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![Current of an upward lightning showing pulses from Mevents while base current is flowing, and showing return strokes after cut-off (adapted from Fuchs[1]).](profile/Lothar-Ruhnke-2/publication/261334754/figure/fig4/AS:667663546187780@1536194802594/Current-of-an-upward-lightning-showing-pulses-from-Mevents-while-base-current-is-flowing.jpg)
Current of an upward lightning showing pulses from Mevents while base current is flowing, and showing return strokes after cut-off (adapted from Fuchs[1]).
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A qualitative model of a branched, positive upward leader is discussed that explains observations of the current cutoff of base current before the end of leader structure development and the fast recovery of nearby electric fields. Screening of cloud electric fields by branching is a powerful mechanism to influence branch cutoffs a well as the birt...
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Citations
... It is a fact that a developing channel with branches can be cut-off from current sources at the ground by screening of the electric field of cloud charges by branched channels [11]. This process results in a cutoff at a branch point or the ground. ...
... Associated with it is a changed distribution of charges along the channel. At steady state conditions high electric fields with develop at the lower end where current were cut off before the tip cut-off, and consequently high potentials will develop near the tip against the environment with possible breakdown conditions for development of recoil leaders and dart leaders as suggested in [11]. This process is likely the dominant mechanism of the generation of M-events and dart leader developments. ...
This paper discusses the electric fields along lightning channels produced by high currents and high temperatures with generation of free electrons by thermal actions. Various experimental data and plasma calculations are reviewed and published relationships between current and associated potential gradient along the channel are discussed. As a result it is shown that a negative relationship exists for all possible lightning currents. A high degree of transient behavior exist for light as well as electric field along a lightning channel that makes it difficult to relate light to current. A further significant result is the realization that the electric field along a lightning channel does not noticeably affect its growth and therefore the assumption of a constant potential is justified for most model calculations on lightning. The slow development of steady conditions for the electric field along a lightning channel produces the concept of a physical description of the generation of recoil and dart leaders.
... So as long as development of new branches continues at the top of the branched leader, there would be two competing trends: a positive current resulted from developing new branches and a "negative current" in the screened parts of the branched leader. Therefore, the electric field on the ground does not need to become zero during the period preceding the current cutoff, as suggested by Ruhnke [2012]. ...
cutoff in lightning channels, which takes place in the development of both intracloud and cloud-to-ground flashes, is still poorly understood. A new evaluation of the conditions leading to current cutoff, and also of the two existing hypotheses of the cutoff mechanism, is the main objective of the paper. We reviewed the literature with results of measurements and modeling of free-burning arcs in a laboratory (the closest analogs of lightning leaders) focusing on the relationship between the internal electric field and current. This relationship governs the leader's behavior in the current cutoff. In our analysis of the mechanisms leading to current cutoff, we identify the two types of current cutoff in lightning channels: the current cutoff in a single, unbranched leader channel, which occurs as the result of reaching the threshold conditions for leader propagation; and the current cutoff in branched leaders, when screening by the leader branches alters the ambient electrical environment, thus diminishing the leader current and causing cutoff at a branching point or at the base of the straight channel that preceded branching. We advance the electrostatic model of the screening effect of branching on current cutoff, introduced by Mazur and Ruhnke (1993), and we provide the evidence of this mechanism from lightning observations. We also critically evaluate the concept of lightning-channel instability, proposed by Heckman (1992), as a suggested mechanism leading to current cutoff. We show that the fundamentals of this concept and therefore the concept in its entirety are invalid.
... Mazur and Ruhnke [15] stopped just short of suggesting a physical mechanism that would lead to base current cutoff without ending development of an upper leader structure. Such a mechanism is discussed in the companion paper [16].Figure 9. The sketch presents the changes in electrical field, seen in the number and density of the electric field lines, near the vertical channel of the ascending and branching positive leader at three different stages (A), and the corresponding changes in the ambient potential and induced charges on the vertical trunk of the leader (B). ...
Current cutoff in lightning channels is an essential part of the lifecycle of all types of lightning flashes, and plays a pivotal role in the way a lightning flash develops. Yet it has not been seriously investigated, and is still poorly understood. We review the factors that affect changes in the current of leaders of both polarities, and examine and evaluate two recent hypotheses of the current cutoff mechanism.
This thesis characterizes the internal lightning process called recoil leader, which is responsible for the positive leader redevelopment based on breakdown in many types of lightning discharges. This process is somehow the least analyzed and understood, at the same time that it has been indirectly observed for almost a century. The literature review treated the following subjects: thunderstorm electrical environment for lightning; internal processes of the negative cloud-to-ground lightning (preliminary breakdown, stepped-leader, upward connecting leader, return stroke, subsequent leaders, continuing current and M component, K and J processes), of the positive cloud-to-ground and the upward lightning; recoil leader; and lightning as a bidirectional leader. Two analyses of the positive leader were performed in parallel, focusing on its redevelopment: one based on high-speed camera records of exposed leader (outside the cloud) on 18 events of positive cloud-to-ground and negative upwards, and the other based on high-speed camera and electric field antennas records of hidden leader (inside the cloud) on 16 events of negative cloud-to-ground lightning. Both analyses were discussed together in the following topics: recoil leader characteristics (regime, propagation direction, initiation location and electric field signature); recoil leader in the positive leader structure (branch propagation during recoil leader, location of a recoil leader series, propagation direction and extent of the recoil leader, recoil leader roles, generation rates, particular behaviors in different lightning types); recoil leader interaction with other process (grouping, interactivity, cyclic interactivity, recoil leader generating rates during different processes of the negative cloud-to-ground lightning); and about the recoil leader triggering mechanism. A particularly important result of the present thesis shows the key role of a minimum potential difference over the decayed branch as an important condition for recoil leader triggering. Based on several evidences synthesized from both analyses, a simplified electrical model of the recoil leader triggering was developed, and applied to different lightning types and conditions. Such potential difference is presented as a function of decayed branch temperature and length. In terms of nomenclature, it is also suggested ‘redevelopment leader’ instead of ‘recoil leader’.
Thirty-two negative cloud-to-ground lightning flashes (CG) were observed in the urban area of São Bernardo do Campo, São Paulo, Brazil. The 10 thunderstorms were monitored simultaneously by a high-speed video camera (1000 frames/second and 1024x512 pixels resolution), electric field mill and Brazilian Lightning Detection Network (BrasilDat). The thunderstorms presented intensity of E-field over 10 kV/m, the peak current of CG flashes was estimated from 15kA to 40kA and the CG flashes recorded by camera were located from 3km to 30km from FEI. The optical characteristics of multiple ground termination flashes (MGT) were observed classifying them in branched stepped leader, continue luminosity of strokes, multiple channel flashes and stepped-to-dart leader observations. The MGT flashes presented the average and maximum number of about 3 -9 ground termination per stroke, respectively. This result is higher than the others observed by the authors, 1.7 and 6, respectively. From the total of MGT flashes recorded, 94% presented branched stepped leader and 60% multiple strokes. From the multiple strokes recorded, the most common presented 2 (37%), 4 and 9 strokes (16%). The single flashes presented the stepped leader and the total flash average duration of 9ms and 20ms, respectively, while the multiple flashes presented the stepped leader and the total flash average duration of 12ms and 314ms, respectively. The single flashes recorded the longest duration, 40ms, of stepped leader and the first stroke. Three flashes presented multiple channel flashes (MCF). While the average inter-stroke time interval of MGT flashes is from 40ms to 50 ms, the inter-stroke time interval of MCF is from 90ms to 200ms. This paper presents: the maximum number of Multiple Ground Termination Stroke (MGTS), the longest inter-stroke distance of MCFs and the observation of stepped-to-dart leader (SD). INTRODUCTION The stepped leader of the cloud-to-ground lightning flashes progresses to ground on steps identifying high brightness on branches. Many studies show recording of the long leader branches by VHF/UHF interferometric technique, time of arrival technique, high speed cameras and E-field records (Rakov and Uman 1994; Shao et al. 1995; Krehbiel et al. 2000, Ballarotti et al. 2005; Kong et al. 2009; Ruhnhe 2012). Some long leader branches can extend to the ground creating an additional stroke (Qie and Kong 2007, Kong et al. 2010). It occurs due to mechanism of current cutoff and of the reestablishment of current in the lightning flashes channel (Rakov et al. 2003; Mazur and Ruhnke 2012). According to Mazur and Ruhnke (2012) the branch leaders are not equal between the CG flashes. The negative CG flashes presents leader branches more extensive and top of leaders less luminous than the positive CG flashes. The difference between
