  We use the Low Frequency ARray (LOFAR) to probe the dynamics of the stepping
process of negatively-charged plasma channels (negative leaders) in a lightning
discharge. We observe that at each step of a leader, multiple pulses of VHF
(30~--~80 MHz) radiation are emitted in short-duration bursts ($<10\ \mu$s).
This is evidence for streamer formation during corona flashes that occur with
each leader step, which has not been observed before in natural lightning and
it could help explain X-ray emission from lightning leaders, as X-rays from
laboratory leaders tend to be associated with corona flashes. Surprisingly we
find that the stepping length is very similar to what was observed near the
ground, however with a stepping time that is considerably larger, which as yet
is not understood. These results will help to improve lightning propagation
models, and eventually lightning protection models.
