Ionospheric E region remote sensing with ELF radio atmospherics

Radio remote sensing of the ionospheric E region can be difficult, particul arly in the nighttime E region valley which lies between two regions of hig her electron density. We show that extremely low frequency (ELF) electromag netic waves launched from below penetrate this region because of their low attenuation and can be reflected from both the D region and the top of the E region valley. This double reflection, also caused by sporadic E layers, produces a measurable effect on the subionospheric propagation of ELF waves , which we demonstrate with numerical Earth-ionosphere waveguide simulation s. This sensitivity opens the possibility of remotely sensing the E region and detecting sporadic E layers with broadband ELF propagation measurements using lightning discharges as the source. From nighttime observations of E LF lightning radiation over the same propagation path on different days, we extract E region electron density profiles that show significant variabili ty not only from day to day but over the course of a single night. This tec hnique is inherently a path-integrated measurement, enabling tomographic la rge-scale ionospheric remote sensing with multiple sources and receivers.