Close electric field signatures of dart leader/return stroke sequences in rocket‐triggered lightning showing residual fields

Abstract

Vertical electric field waveforms due to dart leader/return stroke sequences measured on the ground 15 and 30 m from the negative lightning channel are used to examine the so‐called residual electric field, the difference between the leader electric field change, and the following return stroke electric field change. At these distances, no residual field is expected if the return stroke neutralizes essentially all the charge deposited by the leader within a few hundred meters above ground. There is a clear tendency for strokes having larger peak currents to be associated with larger residual electric fields. The ratio of residual electric fields at 15 and 30 m suggests that the residual field varies as r−1.5, where r is the horizontal distance from the lightning channel. The residual electric field is found from modeling to be associated with an equivalent point charge of the order of hundreds of microcoulombs to a few millicoulombs at a height of 15 to 30 m deposited by the leader but presumably left unneutralized by the return stroke. This residual point charge decays exponentially on a timescale of the order of milliseconds to tens of milliseconds. While the nature of the residual charge is unknown, it could be associated with small branches formed near the descending leader tip just prior to or during the attachment process. In long laboratory spark experiments, such branches have apparently been observed to lose their connection with the main channel.