Cutoff and reestablishment of current in rocket‐triggered lightning

Abstract

.[1] A total of three negative rocket-triggered lightning flashes without return strokes (two from 1997 and one from 1993) are analyzed in this paper in order to study the processes associated with the disintegration of the triggering wire and its replacement by an airplasma channel. It appears that the gap resulting from the vaporization of the triggering wire by the upward-positive leader current is bridged by a leader/return-stroke type process. Electric fields at distances of 50, 110, and 500 m, the corresponding magnetic fields at 500 m, and the currents to ground are examined for the two 1997 flashes. The electric field prior to the triggering wire's vaporization in these flashes exhibits a positive (atmospheric electricity sign convention) millisecond-scale ramp due to the upwardextending positive leader. The electric field changes observed at the three distances just prior to wire vaporization are consistent with an equivalent point charge of about 0.3 C at a height of 1.2 to 1.5 km, suggesting that the charge density distribution at that time is strongly skewed toward the upward positive leader tip. The length of the triggering wire at the time of its vaporization was estimated from still photographs to be about 210-220 m. Following the ramp, a microsecond-scale V-shaped negative pulse, which resembles the close electric field signature of a small dart-leader/return-stroke sequence, is observed. The corresponding magnetic field decreases abruptly, simultaneously with the onset of the leading edge of the V-shaped pulse, to values near zero and remains there for tens of microseconds, indicating the attempted interruption (cutoff) of the upward positive leader current flow to ground through the triggering wire as it is vaporized by this current. Following the abrupt decrease, the magnetic field exhibits a rapid increase at a time corresponding to the trailing edge of the V-shaped electric field pulse, suggesting that the vaporized triggering wire is replaced by an air-plasma channel that becomes part of the upward positive leader channel when electrical connection to ground is restored. For the third triggered lightning flash, from 1993, similar inferences regarding the processes involved in the replacement of the triggering wire by an air-plasma channel are made from high-speed (streak) photography and from measurements of the current to ground and electric field at 30 m. In this flash, the upward positive leader exhibited very pronounced stepping: step current pulses had peaks, as measured at the ground, that were up to a few kiloamperes, and step charges that were up to 100 mC. Characterization of the attempted interruption and the following reestablishment of current to ground in rocket-triggered lightning may have important implications for the understanding of channel current cutoff in natural lightning flashes and may provide new insights into the formation of strokes observed to occur in the same channel within a millisecond or less.INDEX TERMS: 3304

RF and X‐ray source locations during the lightning attachment process

“…Flash UF‐0707, a rocket‐and‐wire triggered flash, was initiated on 31 July 2007 at approximately 19:36 UT. This flash consisted of an initial stage (as in all negative triggered lightning flashes, involving an upward positive leader, destruction of the triggering wire, its replacement with a plasma channel, and subsequent steady current flow between cloud and ground [e.g., Rakov et al , 2003]) followed by two leader/return‐stroke sequences, with each stroke terminating to the launcher mounted on the top of an 11 m tower. The focus in this section is the first of these two strokes.…”

Luminous pulses during triggered lightning

“… Shao et al [1995, p. 2780] classify M components into early and late‐type M events. The early type are “those which are produced by positive streamer–fast negative recoil sequences immediately following return strokes,” and the late type are “those produced by negative breakdown into the channel, later in a continuing current.” Rakov and Uman [2003, p. 181] suggest that the two types “may differ only in the way in which the negative charge source is connected to the conducting channel to ground, not in the processes occurring in that channel.” Rakov et al [2001] show that M components transfer negative charge to ground and consist of a superposition of a downward incident wave and an upward reflected wave with nearly equal current amplitudes. The waves are guided by the preexisting conducting channel.…”

“…A summary of research on pulses and currents in lightning flashes can be found in the book by Rakov and Uman [2003, chapters 4 and 7]. For more recent results and reviews on triggered flashes and flashes to towers see the articles by Rakov et al [2003], Miki et al [2005], Campos et al [2007], Flache et al [2008], Campos et al [2009], Akita et al [2010], and Qie et al [2011].…”

An evaluation of the performance characteristics of the U.S. National Lightning Detection Network in Florida using rocket‐triggered lightning

“…The presence of the wire enhances the electric field produced by the cloud charge source. If the conditions are right, an electric discharge (known as a leader) propagates upward from the top of the wire, resulting in a flow of current in the wire, and when this current is sufficiently high (about 100 amperes) for a sufficient period of time (some milliseconds), the wire explodes and is replaced by a conducting plasma channel [ Rakov et al , 2003]. Following this process, a steady current of a few hundred amperes (known as the initial continuous current or ICC) flows to ground for several hundred milliseconds.…”