The initial stage processes of rocket‐and‐wire triggered lightning as observed by VHF interferometry

Abstract

[1] We examine VHF interferometric images, channel-base currents, and broadband electric field waveforms of the initial stage (IS) in two rocket-and-wire triggered lightning flashes. Two types of negative leaders, termed "long-duration" and "short-duration" leaders, were imaged by the VHF interferometers during the IS of the two flashes. There were three leaders that had relatively long durations of more than a few milliseconds. These three leaders were not accompanied by a significant change of channel-base current during their early stages of development, indicating that they corresponded to intracloud (IC) discharges that were not connected to the grounded triggered-lightning channel. Two of these three leaders eventually connected to the triggered-lightning channel and initiated initial continuous current (ICC) pulses. The third long-duration leader apparently developed from the vicinity of an isolated negative charge region toward an upper-level positive charge region and toward a branch of the grounded channel; it served to bridge the positive charge region and the triggered-lightning channel, resulting in the opposite polarity portion of the bipolar ICC. The short-duration negative leaders had durations of some hundreds of microseconds. These negative leaders apparently recoiled along the conductive channels created by branches of the upward positive leader (UPL); they initiated ICC pulses when the grounded channel was sufficiently conductive. It follows that ICC pulses can be initiated either by recoil leaders or via interception of separate in-cloud leaders by a grounded current-carrying channel.

Characteristics of M‐component in rocket‐triggered lightning and a discussion on its mechanism

“…[] (as introduced in section 4.1) is unreasonable. Here, combined with the observations by different authors [e.g., Shao et al ., ; Rakov et al ., ; Mazur and Ruhnke , ; Yoshida et al ., ], we agree with Winn et al . [] on the issue of the generation (or origin process) of M‐component.…”

“…[] and by Yoshida et al . [] concerned such intercloud processes, and their descriptions of M breakdown and streamer (or leader) developments are useful for the further investigation of the M mechanism through the channel, which will be discussed in the following section.…”

“…Recently, Yoshida et al . [] found that M‐component type processes can be initiated at the upper channel extremity by either recoil leaders or via the interception of separate in‐cloud stepped leaders by a grounded current‐carrying channel.…”

An Advanced Model of Lightning M‐Component

“…Shao et al (), who used a narrowband very high frequency (VHF) interferometer, found that the initiation of M‐components was usually associated with the development of an in‐cloud channel making connection to the channel carrying continuing current (CC) to ground. Similar observations for initial continuous current (ICC) pulses and M‐components in rocket‐triggered lightning were reported by Yoshida et al () and Pilkey et al () from VHF imaging of lightning channels by a wideband VHF interferometer and a Lightning Mapping Array, respectively. An example of electric field signature of in‐cloud leader making connection with the continuing‐current‐carrying channel and producing a hook‐shaped signature characteristic of M‐component is found in Figure 3 of Rakov et al ().…”

“…Yoshida et al (), who analyzed wideband VHF interferometric images of the initial stage in two rocket‐triggered lightning flashes in conjunction with the corresponding channel‐base current records, observed that ICC pulses are initiated by either “short‐duration” or “long‐duration” leaders. The former, whose durations were about a few hundreds of microseconds, were inferred to be recoil leaders developing in remnants of previously formed channels, while the latter, which lasted more than a few milliseconds, appeared to be separate in‐cloud leaders developing in virgin air.…”