1,2,2,4,2,2,2,- 1Retired from University of Groningen, Kapteyn Astronomical Institute, physics, Groningen, Netherlands (o.scholten@rug.nl)
- 2Netherlands Institute for Radio Astronomy (ASTRON), Dwingeloo, The Netherlands
- 3Department of Electrical and Computer Engineering, Duke University, Durham, NC, 27708, USA
- 4Department of Physics and Astronomy and Space Science Center (EOS), University of New Hampshire, Durham NH 03824 USA
- 5Erlangen Centre for Astroparticle Physics (ECAP), Friedrich-Alexander-Universität Erlangen-Nürnberg, 91058 Erlangen, Germany
- 6Deutsches Elektronen-Synchrotron DESY, Platanenallee 6, 15738 Zeuthen, Germany
- 7State Key Laboratory of Severe Weather Meteorological Science and Technology, CMA Key Laboratory of Lightning, Chinese Academy of Meteorological Sciences, Beijing, China
Although strong electric fields have been observed in lightning clouds, these fields are well below the limit to spontaneously initiate a spark that could be the beginning of a lightning flash. Understanding the lightning initiation process is thus one of (if not The) main topics in lightning research.
In this work we present very high frequency (VHF) radio observations using the LOFAR radio telescope [1]. Because of the high resolution and high sensitivity of LOFAR we could observe the faint initiating event for multiple lightning flashes. The new imaging procedure (called ATRI-D) was shown to be able to distinguish different emission sites of VHF pulses on an airplane flying at an altitude of 8 km [2].
The propagating tip of this apparent initiating event carries positive charge, as is generally expected. Our observations show that the propagation speeds of this positive initiating event (PIE) are very similar at about 5 x 10^6 m/s. Very surprisingly, both the e-folding rates in VHF-intensity and peak intensities differ significantly for the investigated flashes and show no correlation with altitude. Additionally, these structures are extremely narrow, with diameters under 0.8 meters, and maintain this confinement over propagation distances exceeding 100 meters. Even more surprising is that subsequent dart leaders do not follow the path of the PIE, implying that the PIE has not formed a well-conducting structure and does not transform into a positive leader.
Lightning initiation is shown to be a very subtle process, in spite of the vigor of a lightning flash, and the high resolution and sensitivity of LOFAR shows, for multiple lightning flashes, that the initiating event is a very weakly radiating, positively charged propagating structure.
1) Olaf Scholten, Steven A. Cummer, Joseph R Dwyer, et al.; A Comprehensive analysis of High Resolution VHF Observations with LOFAR of the Positive Initiating Event for Several Lightning Flashes. ESS Open Archive . December 12, 2025. https://doi.org/10.22541/essoar.176556304.42772793/v1
2) O. Scholten, M. Lourens, et al. (2025) ; Measuring location and properties of very high frequency sources emitted from an aircraft flying through high clouds. Nature Communications, 16 (1), 10572. https://doi.org/10.1038/s41467-025-65667-2
How to cite: Scholten, O., Cummer, S., Dwyer, J., Hare, B., Liu, N., Lourens, M., Nelles, A., Sterpka, C., Turekova, P., and Wu, B.: A Comprehensive analysis of lightning Initiation with LOFAR, EGU General Assembly 2026, Vienna, Austria, 3–8 May 2026, EGU26-6448, https://doi.org/10.5194/egusphere-egu26-6448, 2026.
