strokes with 41.1% have occurred between the first and second return strokes. The maximum number of CPT in one sequence is 3, which can be observed between the first and third return strokes only. In IC flashes, all CPTs were observed to occur in between IC flash pulses.

  • 2.
    Ismail, M. M.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Sharma, S. R.
    Cooray, Vernon
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Hettiarachchi, P.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Rahman, Mahbubur
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Johari, D.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    HF and VHF electric field radiation produced by preliminary breakdown process pertinent to Swedish thunderstorms2015In: 2015 Asia-Pacific International Conference on Lightning (APL), Nagoya, Japan, 2015Conference paper (Refereed)
  • 3.
    Ismail, Mohd Muzafar
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. Univ Teknikal Malaysia Melaka, Fac Elect & Comp Engn, Telecommun Engn, Durian Tunggal 76100, Malacca, Malaysia..
    Rahman, Mahbubur
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Cooray, Vernon
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Fernando, Mahendra
    Univ Colombo, Dept Phys, Colombo 03, Sri Lanka..
    Hettiarachchi, Pasan
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Johari, Dalina
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. Univ Teknol Malaysia, Ctr Power Elect Engn Studies, Fac Elect Engn, Shah Alam 40450, Selangor, Malaysia..
    On the possible origin of chaotic pulse trains in lightning flashes2017In: Atmosphere, E-ISSN 2073-4433, Vol. 8, no 2, article id 29Article in journal (Refereed)
    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.

    Download full text (pdf)
    fulltext
  • 4.
    Ismail, Mohd Muzafar
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Rahman, Mahbubur
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Cooray, Vernon
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Sharma, Shriram
    Hettiarachchi, Pasan
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Johari, Dalina
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Electric field signatures in wideband, 3 MHz and 30 MHz of negative ground flashes pertinent to Swedish thunderstorms2015In: Atmosphere, E-ISSN 2073-4433, Vol. 6, no 12, p. 1904-1925Article in journal (Refereed)
    Abstract [en]

    In this study, the electric field signatures of negative ground flashes pertinent to the Swedish thunderstorms were recorded simultaneously during the summer of 2014 using wide (up to 100 MHz) and narrow (at 3 MHz and 30 MHz as central frequencies) bandwidth antenna systems. The electric field signatures were recorded for a time duration of 250 ms. In the analysis, the whole flash was considered and a total of 98 flashes were chosen where electric field signatures of all wideband, 3 MHz and 30 MHz signals were present. It is observed that preliminary breakdown pulses are stronger radiators at 3 and 30 MHz compared to the return strokes. A comparison of our results with those of the previous studies obtained from different geographical regions clearly shows that the strength of preliminary breakdown pulses is higher in the temperate region (Sweden for instance) and is a function of latitude.

    Download full text (pdf)
    fulltext
  • 5.
    Ismail, Muzafar M.
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. FKEKK Univ Tekn Malaysia Melaka, Durian Tunggal, Malaysia.
    Cooray, Vernon
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Hettiarachchi, Pasan
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Rahman, Mahbubur
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Fernando, M.
    Univ Colombo, Dept Phys, Colombo 3, Sri Lanka.
    Johari, Dalina
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. FKE Univ Teknol Mara, Shah Alam, Malaysia.
    On the possible origin of chaotic pulse trains in lightning flashes2015In: 2015 International Symposium on Lightning Protection (XIII SIPDA), Balneário Camboriú, Brazil, 28th Sept. – 2nd Oct. 2015., 2015, p. 409-412Conference paper (Refereed)
    Abstract [en]

    it is suggested that the chaotic pulse trains observed in lightning flashes are created as a result of the superposition of several regular pulse bursts produced by dart leader type discharge processes in the cloud. The similarity between the individual pulses in both chaotic and regular pulse bursts strengthen this suggestion.

  • 6.
    Johari, Dalina
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Features of the Electric Fields Generated by Lightning with Special Attention to Positive Ground Flashes2017Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    This thesis focuses on the main processes in positive ground flashes and the distant lightning environment for both positive and negative ground flashes. It presents the characteristics of the preliminary breakdown pulses (PBPs), the characteristics of the electric field pulses observed during leader propagation, and the characteristics of the electric fields produced by the first and the subsequent return strokes. It also features the observations of distant positive and negative ground flashes at distances up to 1000 km. The results were based on electric field measurements conducted remotely during summer thunderstorms in Sweden in 2014.

    We found that the majority of the positive ground flashes were preceded by PBPs. Some were preceded by more than one PBP train, and the parameter values for the subsequent PBP trains were found to be smaller than the values for the first PBP train. Three types of PBPs were also identified. The results suggest that the PBPs in positive ground flashes during summer thunderstorms in Sweden are weak, and the inverted dipole charge cloud configuration is consistent with our observation.

    A small percentage of these positive ground flashes were observed to be preceded by pronounced leader pulses. The presence of these pulses suggests that the leaders propagate in a stepped manner. We inferred that these pulses were due to the upward-connecting negative leader since their characteristics were similar to those of a negative stepped leader. On the basis of the leader pulses’ time of initiation and the average speed of the leader, we estimated the distance travelled by the leader. One case of positive return stroke preceded by negative leader pulses was also observed, and the occurrence of these pulses was the first in positive ground flashes to be reported.

    The majority of these positive ground flashes were found to be single-stroke. Comparison between the first and the subsequent return strokes showed that the average durations of the subsequent stroke parameters were smaller than that of the first strokes. The distances reported by the lightning location system suggest that the subsequent strokes probably created new terminations to ground. Two possible reasons were given to explain the reason for the shorter duration of the subsequent return strokes parameters compared to the first strokes.

    Finally, observations of distant positive and negative ground flashes showed that the electric field waveforms have a typical shape, like a distorted ‘W’ (or distorted ‘M’ for negative ground flashes) followed by small oscillations. These small oscillations were more pronounced in negative ground flashes, especially at a greater distance. The heights of the ionospheric reflections estimated for both ground flashes were found to likely correspond to a D-layer of the ionosphere. Two possible reasons were suggested for the small oscillations observed in the waveforms.

    Download full text (pdf)
    fulltext
    Download (jpg)
    presentationsbild
  • 7.
    Johari, Dalina
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. Univ Teknol MARA, Shah Alam, Malaysia.
    Cooray, Vernon
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Hettiarachchi, Pasan
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Rahman, Mahbubur
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Ismail, Mohd Muzafar
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. Univ Tekn Malaysia Melaka, Durian Tunggal, Malaysia.
    Some characteristics of leader pulses in positive cloud-to-ground flashes2015In: 2015 International Symposium On Lightning Protection (Xiii SIPDA), 2015, p. 283-287Conference paper (Refereed)
    Abstract [en]

    Compared to negative ground flashes, not much is known about the characteristics of leaders in positive ground flashes. This paper presents some characteristics of the electric field pulses observed during leader propagation in positive ground flashes. We analyzed in detail the electric field changes produced by 50 positive ground flashes during 2014 summer thunderstorms in Uppsala, Sweden. Pronounced leader pulses were observed in 22% of the cases. They were observed to occur 1.4 ms or less before the first return stroke. Interpulse duration ranged from 13.3-50.3 mu s with a mean value of 24.7 mu s. The largest relative amplitude ranged from 2.7-17.8 % of the return stroke peak. No dependence of the pulses occurrence were observed with distance. However, the relative amplitude were found to decrease with distance. The presence of these pulses suggests that the leaders propagate in a stepped manner. One case of positive ground flashes preceded by negative polarity pulses just before the return stroke is also reported.

  • 8.
    Johari, Dalina
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. Faculty of Electrical Engineering, Centre for Electrical Power Engineering Studies, Universiti Teknologi Mara, 40450 Shah Alam, Selangor, Malaysia.
    Cooray, Vernon
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Rahman, Mahbubur
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Hettiarachchi, Pasan
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Ismail, Mohd Muzafar
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. Faculty of Electronics and Computer Engineering, Telecommunication Engineering Department, Universiti Teknikal Malaysia Melaka, Hang Tuah Jaya, 76100 Durian Tunggal, Malacca, Malaysia.
    Characteristics of leader pulses in positive ground flashes in Sweden2017In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 153, p. 3-9Article in journal (Refereed)
    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 1.4 ms before the first return stroke. Interpulse duration ranged from 13.3 to 50.3 μs with a mean value of 24.7 μs. 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.

  • 9.
    Johari, Dalina
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. Univ Teknol Mara, Fac Elect Engn, Ctr Elect Power Engn Studies, Shah Alam 40450, Selangor, Malaysia.
    Cooray, Vernon
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Rahman, Mahbubur
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Hettiarachchi, Pasan
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Ismail, Mohd Muzafar
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. Univ Tekn Malaysia Melaka, Fac Elect & Comp Engn, Telecommun Engn Dept, Durian Tunggal 76100, Malacca, Malaysia.
    Features of the First and Subsequent Return Strokes in Positive Ground Flashes based on Electric Field Measurements2017In: Electric power systems research, ISSN 0378-7796, E-ISSN 1873-2046, Vol. 150, p. 55-62Article in journal (Refereed)
    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.

  • 10.
    Johari, Dalina
    et al.
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. Univ Teknol Mara, Fac Elect Engn, Ctr Power Elect Engn Studies, Shah Alam 40450, Selangor, Malaysia.
    Cooray, Vernon
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Rahman, Mahbubur
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Hettiarachchi, Pasan
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
    Ismail, Muzafar
    Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. Univ Tekn Malaysia Melaka, Fac Elect & Comp Engn, Telecommun Engn Dept, Durian Tunggal 76100, Malacca, Malaysia.
    Characteristics of preliminary breakdown pulses in positive ground flashes during summer thunderstorms in Sweden2016In: Atmosphere, E-ISSN 2073-4433, Vol. 7, no 3, article id 39Article in journal (Refereed)
    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.

    Download full text (pdf)
    fulltext
1 - 10 of 10
CiteExportLink to result list
Permanent link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf