Underlying Physical Mechanisms in Upward Positive Flashes

TL;DR

This paper reports the first observation of a mixed charge transfer mode in upward positive lightning flashes, revealing complex physical mechanisms and challenging existing classifications through high-speed imaging and electric measurements.

Contribution

It introduces the first direct observation of a mixed charge transfer mode in upward positive flashes and links it to unique pulse characteristics, expanding understanding of lightning physics.

Findings

Recoil leader connects to grounded plasma at <1 km height.

Main pulse exhibits mixed-mode and M-component characteristics.

Sequence of electric field pulses indicates step-like propagation.

Abstract

This study presents the first observation of a mixed mode of charge transfer during an upward positive flash, which was initiated from the S\"antis 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 mixed mode 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 (ICC) 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.