Zhou H., V.A. Rakov, G. Diendorfer, R. Thottappillil, H. Pichler, M. Mair:
A study of different modes of charge transfer to ground in upward lightning
Journal of Atmospheric and Solar-Terrestrial Physics (JASP), 2015
We examine properties of pulses superimposed on the slowly varying initial-stage current in upward flashes initiated from the Gaisberg Tower (GBT), Austria, based on simultaneous measurements of currents, electric field changes, and high-speed video images. These pulses, often referred to as initial continuous current (ICC) pulses, are associated with the M-component mode of charge transfer to ground, if only one branch of the upward lightning channel is active. However, due to multiple branches formed by an upward leader from the tall tower, ICC pulses are often associated with a downward leader/return-stroke process in a decayed (new) channel branch that is connected to another, continuous current carrying channel, with the connection point being some tens to a few hundreds of meters of the tower top. We call this scenario mixed mode of charge transfer to ground, which optically appears as a re-illuminated (previously luminous) or newly illuminated branch connecting to the already luminous channel attached to the tower. If the connection point were a kilometer or more above the tower top (inside the cloud), the resultant ICC pulse measured at the tower top would appear as a “classical” M component, and if it were very close to the tower top (say, within a few meters), the ICC pulse would be characteristic of a return stroke. In contrast to tower-initiated lightning, ICC pulses in rocket-triggered lightning (at least in Florida and China), usually involve only one channel below the cloud base and hence are associated predominantly with the M-component mode of charge transfer to ground. In our data set, ICC pulses associated with the mixed mode of charge transfer to ground exhibit shorter risetimes, larger peaks, and shorter half-peak widths than “classical” M-components, as previously reported for lightning initiated from tall objects by Miki et al. . We found that the mixed mode of charge transfer to ground can also occur in M-components following return strokes in upward lightning.
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