Speed Of Leader Research Articles

Speed and current for lightning stepped leaders near ground as determined from electric field records

The speed and current during the final stages of the lightning stepped leader were determined from features of the electric field waveform. The average speed of the leader tip was found below the height where dE/dt was zero for close lightning. This height was in the range 0.6–2.0 km for flashes at the distances considered here, 3.6–5.7 km. For 10 stepped leaders observed near Tampa, Florida, the mean final one‐dimensional speed was 4.4×105 m/s, the standard deviation was 3.8×105 m/s, and the range was 1.0–14×105 m/s. These speeds are consistent with other stepped leader speeds reported in the literature that were found using optical techniques. The current was estimated when the leader was near ground from dE/dt during the last hundred microseconds before the return stroke. For 62 stepped leaders observed at distances of 3.6–14 km, the mean final leader current was 1.3 kA, the standard deviation was 1.0 kA, the geometric mean was 1.0 kA, and the range was 110 A to 5.0 kA. Errors resulting from uncertainties in distance, charge center height, calibration, ground channel inclination, and channel branching were found to be about 40–50% rms for both speed and current.

News in brief

Lightning thought Professor T E Allibone (Leeds) contributed a discussion on the speed of a lightning leader to an IEE meeting on 18 October 1982 and thereby caused H Aspden of Chilworth, near Southampton, to ponder anew on the question of the laws which should be applied to open circuit interactions. One's natural tendency is to use the Lorentz force, modified only by considerations of inertia, to calculate the velocity the electrons at the leading edge of a lightning stroke should achieve.

Lightning leader characteristics in the Thunderstorm Research International Program (TRIP)

We have used high speed streaking photographic techniques to time‐resolve the luminous compnents 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 11×106 m/s, with a range of 2.9 to 23×106 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 23×106 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 µs. Consequently, with a measured step length of ∼20 m, the luminous pulse must propagate along the step at a speed of at least 2×107 m/s. The mean propagation speed for three stepped leaders is found to be 1.1×106 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.

Radar as a diagnostic tool for lightning

The radar cross section of a cloud-to-ground lightning stroke is calculated for the two limiting cases of overdense and underdense plasma. By using a model of the decaying channel and smoothly joining these limiting cases, curves of the temporal variation of the radar cross section can be obtained. Results are presented for wavelengths of 3.2, 10, 23, and 150 cm. Comparison with radar noise levels shows that radars of wavelength 10 cm or greater can be used to monitor the conditions within lightning strokes for up to about 100 msec after stroke incidence. Radars with wavelengths much less than 10 cm are not so suitable. With radar it should be possible to determine the lightning channel conditions conducive to dart leader formation, the relationship between initial ionization density and dart leader speed, the ratio of the diameters of leaders and return strokes, some information on the electron density inside leaders and its temporal variation, and also, with radars of smaller wavelength, (3 cm or less) the initial shock expansion rate of the return stroke. The radar cross section of the dielectric discontinuity of the hot air of the channel is shown to be negligible, in agreement with Browne (1951).

Prebreakdown phenomena in standard rod gaps subjected to impulse voltages

The prebreakdown phenomena of impulse corona, corona propagation and leader propagation in the standard rod gap in air have been studied for spacings of up to 30 cm. The variations occurring in the measured quantities of corona-current amplitude, time of occurrence, corona-propagation time, corona speed, number and length of steps in the leaders, leader-propagation time and leader speed are correlated to the variations in the conditions of gap length, applied voltage, humidity and irradiation. The results of the prebreakdown phenomena have been shown to explain the characteristics of impulse breakdown of the rod gap.