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

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.