Abstract
Long-range networks for lightning detection and location are affected by skywaves. The electromagnetic waves emitted by lightning discharges propagate in the earth-ionosphere waveguide to large distances, reflected by the Earth and lower ionosphere conductivity.This PhD aims to provide a novel solution to mitigate skywave interference in long-range networks using simulations with recorded waveforms of lightning discharges, named ‘atmospherics’ or ‘sferics’. Traditional long-range lightning detection and location networks use the time of arrival (TOA) technique to determine a single lightning location. This work uses simulations to develop an interferometric method that maps lightning locations into an area by mitigating skywave interference.
To achieve this aim, lightning sferic amplitudes and coherencies are used to calculate waveform banks that form the basis for the simulations. Coherency is a short name for the phase coherency of complex analytic signals. The simulations with the interferometric method using the coherency in a long-range network work generally well. It is found that the coherency waveform bank exhibits a similar number of skywave arrivals and less attenuated ground waves when compared to the amplitude waveform bank. The coherency is quantitatively studied for lightning events and background noise for comparison. It is found that the coherency ratio between the lightning event and background noise increases with the event number. This ratio becomes approximately constant when the event number reaches ∼100. The interferometric method maps lightning events into an area with maximum coherency. The lightning waveforms are filtered by the inverse impulse response of the waveform from the amplitude waveform bank to remove the skywave. Then the filtered waveforms are used to simulate the interferometric method, which exhibits only one maximum area.
The maximum coherency of this area is ∼0.7, while the expected coherency from the theoretical calculation is ∼0.6. The improved understanding of lightning skywaves is subsequently used to calculate the height of one particular lightning discharge and the corresponding reflection height of the lower ionosphere. This lightning discharge caused a ‘blue discharge’ near the top of a thundercloud, i.e., a Transient Luminous Event (TLE) recorded from space at blue wavelengths. The altitude of the blue discharge event is estimated to occur between ∼16.0–18.8 km in height. The corresponding ionospheric height is inferred to be 95.0 (±0.5) km by combining two wave propagation models. The arrival times of skywaves from other lightning discharges in the same storm validate the ionospheric height to be ∼93.4 (±0.6) km.
The interferometric method can be further investigated with lightning data collected during various ionospheric scenarios and geographic locations, potentially leading to a real-time lightning location using long-range interferometry. In addition, the altitude of lightning discharges could also be calculated to determine the lightning location in a three-dimensional long-range system with the interferometric method.
| Date of Award | 28 Mar 2024 |
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| Original language | English |
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| Supervisor | Martin Fullekrug (Supervisor) & Biagio Forte (Supervisor) |