Remote measurement of ELF/VLF radio emissions by lightning and ground-based transmitters

Abstract

<p>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.</p><p>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. </p><p>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.</p>