一次多回击负地闪放电过程的甚高频辐射和传输特征分析

利用短基线时间差甚高频(VHF)辐射源定位系统对一次多回击负地闪放电过程进行详细研究发现, 负地闪的预击穿、梯级先导、直窜先导及回击后云内放电过程伴随有较强烈的VHF辐射。结合同步观测的闪电快、慢电场变化资料, 分析VHF辐射源时空发展特征发现, 预击穿阶段辐射源在云中的放电通道为双向发展, 平均速度均在104 m s-1量级, 预击穿下行分支直接转化为梯级先导, 并产生多个分支通道同时向地面发展, 先导平均速度在105 m s-1量级。继后回击之前先导过程均产生多个分支通道, 直窜先导平均速度在105～106 m s-1量级, 新开辟的梯级先导速度在105 m s-1量级。闪击间及地闪后期云内放电活动较为复杂, 主要表现为辐射源从闪电起始区域发展, 进一步延伸云内闪电通道。地闪后期多次负极性K变化过程(Kitagawa and Kobayashi, 1958)主要表现为负极性流光沿之前的正极性电离通道快速发展, 平均速度在106 m s-1量级。

Analysis on Very High Frequency Radiation and Development Characteristics of a Negative Cloud-to-Ground Lightning Discharge with Multiple Return Strokes

A negative cloud-to-ground lightning flash with multiple return strokes was observed using a short-baseline time-of-arrival lighting VHF (Very High Frequency) radiation location system. Combining the synchronous data of electric field change, the discharge processes of the preliminary breakdown, stepped leaders, dart leaders, and discharges after the last return stroke inside the cloud are analyzed. The results suggest that the preliminary breakdown depicts bi-directional propagations, with the magnitude of the average speeds of two concurrent channels of 10⁴ m s^{-1 in opposite directions. The stepped leaders developed from the downwards channels of preliminary breakdown and split into two downward channels, only one off which contacted the ground. The propagation speeds of the two leader channels were about 105 m s-1}. Dart leaders appeared to initiate the return stroke along the first return stroke discharge channel, at a speed of about 10⁵-10⁶ m s^-1. They were accompanied by several branches developing to the ground simultaneously at a speed of about 10⁵ m s^-1. The discharge processes were notably complex and intensive. VHF radiation was produced during the intermission between return strokes and after the last return stroke. During the discharge between the return strokes and the last return stroke, the radiation started from the source region of preliminary breakdown and extended the discharge channels in cloud. After the last return stroke, several K processes (Kitagawa and Kobayashi, 1958) were found to be caused by the negative streamers propagating backwards along the positive electrified channel. The propagation speeds of the K process were about 10⁶ m s^-1.