Ionospheric Diagnostics via VLF Elliptical Polarization and Cross-Wavelet Analysis: Assessing the Impacts of Geomagnetic Storms

The interaction between the Earth's conducting ground surface (with varying land cover types) and the ionospheric D-region (60–90 km altitude) forms a waveguide that supports the propagation of Very Low Frequency (VLF) and Low Frequency (LF) radio waves (3–30 kHz and 0.5–470 kHz, respectively). Variations in these VLF signals provide a valuable tool for remotely sensing dynamic changes in the D-region's conductivity under both ambient and disturbed ionospheric conditions. This study employs an innovative approach, combining amplitude and phase deviations from colocated perpendicular antenna measurements, to characterize elliptical polarization parameters during an intense geomagnetic storm. This method outperforms conventional techniques that rely solely on isolated phase or amplitude variations from VLF transmitters to detect ionospheric disturbances. Subsequently, the extracted polarization parameters were analyzed using cross-wavelet analysis in conjunction with a high-resolution SYM-H geomagnetic index. Cross-wavelet analysis was chosen for its ability to evaluate localized correlations between two datasets across both time and frequency domains and to identify the leading or lagging parameter in a periodic context. The results, particularly concerning the delayed response of the D-region, not only confirm existing findings but also offer new insights into the underlying mechanisms. These advancements contribute to a deeper understanding of D-region dynamics, improving ionospheric modeling and enhancing the accuracy of space weather prediction frameworks.