Abstract
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.