The basic principle of radio interferometry is that radio signals measured by separate antennas from a single source add coherently when adjusted for propagation time delays, while pulses from different sources or from random noise add incoherently (Taylor et al., 1999). For a lightning source, the combined signals will result in a received power approximately proportional to the square of the number of antennas and inversely proportional to the square of the distance from each antenna to the source. In contrast, signals from random noise will result in received power approximately proportional to the number of antennas. LOFAR is comprised of thousands of VHF antennas that are distributed all over Europe. For lightning studies, antennas are selected from the Netherlands to provide both large and small antenna separations (also known as baselines). The combination of the low-noise antennas and long baselines provides outstanding image resolution due to the fact that the maximum achievable angular resolution is proportional to λ/d, where λ is the wavelength of the radiation and d is the largest baseline length. Interferometers previously used to study lightning typically consisted of 3-4 antennas separated by a few hundred meters resulting in a resolutions on the order of 1.6° azimuth and 3.5° in elevation with no sensitivity along the radial axis (Tilles et al., 2019). In many cases, the algorithm used is closer to a time-of-arrival technique where only the location of the peaks are extracted from the result of the cross-correlations (Rison et al., 2016;Stock et al., 2014). The LOFAR impulsive imager uses a similar technique to the time-of-arrival, but has the advantage of hundreds of antennas and large baselines (Scholten et al., 2021b). As a result, the impulsive imager achieves source densities of over 200 sources per millisecond (Scholten et al., 2021b). Within this work and the previous impulsive imager, we achieve angular resolutions up to 1 arc sec in azimuth and 2 arc sec in elevation. This results in submeter resolution along both the horizontal (azimuth) and elevation axes, while also achieving 5 m resolution along the radial axis. LOFAR beamforming combines hundreds of antennas and selects baselines of up to 100 km, resulting in images with remarkably high signal to noise ratios and resolutions

As seen in VHF, the first lightning signal detectable above background increases exponentially by two orders of magnitude in 15 µs.• Initiation is likely caused by branching streamers with overall constant propagation speed of 4.8 ± 0.1 × 10 6 m/s during the exponential ramp-up phase.• Mechanism is similar to narrow-bipolar events, but much weaker in VHF power.