Features and Origin of Electromagnetic Fields Generated by Lightning Flashes

Abstract: Negative cloud-to-ground (CG) lightning flashes transport negative charge from cloud to ground. Negative ground flashes typically involve various processes identified as preliminary breakdown, stepped leader, return stroke, dart leader, dart-stepped leader, subsequent return stroke, and cloud activity between strokes, such as regular pulse trains and chaotic pulse trains. These processes can be identified through their electromagnetic field signatures. The main focus of this thesis is to document the features and understand the origin of electromagnetic fields, especially the chaotic pulse trains, generated by lightning flashes.Electric field measurements have been used to study lightning flashes in Sweden. The equipment was a parallel flat plate antenna with an analog filter buffer circuit, connected to a digital high speed oscilloscope. Four simultaneous measurements were made: wideband measurement of the E-field (the vertical component) and its time derivative dE/dt, and two narrowband measurements of the E-field, centred around 3 MHz and 30 MHz. Fourier and wavelet transforms were used in the analysis of the measured data.The results show that preliminary breakdown pulses are stronger radiators at 3 MHz and 30 MHz than are the return strokes. A comparison of our results with those of previous studies obtained in different geographical regions clearly shows that the strength of preliminary breakdown pulses decreases with decreasing latitude. It is higher in the temperate regions (Sweden, for instance) and lower in the tropical regions.A comparison of the time derivatives of preliminary breakdown pulses and of the narrow bipolar pulses shows that the physical origin of these two types of pulse is different, even though they may have similar appearances in the broadband fields.This thesis introduces a new procedure to estimate the zero-crossing time of the lightning-generated radiation fields. The procedure is based on the fact that the time integral of the radiation fields generated by a discharge event whose duration is finite is equal to zero, and the zero-crossing time corresponds to the time when the peak of the integral is reached.In addition to tabulating the various statistical parameters and features of Chaotic Pulse Trains (CPTs), it is shown that these pulse trains are created by the simultaneous propagation of several dart-stepped leader type discharges in the cloud. Each dart-stepped leader type discharge generates a Regular Pulse Train (RPT), and these pulse trains combine randomly in time to generate CPTs. This conclusion is based on the results obtained by numerical simulations and by analysing the signatures of these pulse trains using Fourier and wavelet transformations.The results presented in this thesis show that electromagnetic fields, even those measured from a single station, can be used to extract information concerning the physical processes that gave rise to these fields.