Sferic-based tomography for D-region electron density modeling

While important for communication, the D-region ionosphere is difficult to measure using conventional means. Instead, researchers have historically relied on very-low-frequency and low-frequency (VLF, LF) radio waves, often from lightning or VLF transmitters, to indirectly infer D-region electron density. These methods have become highly capable of producing path-average electron density profiles; however, they typically do not provide great spatial resolution. In this work, we present tomography as a method for producing 3D electron density maps over the SE United States and Gulf of Mexico using lightning emissions known as sferics. Our method achieves approximately 3-4 degree spatial resolution, and is capable of capturing both daytime and nighttime conditions as well as the day-night terminator. Additionally, we show strong agreement between electron density and sunlight fraction during the 2017 "Great American Solar Eclipse" indicating our method can capture disturbed conditions.