Waveshapes of continuing currents and properties of M-components in natural positive cloud-to-ground lightning (original) (raw)
Related papers
Atmospheric research, 2007
Continuing current is a continuous mode of charge transfer to ground in a lightning flash. The extent to which the continuing current contributes to the total negative charge lowered to earth is large. In order to study the waveshape of the continuing currents of natural flashes, we developed a computational algorithm that analyzes the pixels of the images obtained by a high-speed camera and plots luminosity-versus-time. Tower measurements have shown that during the continuing current phase of the flash the luminosity of the channel is directly proportional to the current that flows through it. Using this information it was possible to infer the continuing current waveshape for 63 natural discharges and classify them into six different types. Statistics on some characteristics of 345 M-components (that occurred during the same 63 events) are also presented. As far as we know, this is the first study on waveshapes of continuing currents for natural lightning.
Journal Of Geophysical Research: Atmospheres, 2018
We present a study on the characteristics of current and electric field pulses associated with upward lightning flashes initiated from the instrumented Säntis Tower in Switzerland. The electric field was measured 15 km from the tower. Upward flashes always begin with the initial stage composed of the upward-leader phase and the initial-continuous-current (ICC) phase. Four types of current pulses are identified and analyzed in the paper: (1) return-stroke pulses, which occur after the extinction of the ICC and are preceded by essentially no-current time intervals; (2) mixed-mode ICC pulses, defined as fast pulses superimposed on the ICC, which have characteristics very similar to those of return strokes and are believed to be associated with the reactivation of a decayed branch or the connection of a newly created channel to the ICC-carrying channel at relatively small junction heights; (3) "classical" M-component pulses superimposed on the continuing current following some return strokes; and (4) M-component-type ICC pulses, presumably associated with the reactivation of a decayed branch or the connection of a newly created channel to the ICC-carrying channel at relatively large junction heights. We consider a data set consisting of 9 return-stroke pulses, 70 mixed-mode ICC pulses, 11 classical M-component pulses, and 19 M-component-type ICC pulses (a total of 109 pulses). The salient characteristics of the current and field waveforms are analyzed. A new criterion is proposed to distinguish between mixed-mode and M-component-type pulses, which is based on the current waveform features. The characteristics of M-component-type pulses during the initial stage are found to be similar to those of classical M-component pulses occurring during the continuing current after some return strokes. It is also found that about 41% of mixed-mode ICC pulses were preceded by microsecond-scale pulses occurring in electric field records some hundreds of microseconds prior to the onset of the current, very similar to microsecond-scale electric field pulses observed for M-component-type ICC pulses and which can be attributed to the junction of an in-cloud leader channel to the current-carrying channel to ground. Classical M-component pulses and M-component-type ICC pulses tend to have larger risetimes ranging from 6.3 to 430 μs. On the other hand, return-stroke pulses and mixed-mode ICC pulses have current risetimes ranging from 0.5 to 28 μs. Finally, our data suggest that the 8-μs criterion for the current risetime proposed by Flache et al. is a reasonable tool to distinguish between return strokes and classical M-components. However, mixed-mode ICC pulses superimposed on the ICC can sometimes have considerably longer risetimes, up to about 28 μs, as observed in this study.
High-speed video observations of positive lightning flashes
2010
Although positive flashes are usually less frequent than negative lightning, their strokes may have high peak currents followed by long continuing current combining two threatening characteristics for lightning protection. The highest directly measured lightning currents and the largest charge transfers to ground are thought to be associated with positive lightning. This study presents some characteristics of natural cloud-to-ground positive lightning from video images obtained with high-speed cameras (up to 8000 frames per second), from electric field measurements, and from information given by the Brazilian Integrated Lightning Detection Network. The characteristics presented in this work are: stroke and channel multiplicity, percentage of single strokes, average interstroke time interval, continuing current and flash duration, leader speed, and peak current distribution. Besides, we present, for the first time in literature concerning positive lightning, the presence of Mcomponents during the continuing current. We also present measurements of 2-D average velocity for four cloud-toground positive leaders.
High-speed video observations of positive lightning flashes to ground
Journal of Geophysical Research, 2010
Although positive flashes are usually less frequent than negative lightning, their strokes may have high peak currents followed by long continuing current combining two threatening characteristics for lightning protection. The highest directly measured lightning currents and the largest charge transfers to ground are thought to be associated with positive lightning. This study presents some characteristics of natural cloud-to-ground positive lightning from video images obtained with high-speed cameras (up to 8000 frames per second), from electric field measurements, and from information given by the Brazilian Integrated Lightning Detection Network. The characteristics presented in this work are: stroke and channel multiplicity, percentage of single strokes, average interstroke time interval, continuing current and flash duration, leader speed, and peak current distribution. Besides, we present, for the first time in literature concerning positive lightning, the presence of Mcomponents during the continuing current. We also present measurements of 2-D average velocity for four cloud-toground positive leaders.
Journal of Geophysical Research, 1982
The characteristics of some radiation field waveforms of lightning from frontal thunderstorms in Sweden are presented. The waveforms are distinctly different from previously published signatures from intracloud discharges. In general, they are similar to the radiation fields produced by return strokes in negative ground flashes except for the initial polarity, but several important differences are found in the detailed characteristics. The zero-to-peak rise times of these waveforms are found to be in the range 5-25 /as. The waveforms begin with an initial portion or front which rises slowly for 3-20 /as to about half of the field peak amplitude. The observed mean values of 13 /as and 9 /as of zero-to-peak rise time and front duration, respectively, of these waveforms are about twice the corresponding values observed for negative return strokes. The mean radiation field peak value, normalized to 100 km, for these waveforms is 2 times that for negative return strokes. Some waveforms were preceded by small-amplitude pulses which are assumed to be produced by a leader process. The mean separation in time of these pulses is about 26 /as, which may be compared with 14 /as observed for negative return strokes. Another important feature is the presence of 'slow tails' in some of these waveforms, indicating the presence of long-lasting currents in their sources. It is suggested that the sources of the observed waveforms are return strokes bringing down positive charge to earth.
Current waveforms associated with positive flashes recorded on the sntis tower in summer 2010
2011
We present in this paper measured current waveforms associated with positive flashes recorded on the Säntis tower, Switzerland, in Summer 2010. About 20% of the recorded flashes were of positive polarity (transported positive charge to ground), all of them being recorded in July and August. This percentage is considerably larger than the values observed in other studies in summer months (3% to 6.5%). The observed current waveforms can be classified into two types. The first type is characterized by three stages: (1) an initial, slowly rising portion lasting a few milliseconds, (2) the main pulse, and (3) a long continuing current that may contain several pulses of both polarities characteristic of M components. The second type of observed positive flashes is characterized by (i) the absence of any initial slowly rising portion, (ii) lower peak currents, and (iii) presence of successive pulses which may be due to an upward stepped leader. The time-derivatives of the current pulses associated with upward stepped leaders are found to be much larger than those of the main pulse. All the observed flashes contained a single main pulse, except for one flash of the second type, which featured two pulses. Our recorded data constitute the first directly-measured evidence of M-components of both polarities during a continuing current lowering positive charge to ground.
Modeling initial breakdown pulses of CG lightning flashes
Journal of Geophysical Research: Atmospheres, 2014
Electric field change waveforms of initial breakdown pulses (IBPs) in cloud-to-ground (CG) lightning flashes were recorded at ten sites at Kennedy Space center, Florida, in 2011. Six "classic" IBPs were modeled using three modified transmission line (MTL) models called MTLL, MTLE, and MTLK. The locations of the six IBPs were obtained using a time-of-arrival method and used as inputs for the models; the recorded IBP waveforms from six to eight sites were used as model constraints. All three models were able to reasonably fit the measured IBP waveforms; the best fit was most often given by the MTLE model. For each individual IBP, there was good agreement between the three models on several physical parameters of the IBPs: current risetime, current falltime, current shape factor, current propagation speed, and the total charge moment change. For the six IBPs modeled, the ranges, mean values, and standard deviations of these quantities are as follows: current risetime [4.8-25, (12 ±6)] μs, current falltime [15-37, (25 ±6)] μs, current speed [0.78-1.8, (1.3 ±0.3)]×10 8 m/s, and charge moment change [0.015-0.30, (0.12 ±0.10)] C km. Currents in the MTLL and MTLE models moved a negative charge −Q downward and deposited an equivalent positive charge +Q along their paths; the mean Q values were 0.35 C for MTLL and 0.71 C for MTLE. MTLK model deposited negative charge along its lower path and positive charge along its upper path with mean values of 0.27 C. Recent research has begun to fill in some of these missing details. Karunarathne et al. [2013] developed a technique called Position By Fast Antenna or PBFA that locates IBPs in (x, y, z, t). This system uses an array of 10 E-change sensors and locates the IBPs with a time-of-arrival technique based on data from at least five sensors. For IBPs within 10 km of the center of the array, the position errors are less than 100 m in x and y and less than 300 m (150 m) for z = 5.0 km (10 km) altitude; the time error within 20 km of the array center is less that 0.25 μs. Karunarathne et al. [2013] showed that IBPs at the beginning of a typical negative cloud-to-ground (CG) flash move negative charge downward while IBPs at the beginning of a typical intracloud (IC) flash move negative charge upward. Bitzer et al. [2013] have developed a similar E-change sensor array (called HAMMA for Huntsville Alabama Marx Meter Array) that locates many types of fast lightning pulses, including IBPs, using a time-of-arrival technique. Evidence of the physical nature of IBPs has been provided by Stolzenburg et al. [2013a] from data collected using a high-speed video camera operating at 50,000-54,000 frames/s. They recorded the initial luminosity in CG flashes. The luminosity came in bursts lasting 20-200 μs and was associated with an KARUNARATHNE ET AL.