Polarization-Based VLF Remote Sensing of Transient Ionospheric Disturbances

Very-low-frequency (VLF) (3–30 kHz) radio waves can propagate large distances around the globe in the Earth-ionosphere waveguide and be scattered by transient disturbances to the lower ionosphere. Such scattering enables the VLF remote sensing of lightning-related (as well as other) disturbances to the D-region ionosphere (60–100-km altitude). Indeed, recent work studying the polarization of VLF radio waves has yielded insights into the physical mechanisms associated with “large” early/fast VLF events, which are produced by powerful lightning discharges. While the polarization of the ambient VLF field has been investigated in such cases, the polarization of the field scattered by the ionospheric disturbance has not been investigated in detail, primarily because it can be difficult to measure. In this article, we present a simple, reliable method to accurately calculate the polarization of such a scattered field, and we apply the method to analyze three different types of ionospheric VLF scattering events: an early/fast VLF event, a long-recovery early VLF event, and a lighting-induced electron precipitation (LEP) event. In so doing, we demonstrate the significant advances in VLF remote sensing capabilities provided by the scattered field polarization method.