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We report the first observations of negative intracloud (IC) dart‐stepped leaders accompanied by regular trains of microsecond‐scale pulses, simultaneously detected by shielded broadband magnetic loop antennas and the radio telescope Low Frequency Array (LOFAR). Four investigated pulse trains occurred during complicated IC flashes on 18 June 2021, when heavy thunderstorms hit the Netherlands. The pulses within the trains are unipolar, a few microseconds wide, and with an average inter‐pulse interval of 5–7 μs. The broadband pulses perfectly match energetic, regularly distributed, and relatively isolated bursts of very high frequency sources localized by LOFAR. All trains were generated by negative dart‐stepped leaders propagating at a lower speed than usual dart leaders. They followed channels of previous leaders occurring within the same flash several tens of milliseconds before the reported observations. The physical mechanism remains unclear as to why we observe dart‐stepped leaders, which show mostly regular stepping, emitting energetic microsecond‐scale pulses. Plain Language Summary: Lightning phenomena inside thunderclouds can be explored using their electromagnetic radiation. To study these processes at small temporal and spatial scales, we combine broadband magnetic loop antennas with the Low Frequency Array (LOFAR) radio telescope. Measurements of broadband antennas acquired during a severe Dutch thunderstorm showed pulse sequences composed of tens microsecond‐scale unipolar pulses, which were surprisingly regularly distributed. Such regular pulse trains have been rarely reported from previous observations. When we thoroughly lined up the timestamps of both simultaneously measuring observational systems, we found that the regular broadband pulses perfectly match with localized isolated bursts of energetic very high frequency radiation detected by LOFAR. The 3D mapping of the radio sources of these bursts allowed us to place the investigated events into the context of the parent intracloud (IC) lightning flash. The results revealed negative IC dart leaders, which propagated along the preconditioned channels originally formed by previous positive or negative IC leaders. Some of these dart leaders then exhibited unusual stepping manifested by the observed regular pulses. We assume that a favorable combination of the conductivity of preexisting lightning channels and the strength of the ambient electric field inside thunderclouds might be needed to trigger this unusual stepping. Key Points: We observed intracloud negative dart‐stepped leaders producing regular trains of broadband electromagnetic microsecond‐scale pulsesVery High Frequency sources follow channels of previous leaders occurring within the same flash tens of milliseconds before the reported observationsConductivity of decaying channels and strength of the ambient electric field might act together to trigger this unusual stepping process [ABSTRACT FROM AUTHOR]

We combine electromagnetic measurements with meteorological and lightning detection data to explain an observation of unusually strong preliminary breakdown (PB) produced by a thunderstorm system that developed along the Mediterranean Coast of Southern France in the early hours of 19 June 2013. This multi‐cellular storm was composed of several parallel convective lines in the NW‐SE direction. Our analysis focuses on 10 sequences of energetic electromagnetic PB pulses recorded by two receivers located at different distances from this thunderstorm. The peak currents, which generated these strong PB pulses, reached −36 kA. The initial polarity of all observed energetic PB pulses confirmed the movement of the negative charge downward, as in case of PB pulses preceding negative cloud‐to‐ground discharges. The locations of PB pulses appeared in areas with none or very weak lightning activity. Most PB pulses were initiated in small, short‐living, rapidly moving convective storm cells characterized by low reflectivity values (generally <40 dBZ), weak vertical development, and low flash density. Our findings indicate that the observed thunderstorm might possess temporary strong negatively charged pockets located above a strong positive charge region at low‐level. Such charge arrangement, likely explains our observation of unusually strong PB pulses and the absence of RS pulses in electromagnetic recordings. Key Points: Energetic preliminary breakdown pulses were found in broadband electromagnetic recordings from a multi‐cell summer thunderstormAnalysis of electromagnetic and radar data placed the preliminary breakdown process in small short living cells outside or on edges of the main convective linePresence of strong negative charge pockets and a strong lower positive charge region inside the thundercloud could explain the observation [ABSTRACT FROM AUTHOR]

With LOFAR we have been able to image the development of lightning flashes with meter‐scale accuracy and unprecedented detail. We discuss the primary steps behind our most recent lightning imaging method. To demonstrate the capabilities of our technique we show and interpret images of the first few milliseconds of two intracloud flashes. In all our flashes, the negative leaders propagate in the charge layer below the main negative charge. Among several interesting features we show that in about 2 ms after initiation the primary initial leader triggers the formation of a multitude (>10) negative leaders in a rather confined area of the atmosphere. From these only one or two continue to propagate after about 30 ms to extend over kilometers horizontally while another may propagate back to the initiation point. We also show that normal negative leaders can transition into an initial leader like state, potentially in the presence of strong electric fields. In addition, we show some initial breakdown pulses that occurred during the primary initial leader, and even during two "secondary" initial leaders that developed out of stepped leaders. Key Points: Our new LOFAR imaging procedure can locate over 200 sources per millisecond of flash with meter‐scale accuracyThe primary initial leader breaks up into many (>10) negative leaders of which only one or two continue after 30 msSome negative leaders propagate from the positive charge layer back to get close to the initiation point [ABSTRACT FROM AUTHOR]