Investigation of low frequency electromagnetic waves for long-range lightning location

• Main Author: Liu, Zhongjian
• Other Authors: Fullekrug, Martin ; Astin, Ivan
• Published: University of Bath 2017
• Subjects: 621.3
• Online Access: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.760951

Lightning is the strongest natural electromagnetic radiation source, emitting electromagnetic energy in the frequency range from ~4 Hz to ~300 MHz or more. The location of lightning is calculated based on the received electromagnetic waves. The received electromagnetic waves, or lightning sferics, propagate from the lightning radiation source to the receiver along the ground path and reflections by the ionosphere named sky waves. Particularly for a long-baseline (>400 km) lightning receiver array, the received electromagnetic waves are usually a mixture of the ground wave and sky waves, which easily introduce a certain level of location uncertainty. Lightning sferics and the wave propagation velocity are analysed in order to mitigate the interference from long distance wave propagation. The complex lightning sferics are calculated by the Hilbert transform, which provides additional information regarding the instantaneous phase and frequency. The time differences calculated from the instantaneous phases are closer to the phase delay time introduced by the speed of light when compared to other possible signal processing methods. It is also found that the instantaneous frequencies at maximum amplitudes in the waveform bank are distance dependent, which has a potential application, i.e., to determine the distance between the lightning location and the receiver. The radio waves from two submarine communication transmitters at 20.9 kHz and 23.4 kHz exhibit phase propagation velocities that are ~0.51% slower and ~0.64% faster than the speed of light as a result of sky wave contributions and ground effects. Therefore, a novel technique with a variable phase propagation velocity is implemented for the first time using arrival time differences. The lightning locations inferred from variable velocities improve the accuracy of locations inferred from a fixed velocity by ~0.89–1.06 km when compared to the lightning locations reported by the UK MetOffice. The velocity map inferred from the calculated phase propagation velocities reflects the impact of sky waves and ground effects on the calculation of lightning locations as a result of the network configuration. Overall, the wave propagation issues are mitigated by analysis of the complex waveform and the variable phase propagation velocity. Finally, three interferometric methods, 2D lightning mapping, cross-correlation with a short time window, and lightning locations inferred from each sample, are proposed here in order to take advantage of the greater number of samples and information from the recordings.