Broadband and HF Radiation from Cloud Flashes and Narrow Bipolar Pulses

University dissertation from Uppsala : Acta Universitatis Upsaliensis

Abstract: Remote measurement of electric field generated by lightning has played a major role in understanding the lightning phenomenon. Even though other measurements such as photographic and channel base current have contributed to this research field, due to practical reasons remote measurements of electric field is considered as the most useful tool in lightning research.This thesis discusses the remotely measured radiation field component of electric field generated by cloud flashes (ICs) and narrow bipolar pulses (NBPs). The associated HF radiation of these events at 3 MHz and 30 MHz are also discussed. To understand the initiation process of these discharges, a comparative study of the initial pulse of cloud flashes against the initial pulse of cloud to ground flashes was conducted. The result suggests that both discharges might have been initiated by similar physical processes inside the thunderclouds. Comparing the features of initial pulse of cloud and ground flashes with that of pulses that appeared in the later stages of cloud flashes suggests that the initiation process involved in both flashes are not very much different from the initiation of cloud flashes at the later stage. The average spectral amplitudes of electric field of full duration cloud flashes (180 ms) showed f  -1 frequency dependence within the interval of 10 kHz to approximately 10 MHz. This is in contrast to the standard f  -2 decrement (or even steeper ) at high frequency region for other lightning processes such as return strokes. It was suggested that small pulses which repeatedly appeared at the later stage of cloud flashes might have contributed to enhance the spectral amplitude at higher frequencies.Electric fields generated by Narrow Bipolar Pulses (NBPs), which are considered as one of the strongest sources of HF radiation, were measured in the tropics of Malaysia and Sri Lanka.  Their features were also studied and show a good agreement with previously published observations of NBPs from other geographical regions. Thorough analyses and observations of these pulses found previously unreported sharp, fine peaks embedded in the rising and decaying edge of the electric field change of NBPs. Therefore it was suggested that these fine peaks are mostly responsible for the intense HF radiation at 30 MHz.

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