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Underlying Physical Mechanisms in Upward Positive Flashes

Oregel-Chaumont, Toma  
•
Šunjerga, A.
•
Kasparian, J.
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June 21, 2025
Journal of Geophysical Research: Atmospheres

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.

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JGR Atmospheres - 2025 - Oregel‐Chaumont - Underlying Physical Mechanisms in Upward Positive Flashes.pdf

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