Xinsheng Liu

Spatial and temporal properties of optical radiation produced by stepped leaders

The relative light intensities as a function of height and time for two negative downward stepped leaders, A and B, recorded by a high-speed digital 16 × 16 photodiode array photographic system, are studied. For leader A it is found that the light waveform for each segment of the leader channel starts with a series of sharp light pulses followed by several slow-rising and longer-lasting light surges, with both the light pulses and surges superimposed on a continuous luminosity slope that has a long rising front followed by an almost constant light level. Analysis indicates that each light pulse involves a step process; it originates at the leader tip and appears to propagate upward, with the pulse amplitude suffering little degradation within the first several tens of meters to 200 m from the leader tip up (bright tip length) but with a severe attenuation above. The light surges are observed to be almost constant in amplitude above the bright tip, and for one of them an upward propagation speed of the order of 108 m/s is inferred. From appearances of the light pulses it is determined that the leader A has an overall velocity of 4.5–11.2×105 m/s, a step interval of 5–50 μs, and a step length of 7.9–19.8 m. For leader B the step light pulses are found to propagate from the leader tip back up at a speed of 0.14–1.7×108 m/s, and the overall leader velocity, the step interval, and the step length are determined to be about 4.9–5.8×105 m/s, 18–21 μs, and 8.5 m, respectively. In addition, on the basis of the light waveforms of the leader A it is inferred that the current of a stepped leader may consist of two parts: an impulsive current within the bright tip and a continuing current above it. After propagating along the bright tip up, because of increasing resistance and capacitance of the leader channel the impulsive current rapidly transforms into part of the continuing current.

Experiment of artificially triggering lightning in China

Triggering lightning experiment with rocket—wire technique was conducted in Gansu, northwestern China. Ten cases of triggered lightning were obtained in three summers from 1989 to 1991. They were all with positive electric field at ground (positive charge overhead) under the influence of dominant lower positive charge of Gansu thundercloud. Triggered lightnings were weak discharges initiated by upward moving negative leaders followed by continuous current processes. No dart leader—return stroke processes were observed. The discharge lasted about several tens of miliseconds with a peak current of only a few hundred amperes. Simple physical models are used to evaluate the velocity of leader propagation, magnitudes, and altitudes of the charge transferred by triggered lightnings. Results are consistent with the features of lower positive charge of Gansu thunderstorms.

Broadband interferometer observations of a triggered lightning

The development of positive leader of an artificially triggered lightning has been analyzed based on the data of electric field change, location of radiation source and frequency spectrum obtained by using the broadband interferometer system. The results indicate that radiation from positive leader could be detected within close distance in spite of the relatively weak radiation, while the radiation from negative breakdown processes was relatively stronger. Positive leader developed with few branches, and the initial progression velocity was of the order of 105 m/s. The distribution of power spectrum by 25 MHz high pass filter indicated that the radiation frequency from positive leader maximized at 25–30 MHz, while that from negative breakdown processes maximized at 60–70 MHz.

Bidirectional propagation of lightning leader

The characteristics of initial leader of triggered lightning flashes are studied by use of a high-speed digital camera system with the rate of 1 000 frame per second and simultaneous measurements of current and electric field change with 0.1 μs time resolution. The results show that the altitude-triggered lightning was initiated by bidirectional propagation leader because of “current interruption” by air gap, whereas the grounded-triggered one was not. The onset of downward-moving positive leader was prior to that of the upward-going negative leader for 843 ms in an altitude-triggered lightning under the positive ambient electric field. The downward-moving leader, regardless of its polarity, would produce inhomogeneous distribution of luminosity (current) along the channel on the moment of connection to the ground.