Radio Remote Sensing and Imaging of Lightning

Abstract

Lightning is one of the most familiar, impressive, but catastrophic natural phenomena that occur commonly on Earth. It produces perhaps the loudest sound, the most broadband radio emission, and the brightest light in the atmosphere. However, lightning remains relatively poorly understood since it is so transient (usually < 1 second) and so unpredictable that hinders direct measurements inside thunderstorms. For these reasons, radio remote sensing has been widely used for lightning studies. With recent advances in instrumentation and remote sensing technique, some basic problems like how lightning initiates inside the thundercloud begin to be addressed, and new challenging scientific problems are being discovered, such as the Terrestrial Gamma-ray Flashes (TGFs, energy > 20 MeV) associated with lightning, photonuclear reactions triggered by lightning, and needle-like plasma structures on the positive lightning leader, connecting lightning as part of atmospheric physics to high-energy physics, plasma physics, etc.This dissertation aims to address fundamental questions like how lightning initiates and propagates, and how are TGFs related to lightning processes, by applying state-of-the-art radio remote sensing and imaging techniques. We measure and analyze electromagnetic signals produced by lightning from the vicinity to more than a thousand miles away, at radio frequencies from VLF, LF, to VHF and UHF. First, we investigated LF/VLF lightning sferics at the time of TGFs and found a statistically consistent connection between a slow LF pulse (~80 $\mu s$ duration) and TGFs, suggesting that the radio pulse is produced directly by the TGF production process. Second, in light of the slow pulse-TGF connection, we discovered a new type of downward CG-TGF with a reverse positron beam detected by Fermi GBM on the orbit, which could constitute 5--10 % of the previously known TGF population. Third, we employed supervised and unsupervised machine learning approaches to classify energetic lightning radio pulses for unprecedented ground detection of TGFs as well as understanding lightning sferics and ionospheric effects. In the meanwhile, we developed a short-baseline VHF interferometer with 200 MHz bandwidth to image lightning channels in high spatiotemporal resolution, shedding new insights into needles and lightning leader dynamics. Last but not least, we demonstrated and applied a new approach to indirectly measuring electric fields in the discharge region during lightning initiation and positive leader propagation using VHF-UHF radio spectrum, enabling an entirely new and useful capability for probing the ambient condition during lightning discharge processes. Implications of the estimated electric fields for lightning physics and high-energy physics are discussed.