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Features of the Electric Fields Generated by Lightning with Special Attention to Positive Ground Flashes
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity.
2017 (English)Doctoral 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.

Place, publisher, year, edition, pages
Uppsala: Acta Universitatis Upsaliensis, 2017. , p. 49
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1588
Keywords [en]
positive ground flashes, preliminary breakdown pulses, multiple-trains PBPs, positive leader, bidirectional leader, negative leader branch, subsequent return strokes, distant lightning
National Category
Meteorology and Atmospheric Sciences Engineering and Technology
Research subject
Engineering Science with specialization in Atmospheric Discharges
Identifiers
URN: urn:nbn:se:uu:diva-331885ISBN: 978-91-513-0128-0 (print)OAI: oai:DiVA.org:uu-331885DiVA, id: diva2:1151052
Public defence
2017-12-11, Häggsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala, 13:00 (English)
Opponent
Supervisors
Available from: 2017-11-20 Created: 2017-10-20 Last updated: 2018-03-07
List of papers
1. Characteristics of preliminary breakdown pulses in positive ground flashes during summer thunderstorms in Sweden
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
2. Characteristics of leader pulses in positive ground flashes in Sweden
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
3. Features of the First and Subsequent Return Strokes in Positive Ground Flashes based on Electric Field Measurements
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
4. Oscillations Observed in Bipolar Electric Fields from Distant Cloud-to-Ground Lightning Flashes
Open this publication in new window or tab >>Oscillations Observed in Bipolar Electric Fields from Distant Cloud-to-Ground Lightning Flashes
(English)Manuscript (preprint) (Other academic)
Abstract [en]

This paper presents the observations of distant positive and negative ground flashes at distances up to 1000 km recorded during 2014 summer thunderstorms in Sweden. We observed that the electric field waveforms have a typical shape, like a distorted 'W' (or a distorted ‘M’ for negative ground flashes) followed by small oscillations. Compared to positive ground flashes, the small oscillations were more pronounced in negative ground flashes. All of the positive ground flashes were single-stroke while 74% of the negative ground flashes had subsequent return strokes (SRS). The SRS in negative ground flashes showed wave shapes similar to those of the first return strokes. For both the positive and negative ground flashes, preliminary breakdown pulse (PBP) trains were observed in 71% of the cases. The average time separation between the largest PBP and the ground wave were much larger for positive ground flashes compared to that of the negative ground flashes. Based on the difference in time intervals between the ground wave and the first sky wave, the ionospheric reflections heights were estimated to be 71 and 67 km for positive and negative ground flashes, respectively, corresponding probably to a D-layer of the ionosphere. We have also determined the amplitudes of the small oscillations relative to the return stroke peak, the period of the oscillations and their corresponding frequencies. We suggest that the small oscillations observed in the waveforms either come from the low frequency component of the return stroke current that oscillates in the lightning channel or due to the filtering of the frequencies by the propagation through the Earth-ionosphere waveguide.

National Category
Meteorology and Atmospheric Sciences Engineering and Technology
Research subject
Engineering Science with specialization in Atmospheric Discharges
Identifiers
urn:nbn:se:uu:diva-331837 (URN)
Available from: 2017-10-18 Created: 2017-10-18 Last updated: 2017-10-20

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