Dwyer, Joseph

Hare, Brian

Liu, Ningyu

Scholten, Olaf

Sterpka, Christopher

English

We report results from imaging the initiation region of lightning via 3D interferometric beamforming on data collected by the Netherlands-based core of the Low Frequency Array of Antennas (LOFAR). LOFAR achieves 1 nanosecond timing accuracy and meter-scale spatial precision in lightning imaging on pulses observed in the 30-80 MHz band via the 38 Dutch-based stations. This project complements and enhances the previous work of the LOFAR lightning group of Groningen [Hare, B.M., et al., Nature 568, 360363 (2019)], and [Scholten, O., et al., ESSOAr 10503153] in order to improve image detail in regions with weak sources. This project incorporates beamforming techniques to improve upon previously employed methods with the result of improving both spatial and time resolution of lightning sources. In doing so, we have located and imaged the first non-impulsive sources in lightning flashes. These sources are believed to be caused by a streamer-cascade-like initiation event leading to the formation of the first leader in two separate lightning flashes. The initiation starts from essentially background and within a tens of microseconds ramps up a few orders of magnitude before the first impulsive sources connected with lightning leaders are observed. The events are likely an analog of fast breakdown in narrow bipolar events, and here we report their ramp-up rate, propagation speed, and trajectories

Proceedings - University of Groningen

   University of GroningenThe Impulsive Nature of Lightning InitiationSterpka, Christopher; Dwyer, Joseph; Liu, Ningyu; Hare, Brian; Scholten, OlafIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2021Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Sterpka, C., Dwyer, J., Liu, N., Hare, B., & Scholten, O. (2021). The Impulsive Nature of Lightning Initiation.EGU21-13711. Paper presented at EGU General Assembly 2021.https://ui.adsabs.harvard.edu/abs/2021EGUGA..2313711SCopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 29-10-2022The Spontaneous Nature of Lightning InitiationC. Sterpka1, J. Dwyer1, N. Liu1, B. M. Hare2, O. Scholten2 and the LOFAR CR KSP1University of New Hampshire, United States2University of Groningen, Netherlands Under preparation for NaturePrecision Imaging Through Interferometric Beamforming• Sub-meter precision imaging of lightning initiation via interferometric beamforming with LOFAR.• First imaged pulse is 22 μs prior to first IBP.• Galactic background units (gb) are derived from the normalized noise level on single antenna.• (A-D) Source moves at constant velocity approximately 100 m while increasing in intensity.• (E) Source vanishes and new source appears at the inception point of negative leader.Exponential Ramp-up• VHF power versus time showing exponential ramp-up.• e-folding rate: 2.7 +/- 0.4 μs• 15 μs two order of magnitude rise followed by 2 μs order of magnitude drop, but velocity remains constant.• Pulse at end (E) is the inception point of the negative leader, 4 μs later.Velocity Fit• Initiation discharge starts and ends within 10 m of theinception point of the negative leader (red dot and dashed line)• Propagation speed of avalanche discharge is 4.8 +/-0.1 x 106 m/s  (green dots are VHF source locations and blue line is a linear fit)• Velocity is constant throughout the entire ramp-up (time aligns with ramp-up in previous slide)Proposed Discharge Mechanism• (A) Initiation starts with a single streamer, producing first VHF signal.• (B-D) Streamer branches multiple times, producing significant charge separation and larger VHF signal.• (E) Hot leader channel forms near the start of the avalanche due to accumulation of negative charge at tip.• First source is a rapidly propagating and intensifying group of streamers that subsequently produces a hot leader channel near the initiation point, similar to original Griffiths and Phelps 1976.  * A-E align with images shown in slide 2

The Impulsive Nature of Lightning Initiation

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The Impulsive Nature of Lightning Initiation

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Proceedings - University of Groningen