Broadband Radio Mapping and Imaging of Lightning Processes (2020)

Though thunderstorms and lightning are commonplace on Earth, it is still unclear how lightning initiates, propagates, and how it is involved in generating intense bursts of gamma-rays that can be detected by spacecraft. Lightning is a hot, highly-ionized plasma channel, capable of carrying up to hundreds of kiloamperes electric current, and extending many kilometers in length for hundreds of milliseconds at a time. Despite its immensity, lightning can be difficult to observe, as it primarily initiates and propagates deep within thunderclouds, optically obscured by thousands of cubic kilometers of cloud water and ice. Broadband radio interferometry has been developed to study lightning at radio frequencies, offering us a way to “see” inside the clouds. The technique, which is still in its infancy for lightning research, allows for lightning radio emissions to be mapped and/or imaged with extremely fine time resolution. In this dissertation, a newly-developed three-element, broadband VHF (~14-88 MHz), 16-bit radio interferometer (INTF) is used to investigate extremely transient thunderstorm electrical phenomena involved in lightning initiation, propagation, and high-energy photon production. The investigations demonstrate the novel science that can be done with the INTF system, and reveal previously unforeseen dynamics of lightning formation. Specifically, we image and map the VHF emissions of narrow bipolar events (NBEs), initial breakdown pulses (IBPs), and an energetic in-cloud pulse (EIP) with sub-microsecond resolution. NBEs have long been of interest to the lightning community because they are the most powerful natural emitters of high-frequency and very-high-frequency radio waves on Earth. Moreover, NBEs are readily identifiable by their narrow (~10 µs wide), bipolar sferics (~3 kHz-3 MHz radio emissions). NBEs are not lightning, but appear to be a precursor to lightning, occurring either in complete isolation, or at the beginning of a lightning flash. IBPs, in contrast, never occur in isolation, but rather are the hallmark of lightning channel formation. IBPs typically occur in long trains of sferic pulses, and indicate the imminence of lightning during the first milliseconds after lightning initiation. An IBP is also identified by its sferic, having a bipolar waveform some tens of microseconds wide, the initial pulse of which is superimposed by ~1 µs-wide subpulses. Lastly, EIPs are high-peak-current (>200 kA) events that are involved in the generation of terrestrial gamma-ray flashes (TGFs), which are intense bursts of gamma-rays that radiate out the tops of thunderclouds and are detected in space. EIPs have a signature high-amplitude, ~50 µs-wide sferic, which is time-aligned with satellite-borne gamma-ray detections. EIPs can thus serve as a proxy for TGFs, offering a way to investigate TGFs using ground-based radio sensors, without necessarily needing satellite data. The physical natures of NBEs, IBPs, and EIPs have been active areas of research over the last decade. For over half a century, the role that IBPs play in initial hot channel formation has been under debate. More recently, intense investigation has been focused on exactly how NBEs are involved in lightning initiation. Just in the last few years, EIPs were discovered, offering a new way to investigate the role that lightning plays in TGF generation. By investigating NBEs with the INTF, we discovered a newly-identified form of streamer-based breakdown, termed fast negative breakdown, that does not fit with our current understanding of lightning initiation. Streamers are cold filamentary plasma channels, and based on conventional dielectric theory, it was hypothesized that lightning should be initiated by positive streamers, which carry electric current in their propagation direction. However, fast negative breakdown carries electric current opposite its propagation direction, propagating ~500 m through virgin air with an unusually fast speed of ~10^7 m/s. Aside from breakdown polarity, fast negative breakdown is in many ways similar to recently reported fast positive breakdown that generates the majority of NBEs, and that is expected from conventional dielectric theory. We additionally show that similarly fast breakdown is involved in the production of both IBPs and EIPs. Using the INTF, we show that the IBP process is dominated by a fast-propagating ∼10^7 m/s streamer-based negative breakdown that propagates the channel about ~100 m into virgin air, similar to the fast negative breakdown associated with NBEs. We show that the streamer-based channel extension leads to a sustained electric current, indicating the existence of a hot conductive lightning channel. Fast-propagating ~10^7-10^8 m/s breakdown of both polarities is also a prominent feature during the EIP, but occurs over a larger (>1-km altitude) volume than during NBEs or IBPs. We show that repeated downward- and upward-propagating fast positive and negative breakdown are somehow coupled to the generation of relativistic electrons and associated ionization. We conclude that the electric current that produces the EIP sferic is generated by a newly discovered type of self-sustaining discharge termed a relativistic feedback discharge (RFD), which involves multiple generations of relativistic electron avalanches and back-scattered positrons and X-rays. Our study further demonstrates that TGFs can be produced by RFDs. The INTF was developed by New Mexico Tech, and deployed and operated at Kennedy Space Center (KSC) in Florida during summer 2016 to obtain the data used herein.