Parametric Modeling and Analysis of Lightning Sferic Waveforms for D-region Remote Sensing

The D-region is a layer of the Earth’s ionosphere that extends from an altitude of approximately 50 to 90 km. This region plays a vital role in radio frequency (RF) propagation across a wide variety of frequency bands and significantly impacts the operation of numerous aeronautical, military, radar, broadcast, and, of course, amateur radio systems. Thus, a thorough understanding of how the D-region’s characteristics vary spatially and temporally is highly desirable both from a practical and scientific standpoint, as it can aid in enhancing the understanding and prediction of RF propagation and space weather events. The D-region and the Earth’s surface form what is known as the Earth-ionosphere waveguide (EIWG), facilitating long-distance propagation of very low frequency (VLF) signals within this waveguide. Lightning discharges emit broadband, pulse-like VLF radiation, known as sferics, which can travel hundreds or thousands of miles in the EIWG with little degradation. The characteristics of lightning sferic waveforms are impacted by the D-region along the path of propagation between the source of the strike and the receiver. Thus, information about the state of the D-region can be extracted from received sferic waveforms, making them exceedingly useful for remote sensing of the D-region. In this work, we present an effective parameterized time-domain model of lightning sferics to aid in the analysis of the D-region. The model is then applied to large VLF datasets obtained under typical ionospheric conditions and during abnormal geophysical events. The resulting temporal and spatial variations in sferic parameters are analyzed and are shown to respond in conjunction with sudden ionospheric disturbances such as solar flares.