Luminosity characteristics of lightning M components (original) (raw)
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Journal of Geophysical Research: Atmospheres, 2019
We present a new engineering model for the M component mode of charge transfer to ground that can predict the observed electric field signatures associated with this process at various distances, including (a) the microsecond-scale pulse thought to be due to the junction of in-cloud leaders and the grounded, current-carrying channel and (b) the ensuing slow, millisecond-scale pulse due to the M component proper occurring below the junction point. We examine the features of 13 microsecond-scale, fast electric field pulses associated with M component processes in upward negative lightning initiated from the Säntis Tower and recorded 14.7 km from it. Eleven out of the 13 pulses were found to be unipolar with pulse widths in the range of 9.8 to 35 μs, and the other two were bipolar. To model the process that gives rise to microsecond-scale pulses, we hypothesize that the current pulses propagating away from the junction point along the main lightning channel (below the junction point) and along the feeding in-cloud leader channel (branch) carry the same amount of charge. We further assume that the pulse traversing the branch is similar to a subsequent return-stroke (RS) pulse. In the model, the RS-like process is represented by the MTLE model. The millisecond-scale field signature that follows the initial fast pulse in M components at close distances is simulated in our model using the guided-wave M component model. The proposed model successfully reproduces the vertical electric field waveforms associated with M-component processes in upward lightning flashes initiated from the Säntis Tower at 14.7-km distance from the lightning channel, in which both the fast, microsecond-scale and the following slower, millisecond-scale pulses were observed. The model also reasonably reproduces the known features of electric field signatures at close distances (up to 5 km), where the amplitude of the millisecond-scale hook-like pulse is much larger than that of the microsecond-scale pulse, and at far distances (of the order of 100 km), where the microsecond-scale pulses are dominant.
Geophysical Research Letters, 2008
1] We analyzed high-speed video images and corresponding current records for eight upward lightning flashes initiated by the Peissenberg tower (160 m) in Germany. These flashes contained a total of 33 measurable initial stage (IS) current pulses, which are superimposed on steady IS currents. Seven IS pulses had relatively short (<8 ms) 10-to-90% risetimes and 26 IS pulses had relatively long (>8 ms) risetimes. Six (86%) of seven IS current pulses with shorter risetimes each developed in a newly-illuminated branch, and 25 (96%) of 26 IS pulses with longer risetimes occurred in already luminous (current-carrying) channels. These results support the hypothesis that longer risetimes are indicative of the M-component mode of charge transfer to ground, while shorter risetimes are associated with the leader/return stroke mode. Similar results were obtained for M-component pulses that are superimposed on continuing currents following returnstroke pulses. Citation: Flache, D., V. A. Rakov, F. Heidler, W. Zischank, and R. Thottappillil (2008), Initial-stage pulses in upward lightning: Leader/return stroke versus M-component mode of charge transfer to ground, Geophys. Res. Lett., 35, L13812,
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.
FORTE observations of simultaneous VHF and optical emissions from lightning: Basic phenomenology
Journal of Geophysical Research, 2000
Preliminary observations of simultaneous VHF and optical emissions from lightning as seen by the Fast on-Orbit Recording of Transient Events (FORTE) spacecraft are presented. VHF/optical waveform pairs are routinely collected both as individual lightning events and as sequences of events associated with cloud-to-ground (CG) and intracloud (IC) flashes. CG pulses can be distinguished from IC pulses on the basis of the properties of the VHF and optical waveforms but mostly on the basis of the associated VHF spectrograms. The VHF spectrograms are very similar to previous ground-based HF and VHF observations of lightning and show signatures associated with return strokes, stepped and dart leaders, attachment processes, and intracloud activity. For a typical IC flash, the FORTE-detected VHF is generally characterized by impulsive broadband bursts of emission, and the associated optical emissions are often highly structured. For a typical initial return stroke, the FORTE-detected VHF is generated by the stepped leader, the attachment process, and the actual return stroke. For a typical subsequent return stroke, the FORTE-detected VHF is mainly generated by dart leader processes. The detected optical signal in both return stroke cases is primarily produced by the in-cloud portion of the discharge and lags the arrival of the corresponding VHF emissions at the satellite by a mean value of 243 s. This delay is composed of a transit time delay (mean of 105 s) as the return stroke current propagates from the attachment point up into the region of in-cloud activity plus an additional delay due to the scattering of light during its traversal through the clouds. The broadening of the light pulse during its propagation through the clouds is measured and used to infer a mean of this scattering delay of about 138 s (41 km additional path length) for CG light. This value for the mean scattering delay is consistent with the Thomason and Krider [1982] model for light propagation through clouds.
Luminosity of initial breakdown in lightning
Journal of Geophysical Research: Atmospheres, 2013
Time correlated high-speed video and electromagnetic data for 15 cloud-to-ground and intracloud lightning flashes reveal bursts of light, bright enough to be seen through intervening cloud, during the initial breakdown (IB) stage and within the first 3 ms after flash initiation. Each sudden increase in luminosity is coincident with a CG type (12 cases) or an IC type (3 cases) IB pulse in fast electric field change records. The E-change data for 217 flashes indicate that all CG and IC flashes have IB pulses. The luminosity bursts of 14 negative CG flashes occur 11-340 ms before the first return stroke, at altitudes of 4-8 km, and at 4-41 km range from the camera. In seven cases, linear segments visibly advance away from the first light burst for 55-200 ms, then the entire length dims, then the luminosity sequence repeats along the same path. These visible initial leaders or streamers lengthen intermittently to about 300-1500 m. Their estimated 2-D speeds are 4-18 Â 10 5 m s À1 over the first few hundred microseconds and decrease by about 50% over the first 2 ms. In other cases, only a bright spot or a broad area of diffuse light, presumably scattered by intervening cloud, is visible. The bright area grows larger over 20-60 ms before the luminosity fades in about 100 ms, then this sequence may repeat several times. In several flashes, a 1-2 ms period of little or no luminosity and small E-change is observed following the IB stage prior to stepped leader development.
Journal of Geophysical Research, 1999
Using a high-speed digital optical system, we determined the propagation characteristics of two leader/return-stroke sequences in the bottom 400 rn of the channel of two lightning flashes triggered at Camp Blanding, Florida. One sequence involved a dart leader and the other a dart-stepped leader. The time resolution of the measuring system was 100 ns, and the spatial resolution was about 30 m. The leaders exhibit an increasing speed in propagating downward over the bottom some hundreds of meters, while the return strokes show a decreasing speed when propagating upward over the same distance. Twelve dartstepped leader luminosity pulses observed in the bottom 200 m of the channel have been analyzed in detail. The luminosity pulses associated with steps have a 10-90% risetime ranging from 0.3 to 0.8 •s with a mean value of 0.5 •ts and a half-peak width ranging from 0.9 to 1.9 •ts with a mean of 1.3 •ts. The interpulse interval ranges from 1.7 to 7.2 •ts with a mean value of 4.6 •ts. The step luminosity pulses apparently originate in the process of step formation, which is unresolved with our limited spatial resolution of 30 m, and propagate upward over distances from several tens of meters to more than 200 m, beyond which they are undetectable. This finding represents the first experimental evidence that the luminosity pulses associated with the steps of a downward moving leader propagate upward. The upward propagation speeds of the step luminosity pulses range from 1.9x 107 to 1.0x 10 • m/s with a mean value of 6.7x 107 m/s. In particular, the last seven pronounced light pulses immediately prior to the return stroke pulse exhibit more or less similar upward speeds, near 8x 107 m/s, very close to the return-stroke speed over the same portion of the channel. On the basis of this result, we infer that the propagation speed of a pulse traveling along the leaderconditioned channel is primarily determined by the channel characteristics rather than the pulse magnitude. An inspection of four selected step luminosity pulses shows that the pulse peak decreases significantly as the pulse propagates in the upward direction, to about 10% of the original value within the first 50 m. The return-stroke speeds within the bottom 60 rn or so of the channel are 1.3x 108 and 1.5x 108 m/s for the two events analyzed, with a potential error of less than 20%.
Lightning leader characteristics in the Thunderstorm Research International Program (TRIP)
Journal of Geophysical Research, 1982
We have used high speed streaking photographic techniques to time-resolve the luminous components of cloud-to-ground lightning flashes. All recordings were made during our participation in the Thunderstorm Research International Program (TRIP), conducted at the Kennedy Space Center, Florida, during the summers of 1977 and 1978, and at the Langmuir Laboratory, near Socorro, New Mexico, during the summer of 1979. Twenty one dart leaders, four dart-stepped leaders and three stepped leaders were recorded, the majority under daylight conditions. The mean two-dimensional propagation speed of the dart leaders, evaluated over channel lengths less than or equal to 0.8 km above ground, is 11 x 10 6 m/s, with a range of 2.9 to 23 x 10 6 m/s. Several of the dart leaders reveal a decrease in propagation speed as ground is approached. However, four of the dart leaders in two separate flashes show an increase in speed near the ground, an observation not previously reported in the literature. In two multistroke flashes, we examine the variation of dart leader propagation speed along the channel and find very similar behavior for different strokes in the same flash. The speed variations that we observe may be predominantly caused by geometrical variations of the channel. The dart leader propagation speeds reported in this study are compared with the earlier works of Schonland, McEachron, and Kitagawa and Brook. Agreement among the studies is good, with a common range of observed dart leader propagation speed of 2 to 23 x 106 m/s. The major discrepancy among these studies is the observation, by Schonland, of a distribution of dart leader propagation speeds positively skewed toward the lower limit of reported values. Eleven of the dart leaders are analyzed at upper and lower levels along the visible channel to give 22 dart 'lengths.' They range from 7 to 75 m with a mean of 34 m. For these 22 determinations, we calculate a correlation coefficient of 0.85 between the dart length and the dart leader propagation speed. The correlation of greater dart length with higher propagation speed is consistent with the slower decay of channel luminosity due to the greater initial input of energy to the channel by the faster and, presumably, more energetic dart leader. Four dart-stepped leaders are examined in detail with regard to variation of propagation speed, step length, stepping interval, and luminous intensity during propagation between the cloud base and ground. Significant differences in the tendencies of these parameters are found within these four leaders. For example, one dart-stepped leader recording shows a decreasing propagation speed and an increasing step interval near ground, whereas another shows the opposite behavior. In the best event recorded, several of the individual steps reveal a photographic film density structure, with the lower portion of the step exhibiting a distinct, bright tip that fans out into a symmetrically diffuse image in the upper portion of the step. Our analysis indicates that this spread in the upper portion of the step image is not the result of streaking photography distortion but, rather, represents the luminous structure of the step. We estimate that the step image is recorded in less than 1/xs. Consequently, with a measured step length of---20 m, the luminous pulse must propagate along the step at a speed of at least 2 x 10 7 m/s. The mean propagation speed for three stepped leaders is found to be 1.1 x 106 m/s. All three stepped leaders are very faint, and were recorded only in the last 100-200 m above ground. Two stepped leaders and one dart-stepped leader do not propagate completely to ground before initiation of the return stroke. Apparently, these leaders are met by an upward propagating discharge at heights above ground of 20, 30, and 20 m, respectively. Other stepped and dart-stepped leader cases are indeterminate because an obstacle or the horizon prevent the recording of the leaders near the ground. Connecting discharges are not observed for any of the dart leader events with a resolution of 10 m at 5 km, implying that upward discharges initiated by the approach of dart leader do not occur or are substantially less than a few tens of meters in length. Dart leaders, apparently, propagate completely to ground.
The optical and radiation field signatures produced by lightning return strokes
Journal of Geophysical Research, 1982
The optical signals radiated by Florida lightning in the 0.4-to 1.1-/zm wavelength interval have been recorded in correlation with wide-band electric field signatures. The initial light signal from a return stroke tends to be linear for about 15/zs and then rises more slowly to a peak that is delayed by about 60/zs from the electric field peak. The transition between the fast linear portion and the slower rise may be due to the return stroke entering the cloud base. A small percentage of the records indicate that two different branches of the same stepped leader can initiate separate return strokes. The light pulses from cloud discharges tend to be smaller and more slowly varying than those from return strokes. The total optical power radiated by first strokes in the 5-to 35-km range has a mean and standard deviation of 2.3 _+ 1.8 x 10 9 W at peak. Normal subsequent strokes produce 4.8 _+ 3.6 x 108 W at peak, and subsequent strokes preceded by a dart-stepped leader produce 5.4 _+ 2.2 x 108 W. The characteristic widths of 23 subsequent stroke signals range from 103 to 235/•s, with a mean and standard deviation of 158 _+ 33/•s. Analyses of the initial linear slopes of the light signals suggest that the space-and timeaveraged radiance of first strokes is about 1.0 _+ 0.9 x 10 6 W/m during the bright phase and that normal and dart-stepped subsequent strokes produce about 2.5 _+ 1.8 x 105 W/m and 4.3 _+ 3.1 x 105 W/m, respectively. Further analyses suggest that the dependence of the average radiance on the peak electric field, and probably the peak current, is neither linear nor quadratic.
Journal Of Geophysical Research: Atmospheres, 2015
Utilizing time-correlated high-speed video and electric field change data, a seven-stroke lightning flash is described in which the fifth return stroke (RS) occurs 0.80 ms after the fourth RS connects to a different ground location 3.3 km away. The fifth RS is 0.34 ms after an M component starts down the different channel. The fifth stroke involves a dart leader traveling concurrently, though slower than the M component, in a prior channel to ground. There was no indication of leader advance along this path earlier during the fourth RS. The fourth stroke involves a stepped leader that started from the end of an observed prior dart leader branch which did not previously propagate to ground. The concurrent M component and dart leader are preceded by an in-cloud event evidenced by a large-amplitude, fast electric field change pulse, at 6.1 km estimated altitude, inferred as the connection to the channel for the M component. The M component current apparently initiates the dart leader about 40 μs later. A visible channel length of 10,400 m allows for the 2-D propagation speed of the M component luminosity to be estimated in the range of 1.0 to 1.2 × 10 8 m s À1. The concurrent dart leader travels a visible length of 3445 m with 2-D speed of 1.7 × 10 7 m s À1 , similar to other dart leaders in this flash. Luminosity evolution along the channel through the RS and M component is also described. Estimated optical risetimes of three separate M components are 80-200 μs at 520 m above ground. Jordan et al. [1995] used moving film ("streak" camera) records to describe the luminosity evolution at three altitudes along the channel during two M components of a natural cloud-to-ground (CG) flash. In one case, the luminosity was nearly constant with height below cloud base (1 km), while the other case showed a slight decrease with height. Jordan et al. [1995] also found that the M components were brighter than the RS at 600 and 1100 m altitude, while the RS was brighter at the ground. For one case, Jordan et al. [1995] estimated the downward (1-D vertical) speed in the range of 0.7-1.3 × 10 8 m s À1. Malan and Collens [1937] found that the most frequent luminosity duration of M components in their study was 100 μs, and they estimated STOLZENBURG ET AL.