Upward leaders from instrumented lightning rods competing to connect a
downward leader during a lightning attachment process
Abstract
In this paper we analyze electric-field and current measurements of
competing upward leaders induced by a downward negative lightning flash
that struck a residential building. The attachment process was recorded
by two high-speed cameras running at 37,800 and 70,000 images per second
and the current measured in two lightning rods. Differently from
previous works, here we show, for the first time, the behavior of
multiple upward leaders that after initiation compete to connect the
negative downward moving leader. At the beginning of the propagation of
the leaders that initiate on the instrumented lightning rods, current
pulses appear superimposed to a steadily increasing DC current. The
upward leader current pulses increase with the approach of the downward
leader and are not synchronized but present an alternating pattern. All
leader speeds are constant. The upward leaders are slower than the
downward leader speed. The average time interval between current pulses
in upward leaders is close to the interstep time interval found by
optical or electric field sensors for negative cloud-to-ground stepped
leaders. The upward leaders respond to different downward propagating
branches and, as the branches alternate in propagation and intensity, so
do the leaders accordingly. Right before the attachment process the
alternating pattern of the leaders ceases, all downward leader branches
intensify, and consequently upward leaders synchronize and pulse
together. The average linear densities for upward leaders (49 and 82
µC/m) were obtained for the first time for natural lightning.