First Measurements of Low-Medium Frequency Magnetic Radiation for an Altitude-Triggered Lightning Flash

An altitude-triggered lightning flash was conducted on the Field Experiment Base on Lightning Sciences, China Meteorological Administration, in the summer of 2021. The low-medium frequency magnetic field (B-field) of altitude-triggered lightning was measured for the first time with a sensitive magnetic sensor. The B-field waveforms of precursors and sustained upward positive leader (UPL) show the obvious damped-oscillatory waveform, which is caused by propagation losses and reflections of lightning current at both extremities of steel wire. The propagation of UPL during the inception of attempted downward negative leader (DNL) is revealed by B-field signals with microsecond resolution, showing that the bidirectional leaders develop in an uncoordinated manner. The sustained DNL occurs about 5.5 ms after the inception of sustained UPL in our observations, and the measurements indicate that the stepping of DNL was much more frequent than that of UPL during the stage of sustained propagation.

Automated summary

An altitude-triggered lightning flash was conducted in a field experiment in China, and the low-medium frequency magnetic field of the lightning was measured for the first time using a sensitive magnetic sensor. The measurements revealed that the magnetic field waveforms of precursors and sustained upward positive leaders showed a clear damped-oscillatory pattern, which was caused by propagation losses and reflections of the lightning current. The observations also indicated that the bidirectional leaders developed in an uncoordinated manner, with the sustained downward negative leader occurring about 5.5 ms after the sustained upward positive leader.