LOFAR Observations of Dart Leader Starts

Dart leaders are a poorly understood lightning phenomenon where a current pulse propagates quickly (~10^7 m/s) along a previously established, now decayed, plasma channel, resulting in a re-heating of the channel. It is not understood how dart leaders propagate or how they get started. Therefore, in this work we have imaged the beginning of multiple dart leaders with the LOFAR radio telescope. We have observed two interesting phenomena related to the start of dart leaders. Firstly, we regularly observe other discharges, such as needles or `mini’ dart leaders, hundreds of microseconds before the start of the dart leader. The `mini’ dart leaders are particularly fascinating, as they propagate up side branches over a few hundred meters (towards the positive leader branch tip) before stopping. The main dart leader then initiates after the `mini’ dart leader. The exact connection between these preceding discharges (needles and mini-darts) and the main dart leaders, if one triggers the other, or why mini-darts ought to occur at all, are difficult to understand. In addition, previous work has shown that dart leaders tend to start with an exponential rise in VHF power and speed. In this work we find that some dart leaders have a period at their beginning where they propagate relatively slowly with weak VHF emission before a period of exponential growth. In one particular case, a dart leader initiated on a side branch, propagated slowly (~5x10^6 m/s) and weakly for about for about 100 µs until it connected with the main leader branch, and only then accelerated to a high speed (~ 1.7x10^7 m/s) over a period of about 50 µs. Finally, we will attempt to relate our measurements to recent hypothesis that dart leaders are a new kind of propagation that essentially amounts to a heating wave; the dart leader charge pushes a weak current in-front of it that heats up the plasma channel which in turn allows the current to increase further and the main charge packet to move forward.