Mixed Mode of Charge Transfer During an Upward Positive Flash at S&#228;ntis Tower
Authors: Oregel-Chaumont Toma , Šunjerga Antonio , Rubinstein Marcos , Rachidi Farhad 
Published: March 2024

The term &#8220;mixed mode of charge transfer to ground for initial continuous current (ICC) pulses&#8221; in the context of upward lightning flashes was first proposed by Zhou et al. 2011 [1] to describe fast pulses, distinct from the classical M-component mode of charge transfer, superposed on the slowly varying initial-stage current of upward negative flashes they observed at the Gaisberg Tower in Austria. The pulses in question were associated with leader/return-stroke processes occurring in decayed or newly created branches of the plasma channel connecting to the grounded, current-carrying channel, with junction points below the cloud base (height < 1 km) [1,2].Herein, we report, to the best of our knowledge, the first observation of a mixed-mode-type pulse during the initial stage of an upward positive flash that was initiated from the S&#228;ntis Tower in Switzerland. The Mt. S&#228;ntis Lightning Research Facility, which recorded the flash, consists of a current measurement system installed in the mountaintop tower (2.5 km ASL), slow and fast electric field sensors and X-ray detectors 20 m from the tower base, an additional fast E-field sensor 15 km away, as well as full HD cameras and a high-speed camera (HSC) at various distances, among other systems (see &#352;unjerga et al. 2021 for details [3]).The observed flash, categorized as a Type 1 from its current waveform (see Romero et al. 2013 for definition [4]), occurred at 16:24:03 UTC on July 24th, 2021, during the Laser Lightning Rod project [5]. Its &#8220;return stroke&#8221;-like main pulse was confirmed from HSC footage to have been triggered by a downward-connecting leader with a junction height of approximately 369&#177;5 m AGL, well below the defined cut-off of 1 km. Interestingly, though the 12 kA peak current is reasonable for a mixed-mode pulse, the current and E-field risetimes were both >10 &#956;s, more characteristic of a M-component-type ICC pulse [2].These observations are important to improving our understanding of the charge transfer mechanisms in upward lightning flashes, which regularly damage wind turbines, telecommunications towers, and airplanes during take-off and landing.&#160;References:[1] Zhou, H., Diendorfer, G., Thottappillil, R., Pichler, H., Mair, M. (2011). Mixed mode of charge transfer to ground for initial continuous current pulses in upward lightning. In 2011 7th Asia-Pacific International Conference on Lightning (pp. 677&#8211;681). Chengdu, China: IEEE. https://doi.org/10.1109/APL.2011.6110212[2] Zhou, H., Rakov, V. A., Diendorfer, G., Thottappillil, R., Pichler, H., Mair, M. (2015). A study of different modes of charge transfer to ground in upward lightning. Journal of Atmospheric and Solar-Terrestrial Physics, 125&#8211;126, 38&#8211;49. https://doi.org/10.1016/j.jastp.2015.02.008[3] &#352;unjerga, A., Mostajabi, A., Paolone, M., Rachidi, F., Romero, C., Hettiarachchi, P., &#8230; Smith, D. (2021). S&#228;ntis Lightning Research Facility Instrumentation. International Conference on Lightning Protection, 6.[4] Romero, C., Rachidi, F., Rubinstein, M., Paolone, M., Rakov, V. A., Pavanello, D. (2013). Positive lightning flashes recorded on the S&#228;ntis tower from May 2010 to January 2012: POSITIVE LIGHTNING S&#196;NTIS TOWER. Journal of Geophysical Research: Atmospheres, 118(23), 12,879-12,892. https://doi.org/10.1002/2013JD020242[5] Houard, A., Walch, P., Produit, T., Moreno, V., Mahieu, B., &#352;unjerga, A., &#8230; Wolf, J.-P. (2023). Laser-guided lightning. Nature Photonics, 17(3), 231&#8211;235. https://doi.org/10.1038/s41566-022-01139-z