Browsing by Subject "Atmospheric electricity"

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.

Electromagnetic waves in the very low frequency (VLF, 3-30 kHz) and extremely low frequency (ELF, 3-3000 Hz) bands propagate extremely well in the cavity between the earth and the ionosphere with low attenuation. Because of this, radio waves emitted in this frequency range can be measured at extremely large distances (thousands of kilometers) from the sources of such emissions. Two main sources of signals at these frequencies are lightning events and VLF transmitters designed for communicating with submarines and other naval vessels. Measurement of the signals from both of these sources can be used to discover information about the source, in the case of lightning, or to measure the factors affecting propagation and other signal properties, as with VLF transmitter signals. This document provides a summary of the work undertaken to measure both of these signal sources and to outline goals and briefly outlines some objectives for future work.

A brief background on the atmospheric ELF and VLF environments is given in chapter 1, including a description of the conditions that allow for excellent propagation. A brief introduction to the lightning processes, as well as classification and measurement techniques is included as background information. Details describing current VLF transmitters examined in this work and basics of minimum-shift keying are also described.

Chapter 2 describes the design process and operating characteristics of a sensor designed for measuring magnetic fields in the ELF and VLF frequency ranges of interest in this work. This sensor system is robust and suitable for long-term deployment in thunderstorm environments. Chapter 3 details a method of measuring faint average signals generated by some lightning processes at large distances. Such an averaging process allows for the extraction of extremely small-magnitude processes that are otherwise not visible and enables the comparison of lightning on a larger scale. Averaged waveforms for four separate thunderstorms are compared and post-first stroke flash parameters are analyzed. Chapter 4 applies the averaging procedure to a specific type of lightning known as narrow bipolar events (NBEs). NBEs play an important role in the initiation of other types of lightning but not all NBEs initiate other lightning. This work divides positive NBEs according to whether they initiate other lightning events and examines the differences between them, helping to investigate the processes and conditions that give rise to lightning. Chapter 5 describes a method of unambiguously determining the position of a receiver through the measurement of terrestrial MSK-encoded VLF transmitters. Such a system has many advantages over other methods of navigation and simulated and field-tested capabilities and limitations are discussed, as well as factors affecting system accuracy. Finally, proposals and suggestions for future work are given in chapter 6.