Characteristics of recoil leaders as observed by LOFAR

Abstract

Lightning dart leaders and recoil leaders are current pulses that propagate along previously established lightning channels reionizing the channel. They are poorly understood since it is not known how a previously ionized channel can undergo dielectric breakdown multiple times. In this work we investigate five recoil leaders on two different channels using lightning observations of the LOFAR radio telescope in vhf (where we observe 30--80 MHz) using our TRI-D interferometric imaging algorithm. We show that while recoil leaders do slow down over time on average, they can also clearly accelerate. In addition, the vhf power emitted by the recoil leader is closely correlated with speed. We also investigate the vhf-emitting width of the recoil leaders and show that it is thinner than our meter-scale resolution. This shows that recoil leaders have significant streamer activity in their channel core or at most very inner region of the corona sheath, and that there is either very little, or weakly vhf-emitting, streamer activity in the majority of the corona sheath. Finally, we show that recoil leaders can have small microsecond-scale fluctuations in emitted vhf power that can occur in the same spot across subsequent recoil leaders on the same channel, demonstrating that recoil leader propagation is affected by small-scale channel geometry.