Oscillations Observed in Bipolar Electric Fields from Distant Cloud-to-Ground Lightning Flashes

• 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.

Ämnesord

NATURVETENSKAP  -- Geovetenskap och miljövetenskap -- Meteorologi och atmosfärforskning (hsv//swe)

NATURAL SCIENCES  -- Earth and Related Environmental Sciences -- Meteorology and Atmospheric Sciences (hsv//eng)

Nyckelord

Engineering Science with specialization in Atmospheric Discharges

Teknisk fysik med inriktning mot atmosfäriska urladdningar

Publikations- och innehållstyp

vet (ämneskategori)

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