Characterizing D-Region Roughness using LF/MF Signals of Opportunity
Higginson-Rollins, Marc Alexander
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The D-region of the ionosphere, which ranges from about 60-100 km, is too high for continuous in-situ measurements, such as with high-altitude balloons, and too low for satellite-based measurements. As such, the small-scale roughness of the D-region electron density is not well understood. The bulk of VLF (Very Low Frequency; 3-30 kHz) research studying the D-region has been focused on global or regional studies, and typically all assume a stratified ionosphere. Some works examined more localized variation, but these studies are by no means exhaustive and fail to truly characterize roughness on a scale less than ~10-100 km, particularly under ambient conditions. Early work done using the partial reflection technique touches on the idea of small-scale roughness, but the work was limited in scope and focused on understanding the mechanism of weak partial reflections rather than characterizing the roughness. This work presents a technique to characterize the roughness of the D-region electron density using Low Frequency (LF; 30-300 kHz) and Medium Frequency (MF; 300-3000 kHz) signals of opportunity. The horizontal roughness is measured using the correlation length scale and the vertical roughness is measured using the differential phase height. Field campaign measurements of both metrics are presented. The Monte Carlo Method (MCM) is applied to a Finite-Difference Time-Domain (FDTD) model to connect correlation length scale measurements to a parameterization of the electron density roughness, the ionospheric length scale. The ionospheric length scale values for two sets of field measurements are presented. Results show that this modeling technique can successfully characterize some of the observed data.