We have used the LOw-Frequency ARray (LOFAR) to search for the growing tip of an intra-cloud (IC) positive leader. Even with our most sensitive beamforming method, where we coherently add the signals of about 170 antenna pairs, we were not able to detect any emission from the tip. Instead, we put constraints on the emissivity of very-high frequency (VHF) radiation from the tip at 0.5 pJ/MHz at 60 MHz, integrated over 100 ns. The limit is independent on whether this emission is in the form of short pulses or continuously. We conclude that IC positive leaders propagate in a continuous process which is in sharp contrast to what is seen to the step-wise propagation seen in some cloud-to-ground positive leaders and for all negative leaders.

We have used the LOw-Frequency ARray (LOFAR) to image a few lightning flashes during a particularly severe thunderstorm. The images show an exceptional amount of VHF activity at altitudes above 10 km. Much of this is in the form of small-scale discharges occurring seemingly randomly around the centers of active storm cells. Because of their small and incidental structure we refer to these as ‘speckles’. A detailed investigation shows strong evidence that these speckles are indicative of positive leader channels and that they are equivalent to the needle activity seen around positive leader tracks at lower altitudes.

Four upward propagating streamers in 2 carrot sprites over northwest Texas were recorded at 100,000 frames per second at 07:46:35 UT, 2 June 2019, from the New Mexico Tech Langmuir Laboratory near Socorro, New Mexico. The streamers reached velocities of 50-80×106 m/s with accelerations up to 25×1010 m/s/s. The upward motion ended with the top of the streamers near 90 km altitude. At this time the streamers reached maximum brightness. The streamer head then dissolved and the brightness decayed exponentially with time constants between 0.078 and 0.097 ms. We interpret the dissolution and decay to be the result of interaction with the bottom of the ionosphere. Assuming the decay to be dictated by electric field relaxation, the ambient D region electron density may be derived. The analysis suggests that sprite observations can provide multipoint measurements of the D region ionosphere impacted by powerful lightning capable of triggering sprites.

Recent observations from LOFAR indicate that multiple, spatially distributed corona bursts can occur in lightning processes on the order of 10 microseconds. The close proximity of the corona bursts in space and time poses a great observation challenge for dedicated lightning radio interferometers, typically with $<$100 m baselines. This paper reports simulations to show the interferometry results that would be obtained with a typical lightning interferometer for such a lightning process. In particular, spatially-separated corona bursts at fixed locations may be seen as a fast ($>10^7$ m/s) propagating source for an instrument with resolution greater than the spatial separation of the bursts. The implications and suggestions for lightning interferometry studies are discussed in the paper.

Brief bursts of high frequency (HF) and very high frequency (VHF) radio emissions unaccompanied by strong low frequency radiation have been observed during initiation and propagation of lightning or thunderstorm electrical breakdown without leading to fully-fledged lightning. This paper investigates a physical mechanism to generate such radio bursts by electrical discharge activity inside a thundercloud. When a discharge consists of many high frequency emission sources, such as streamers, that generate currents in random directions, its radiation spectrum peaks in the HF and VHF bands, and the spectral magnitudes in low frequencies are much smaller or even negligible. Combined with recent observational findings, the present study suggests that lightning initiation may begin with a short burst of many randomly occurring small-scale discharges in a localized thundercloud region.

Here, we present new radio interferometer beamforming observations of lightning initiation using data from the Low Frequency Array (LOFAR). We show that the first lightning source in the flash increases exponentially in intensity by two orders of magnitude in 15 microseconds, while propagating 88 meters away from the initiation location at a constant speed of 4.8 ± 0.1 x 10^6 m/s. A second source replaces the first source at the initiation location, and subsequent propagation of the lightning leader follows. We interpret the first source to be a rapidly propagating and intensifying positive streamer discharge that subsequently produces a hot leader channel near the initiation point. How lightning initiates is one of the greatest unsolved problems in the atmospheric sciences, and these results shed light on this longstanding mystery.

We report on ultra-slowly propagating discharge events with speeds in the range 1-13 km/s, much lower than any known lightning process. The propagation speeds of these discharges are orders of magnitude slower than leader or streamer speeds, but faster than the ion drift speed. For one particular event, a lightning leader forms about 40 ms later within 50 m of the discharge, likely within the same high field region. A second slow event forms 9 ms prior to the initiation, and leads into the negative leader. Most slow events appear to not be directly involved with lightning initiation. This suggests that the classic streamer cascade model of initiation is not always a definitive process. In this work we describe these discharge events displaying unique behavior, their relation to common lightning discharges, and their implications for lightning initiation.