Properties of the unusually short pulse sequences occurring prior to the first strokes of negative cloud-to-ground lightning flashes

摘要

We analyze broadband magnetic and electric field measurements of pulse sequences occurring prior to first return strokes of negative cloud-to-ground lightning flashes. Our observations take place a few tens of kilometers from the thunderstorm but we also report the first simultaneous observations of preliminary breakdown pulses from a distance of 400 km. Their amplitudes reach up to 50% of the corresponding return stroke peak and typically decrease by ~40% during the sequences. A typical time interval between neighboring pulses was several tens of microseconds. We observe an unusually short duration of the prestroke activity (1-7 ms) reported for the first time during a summer thunderstorm, with a low height of initiation (3-4 km). A very fast propagation speed (~10~6 m/s) is probably maintained for the entire prestroke process. A possible explanation can be based on a hypothesis of unusually strong negative charge sources in the observed thundercloud.