Development and Application of a Low Frequency Near-Field Interferometric-TOA 3D Lightning Mapping Array

Abstract:

Lightning processes radiated extremely wideband electromagnetic signals. Lightning images mapped by VHF interferometry and VHF time of arrival lightning mapping arrays enable us to understand the lightning in-cloud detail development during the extent of flash that can not always be captured by cameras because of the shield of cloud. Lightning processes radiate electromagnetically over an extremely wide bandwidth, offering the possibility of multispectral lightning radio imaging. Low frequency signals are often used for lightning detection, but usually only for ground point location or thunderstorm tracking. Some recent results have demonstrated lightning LF 3D mapping of discrete lightning pulses, but imaging of continuous LF emissions have not been shown. In this work, we report a GPS-synchronized LF near field interferometric-TOA 3D lightning mapping array applied to image the development of lightning flashes on second time scale. Cross-correlation, as used in broadband interferometry, is applied in our system to find windowed arrival time differences with sub-microsecond time resolution. However, because the sources are in the near field of the array, time of arrival processing is used to find the source locations with a typical precision of 100 meters. We show that this system images the complete lightning flash structure with thousands of LF sources for extensive flashes. Importantly, this system is able to map both continuous emissions like dart leaders, and bursty or discrete emissions. Lightning stepped leader and dart leader propagation speeds are estimated to 0.56–2.5x10⁵ m/s and 0.8–2.0x10⁶ m/s respectively, which are consistent with previous reports. In many aspects our LF images are remarkably similar to VHF lightning mapping array images, despite the 1000 times difference in frequency, which may suggest some special links between the LF and VHF emission during lightning processes.