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Oscillations Observed in Bipolar Electric Fields from Distant Cloud-to-Ground Lightning Flashes
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. (Lightning research group)
Uppsala University, Disciplinary Domain of Science and Technology, Technology, Department of Engineering Sciences, Electricity. (Lightning research group)
(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: urn:nbn:se:uu:diva-331837OAI: oai:DiVA.org:uu-331837DiVA: diva2:1150358
Available from: 2017-10-18 Created: 2017-10-18 Last updated: 2017-10-20
In thesis
1. Features of the Electric Fields Generated by Lightning with Special Attention to Positive Ground Flashes
Open this publication in new window or tab >>Features of the Electric Fields Generated by Lightning with Special Attention to Positive Ground Flashes
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. 49 p.
Series
Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology, ISSN 1651-6214 ; 1588
Keyword
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:nbn:se:uu:diva-331885 (URN)978-91-513-0128-0 (ISBN)
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: 2017-11-20

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