Rubin Jiang

Simultaneously measured current, luminosity, and electric field pulses in a rocket‐triggered lightning flash

[1] Current, luminosity, and electric field pulses in a rocket‐triggered negative lightning flash have been analyzed based on the channel base current, high‐speed video images, and electric field changes at 30 m from the channel. Among the 31 distinct current pulses, there are 4 return strokes, 18 typical M components, 5 large M components with unusual large peak current in a range of kiloamperes, 3 initial continuing current (ICC) pulses, and 1 stroke–M component (RM) event which exhibits both return stroke and M component features. The geometric mean of peak current is 13.5 kA, half peak width is 28.4 ms, and risetime from 10% to 90% peak is 1.1 ms for the 4 return strokes, while the corresponding values are 243 A, 400 ms, and 319 ms, respectively, for the 18 typical M components and 5.1 kA, 76.3 ms, and 34.6 ms, respectively, for the 5 large M components. The electric field and current waveforms of ICC pulses exhibit features similar to those of the M components, indicating the similarity of their mechanisms. Detectable optical luminosity is found just prior to all the pulse events, even return strokes. The M components are superimposed on a slowly varying continuing current, while the directly measured current prior to the return strokeisnotsignificant.ThesimultaneouselectricfieldandcurrentwaveformofRMimplies a superposition of dart leader and M incident wave in the channel, and the possible reason is that two branches with common lower portions coexist simultaneously in the upper part of the discharge channel.

Burst of intracloud current pulses during the initial continuous current in a rocket‐triggered lightning flash

For a rocket-triggered lightning flash on 2 August 2013, the measurement with one pair of broadband low-frequency (20–400 kHz) magnetic induction coils registered a long sequence of over 600 intermittent pulses during the initial continuous current. The timescale of these pulses is about 3-8 µs, and the typical interpulse interval is about 30 µs. The source discharges of these pulses, which are not readily detectable in the frequency range (140–300 MHz) of a short-baseline very high frequency (VHF) lightning imaging system, are attributed to the stepping processes when the positive leader propagated into the cloud region. The reversals in the polarity of magnetic pulses are related to the variation in the vertical direction of positive leader propagation as resolved by the VHF imaging system.

Characteristics of a rocket-triggered lightning flash with large stroke number and the associated leader propagation

A negative lightning flash with 16 leader-return stroke sequences, triggered in the summer of 2013 using the classical rocket-and-wire triggering technique, was examined with simultaneous two-dimensional (2D) imaging of very high-frequency (VHF) radiation sources, channel-base current measurement, broadband electric field waveforms and high-speed video images. A total of 28.0 C negative charge was transferred to ground during the whole flash, and the charge transferred during the initial stage was 4.9 C, which is the weakest among the triggered lightning flashes at the SHandong Artificially Triggering Lightning Experiment (SHATLE). The peak current of 16 return strokes ranged from 5.8 to 32.5 kA with a geometric mean of 14.1 kA. The progression of upward positive leader and downward negative (dart or dart-stepped) leaders was reproduced visually by using an improved short-baseline VHF lightning location system with continuous data recording capability. The upward positive leader was mapped immediately from the tip of the metal wire during the initial stage, developing at a speed of about 104 m/s without branches. The upward positive leader and all the 14 negative leaders captured by the 2D imaging system propagated along the same channel with few branches inside the cloud, which might be the reason for the relatively small charge transfer. The 2D imaging results also show that dart leaders may transform into dart-stepped leaders after a long time interval between successive strokes.

Characteristics of lightning leader propagation and ground attachment

The grounding process and the associated leader behavior were analyzed by using high-speed video record and time-correlated electric field change for 37 natural negative cloud-to-ground flashes. Weak luminous grounded channel was recognized below the downward leader tip in the frame preceding the return stroke, which is inferred as upward connecting leader considering the physical process of lightning attachment, though not directly confirmed by sequential frames. For stepped leader-first return strokes, the upward connecting leaders tend to be induced by those downward leader branches with brighter luminosity and lower channel tip above ground, and they may accomplish the attachment with great possibility. The upward connecting leaders for 2 out of 61 leader-subsequent stroke sequences were captured in the frame prior to the return stroke, exhibiting relatively long channel lengths of 340 m and 105 m, respectively. The inducing downward subsequent leaders were of the chaotic type characterized by irregular electric field pulse train with duration of 0.2–0.3 ms. The transient drop of the high potential difference between stepped leader system and ground when the attachment occurred would macroscopically terminate the propagation of those ungrounded branches while would not effectively prevent the development of the existing space stem systems in the low-conductivity streamer zone apart from the leader tip. When the ungrounded branches are of poor connection with the main stroke channel, their further propagation toward ground would be feasible. These two factors may contribute to the occurrence of multiple grounding within the same leader-return stroke sequence.

High‐speed video observation of stepwise propagation of a natural upward positive leader

Using a high-speed video camera operated at 150,000 frames per second, we have documented the upward propagation of an initial positive leader from a 325 m meteorological tower in Beijing. The stepwise development of the upward positive leader was clearly revealed during its self-sustained propagation toward the cloud base, providing the first optical evidence for the stepping processes of a positive leader. The leader developed with definitive intermittent pauses and re-establishment with abrupt jump of the leader top. Obvious brush-like corona zone emitting outward from the leader top was identified in the frame of stepping, and the luminosity waves propagated downward along the already formed channel from the leader top immediately following the stepping, indicating that a current wave was generated at the leader top and subsequently traveled down the channel. The channel luminosity decreased during the leader pause stage, with the corona zone dimmed correspondingly. The positive leader experienced an average interstep interval of 61.7 µs (ranging between 30 µs and 120 µs) and an average 2-D speed of 8.1 × 104 m/s. The transient speed of the step jump was estimated to be larger than 7.3 × 105 m/s with an average step length of 4.9 m. The need of sufficient positive charge accumulation at the leader top could be the main cause for the stepping characteristics.

High‐speed video evidence of a dart leader with bidirectional development

An upward negative cloud-to-ground lightning flash initiated from a high structure was detected by a high-speed camera operated at 10,000 fps, together with the coordinated measurement of electric field changes. Bidirectional propagation of a dart leader developing through the preconditioned channel was observed for the first time by optical means. The leader initially propagated downward through the upper channel with decreasing luminosity and speed and terminated at an altitude of about 2200 m. Subsequently, it restarted the development with both upward and downward channel extensions. The 2-D partial speed of the leaders upward propagation with positive polarity ranged between 3.2 × 106 m/s and 1.1 × 107 m/s with an average value of 6.4 × 106 m/s, while the speeds of the downward propagation with negative polarity ranged between 1.0 and 3.2 × 106 m/s with an average value of 2.2 × 106 m/s. The downward propagation of the bidirectional leader eventually reached the ground and induced a subsequent return stroke.

Characteristics of a negative lightning with multiple‐ground terminations observed by a VHF lightning location system

Propagation characteristics of a negative cloud-to-ground lightning with four leader-return stroke sequences exhibiting four ground terminations, referred to as multiple termination flash, are analyzed using a two-dimensional mapping system of lightning radiation sources in VHF band along with broadband electric field change records. The four stepped leader-stroke sequences were produced from two different branches of the preliminary breakdown process inside the cloud, and the time intervals between each adjacent two leader-stroke sequences were 82.9 ms, 81.4 ms, and 152.2 ms, respectively. The first two stroke sequences were multiple termination strokes (MTSs), and each MTS showed two different ground terminations induced by two leader branches. The corresponding electric field changes exhibited two fast-rising waveforms separated by 1.5 ms and 2.7 ms, respectively, and the second peak was 3–4 times smaller than the first. The second grounding branch in each MTS occurred when its channel was in poor connection conditions to the main stroke channel. The third and fourth leader-stroke sequences connected to the ground with one of the different grounded channels of the second MTS. The dart leaders before these two strokes exhibited chaotic pulse trains (CPTs) with significant slow electric field variations accompanying strong and continuous VHF radiations. The CPTs only occurred in the dart leader stage which propagated with a speed in an order of 107 m/s. Different from dart leader, attempted leader was associated with regular pulse trains and propagated at a speed order of 106 m/s along the previous channel.

Step-like characteristics of an upward positive leader in triggered lightning

An upward positive leader in a rocket-triggered lightning flash was analysed based on high-speed video images with a time resolution of 10,000 fps and channel base current. The luminous intensity at the leader tip was stronger than that at the channel behind it. The average 2-D propagation speed of the leader was 1.0×105 m/s during the early stage of its ascending, with partial speeds ranging from 2.0×104 m/s to 1.8×105 m/s between 130 m and 730 m above ground. The leader speed showed an accelerating tendency with height and time, however, an obvious irregularity of the speed was observed during the ascending of positive leader, which indicates a possible discontinuity of its extending process. Eleven current pulses, with peak values ranging from tens of amperes (A) to about 150 A, were clearly observed during the inception process of the upward positive leader. During the following ascending process, there were obvious fluctuations superimposed on the slow varying continuous current. Both the optical images and current waveforms indicate step-like properties of the upward positive leader.

Characterization of initial current pulses in negative rocket-triggered lightning with sensitive magnetic sensor†

We report the new measurement of initial current pulses in rocket-triggered lightning with a broadband magnetic sensor at 78 m distance. The high sensitivity of our sensor makes it possible to detect weak ripple deflections (as low as 0.4 A) that are not readily resolved in the typical measurements of channel-base current in rocket-triggered lightning experiments. The discernible magnetic pulses within 1 ms after the inception of a sustained upward positive leader from the triggering wire can be classified into impulsive pulses and ripple pulses according to the discernibility of separation between individual pulses. The timescale (usually >20 µs) of ripple pulses is substantially longer than the leading impulsive pulses (with timescales typically <10 µs), and the amplitude is significantly reduced, whereas there is no considerable difference in the inter-pulse pulse. Along with our previous finding on the burst of magnetic pulses during the initial continuous current (ICC) in rocket-triggered lightning, the new measurements suggest that the stepwise propagation might be a persistent feature for the upward positive leader in rocket-triggered lightning, and the stepping of positive leader early in triggered lightning could be characterized with the observation of ripple pulses. The precedence of impulsive magnetic pulse measured at 78 m range relative to the arrival of corresponding current pulse at the channel base indicates that the ionization wave launched by individual stepping of positive leader propagates downward along the triggering wire at a mean velocity of 1.23 × 108 m/s to 2.25 × 108 m/s.

High speed video observation on random stepping and branching of negative leader

Based on the high speed images recorded at a frame rate of 180 kfps, the random stepping and branching behavior of negative lightning leader occurred in the natural atmosphere was analyzed in detail. The images clearly showed the luminous segments (indicative of space leader) that were separated from the leader tip and then their junction to the channel which resulted in enhanced luminosity. The occurrence of multiple space leaders (with different directions) from a same channel tip was an ordinary phenomenon in step process, and their concurrent connection with the already channel was the origin of leader branching, though most of the attempted branches failed to extend further and eventually extinguished. Those same-rooted branches exhibited alternate stepping during the initial extension.