Underlying Physical Mechanisms in Upward Positive Flashes

This study presents the first observation of a mixed mode (MM) of charge transfer during an upward positive flash, which was initiated from the Säntis Tower in Switzerland. High-speed camera footage, along with current and electric field measurements, revealed a downward-propagating recoil leader connecting to the grounded current-carrying plasma channel at a junction height of <1 km above the tip of the tower. This event triggered the “return stroke”-like main pulse associated with Type 1 upward positive flashes, leading us to propose a MM of charge transfer (normally observed in upward negative flashes) as the physical mechanism at play. Furthermore, the observed “Main pulse” shared characteristics with both mixed-mode and M-component-type initial continuous current pulses, challenging existing classification criteria, and supporting the notion of a unique mode of charge transfer with a range of junction length-dependent pulse characteristics, as opposed to two distinct modes. The recoil leader itself was accompanied by a sequence of fast electric field pulses indicative of step-like propagation, also an observational first. These findings contribute to improving our understanding of the mechanisms of charge transfer in upward lightning flashes.