Propagation of preliminary breakdown pulses preceding cloud-to-ground lightning discharges

A sequence of electromagnetic pulses lasting from a few milliseconds to several tens of milliseconds is usually observed prior to the first return stroke of a cloud-to-ground lightning flash. The sequence is composed of three parts. It begins with a group of relatively large bipolar pulses which are believed to be connected with preliminary breakdown processes. They are usually followed by a relatively low and irregular pulse activity. The sequence ends with a train of smaller and faster pulses attributed to the stepped leader.