Abstract
Recently, a new lightning phenomena, termed needles, has been observed
in both VHF and in optical along positive lightning leaders. They appear
as small ($<$100 m) leader branches that undergo dielectric
breakdown at regular intervals (called twinkles). Providing a coherent
and consistent explanation for this phenomenon is challenging as each
twinkle is a form of negative breakdown that propagates away from the
positive leader. In this work we provide detailed observations of
needles in VHF, observed during two lightning flashes. We show
distributions of different needle properties, including twinkle
propagation speeds, time between twinkles, and needle lengths, among
others. We show a return stroke and multiple recoil leaders that quench
needle activity. We also show that nearby needle activity does not seem
to correlate together, and that needle twinkling can slow down by 10 to
30 percent per twinkle. We conclude by presenting possibilities for how
the positive leader could induce negative propagation away from the
positive channel, and we argue that twinkles can propagate like a
stepped leader or like a recoil leader depending on the temperature of
the needle, which implies that needle twinkles can probably propagate
without emitting VHF.