Analysis on variation of return stroke velocity with frequency at the channel bottom of cloud to ground lightning
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摘要: 回击通道的光学观测对于研究回击发展过程和物理机制、完善工程回击模型都具有重要的意义。采用雷电发展过程高速光学观测系统(LiPOS)测量了人工引雷中距离通道底部21 m至309 m的一组通道发光波形,基于系统辨识的输出误差(OE)模型建立了回击向上传播过程光辐射脉冲间的传递函数,获得了1 kHz~1 MHz频段内群速度和相速度曲线。时域分析表明,通道辐射光脉冲波形前沿上升时间由靠近底部的1.1 μs变化为309 m高度处的1.84 μs,色散特性是其变化的重要原因。频域分析表明,OE模型能够有效抑制测量噪声的影响、获得清晰的分析曲线,100 kHz以下低频段内群速度呈显著的非单调变化,500 kHz以上区域群速度稳定在58%光速左右。将分析结果与相关文献的研究进行了对比和讨论。研究结果对于定量评价回击电流传播色散特性具有重要参考价值。Abstract: Observation of the return stroke channel is a key approach in understanding return stroke's developing process and mechanism, and therefore to improve the engineering model. A lightning progression feature photic observation system (LiPOS) is used to observe the bottom luminosity from 21 m to 309 m in the artificially triggered lightning. The output-error (OE) model in system identification is applied to establish the upward propagation transfer function between different luminosity pulses. The curves of phase velocity and group velocity between 1 kHz to 1 MHz are obtained. Time-domain analysis shows that the risetime of the leading edge changes from 1.1 μs at the bottom to 1.84 μs at 309 m height, and the dispersion in velocity is the key factor to induce this effect. The frequency-domain analysis shows that the OE model can suppress the measurement noise and give a clear frequency distribution curve. Below 100 kHz, the group velocity curve has a range with strong non-monotonic variation, and the curve becomes flat and reaches 58% of light velocity above 500 kHz. The analyzed results are finally compared to the research work in the reference literature and some discussions are presented. The results may find further applications in evaluating dispersions in return strokes quantitively.
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Key words:
- lightning /
- return stroke velocity /
- dispersion /
- optical observation /
- transfer function
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表 1 S5~S8的前沿上升时间
Table 1. Risetime of the waveforms of channel S5~S8
channel height/m rise time/μs S5 20.64 1.10 S6 116.64 1.32 S7 212.64 1.56 S8 308.64 1.84 表 2 不同传播高度对应的时域平均速度
Table 2. Risetime of the waveforms of channel S5~S8
indicators time domain delay/μs time domain speed/(108 m·s−1) 10% 50% oblique
intercept10% 50% oblique
interceptS5~S6 0.52 0.52 0.50 1.85 1.85 1.92 S5~S7 1.24 1.24 1.10 1.55 1.55 1.75 S5~S8 1.98 2.18 1.86 1.45 1.32 1.55 -
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