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The common phenomenon of lightning still harbors many secrets and only recently a new propagation mode was observed for negative leaders. While propagating in this `Intensely Radiating Negative Leader' (IRNL) mode a negative leader emits 100 times more very-high frequency (VHF) and broadband radiation than a more normal negative leader. We have reported that this mode occurs soon after initiation of all lightning flashes we have mapped as well as sometimes long thereafter. Because of the profuse emission of VHF the leader structure is very difficult to image. In this work we report on measurements made with the LOFAR radio telescope, an instrument primarily built for radio-astronomy observations. For this reason, as part of the present work, we have refined our time resolved interferometric 3-Dimensional (TRI-D) imaging to take into account the antenna function. The images from the TRI-D imager show that during an IRNL there is an ionization front with a diameter in excess of 500~m where strong corona bursts occur. This is very different from what is seen for a normal negative leader where the corona bursts happen at the tip, an area of typically 10~m in diameter. The observed massive ionization wave supports the idea that this mode is indicative of a dense charge pocket.
Bibliographical noteFunding Information:
B. M. H. is supported by the NWO [Grant No. VI.VENI.192.071]; The work of the IAP team was supported by European Regional Development Fund-Project CRREAT [Grant No. CZ.02.1.01/0.0/0.0/15-003/0000481] and by the GACR [Grant No. 20-09671S]. This paper is based on data obtained with the International LOFAR Telescope (ILT). LOFAR is the Low Frequency Array designed and constructed by ASTRON. It has observing, data processing, and data storage facilities in several countries, that are owned by various parties (each with their own funding sources), and that are collectively operated by the ILT foundation under a joint scientific policy. The ILT resources have benefitted from the following recent major funding sources: CNRS-INSU, Observatoire de Paris and Université d’Orléans, France; BMBF, MIWF-NRW, MPG, Germany; Science Foundation Ireland (SFI), Department of Business, Enterprise and Innovation (DBEI), Ireland; NWO, The Netherlands; The Science and Technology Facilities Council, UK. The data are available from the LOFAR Long Term Archive (for access see ). To download this data, please create an account and follow the instructions for “Staging Transient Buffer Board data” at . In particular, the utility “wget” should be used as follows: wget https://lofar-download.grid.surfsara.nl/lofigrid/SRMFifoGet.py?surl=“location” where “location” should be specified as: srm://srm.grid.sara.nl/pnfs/grid.sara.nl/data/lofar/ops/TBB/lightning/ followed by L703974_D20190424T210306.154Z_“stat”_R000_tbb.h5 (for Flash B) L703974_D20190424T213055.202Z_“stat”_R000_tbb.h5 (for Flash A) and where “stat” should be replaced by the name of the station, CS001, CS002, CS003, CS004, CS005, CS006, CS007, CS011, CS013, CS017, CS021, CS024, CS026, CS028, CS030, CS031, CS032, CS101, CS103, RS106, CS201, RS205, RS208, RS210, CS301, CS302, RS305, RS306, RS307, RS310, CS401, RS406, RS407, RS409, CS501, RS503, RS508, or RS509. The source code used for improved TRI-D imaging can be found at . All figures in this work have been made using the Graphics Layout Engine (GLE) plotting package. The broadband SLAVIA antenna data used in this study can be retrieved from .
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