Measurement of preliminary breakdown pulse trains in cloud-to-ground lightning using lightning locating systems

The first stroke leader in a cloud-to-ground lightning discharge is thought to be preceded by the initial or preliminary breakdown process which produces a train of relatively large microsecond-scale electric field pulses. In addition to producing the information used to geolocate various types of lightning events (cloud-to-ground return strokes, M-components, ICC pulses, and cloud lightning pulses) and report their parameters such as estimated peak currents, and field waveform characteristics such as risetimes and peak-to-zero times, the sensors in the U.S. National Lightning Detection Network (NLDN) can record the magnetic field waveforms of lightning events. This makes them a valuable tool in studying the phenomenology and physics of lightning discharges. In this study, we use multiple-station measurements of preliminary breakdown pulse trains obtained using NLDN sensors at various distances from the lightning event. Assuming that the preliminary breakdown pulse train in a negative cloud-to-ground discharge is generated when a negatively-charged channel extends downward from the main negative charge region and interacts with the lower positive charge region, we use a modified transmission line model to reproduce the bipolar pulse train electric-field signature associated with preliminary breakdown. The model-predicted waveforms are compared with those measured at different distances by the NLDN sensors.