Title:  D-region Tomography: A Technique for Ionospheric Imaging using Lightning Generated Sferics and Inverse Modeling

Committee:

Dr. Morris Cohen, ECE, Chair , Advisor

Dr. Justin Romberg, ECE

Dr. Sven Simon, EAS

Dr. Malcom Bibby, ECE

Dr. Ryan Said, Vaisala

Abstract:

The ionosphere is an ionized layer of the atmosphere constituted of plasma primarily driven by radiation from the sun. Understanding the ionosphere is mission critical for many radio technologies in both the military and commercial space. The lower ionosphere is relatively poorly understood partly due to the difficulty of making direct measurements and partly due to the global spatial scales in which many phenomena occur. Very low frequency (VLF) waves reflect between the D-region (60 to 90 km altitude) of the ionosphere and the surface of the Earth traveling to global distances in a guided fashion. Lightning is a ubiquitous source of terrestrial VLF radiation from thunderstorms which are distributed throughout the world. We develop an algorithm to normalize the radiated VLF waveforms from lightning, or ‘sferics’, observed on a network of VLF receivers to produce stable broadband sources. We extend the current Wait and Spies 2-parameter model for the electron density of the D-region to better fit features commonly observed by other lower ionospheric measurement techniques. We call the new model the ‘split model’ after the observation that the D-region seems to be split into two exponentially increasing portions and use four parameters to describe the model. Using the US Navy’s Long-Wavelength Propagation Capability (LWPC) code and inverse modeling techniques, we infer a best-fit parameterized electron density curve vs altitude that we interpret as the average along a given transmitter-to-receiver path. We develop and demonstrate the D-region tomography algorithm which uses the set of path-averaged electron density inferences to solve for a 4D image of the electron density in space and time.