Integrated VLF and HF diagnostics of storm-time ionospheric irregularities from the D- to F-Region

Kurzfassung

This study investigates geomagnetic storm-induced ionospheric irregularities spanning the lower (D-region) to upper (E- and F-regions) ionosphere using coordinated ground- and space-based radio diagnostics. Variations in very low frequency (VLF) radio wave amplitudes were analysed to characterise D-region responses, while high-frequency (HF) ionosonde parameters and total electron content (TEC) measurements were used to assess storm-time perturbations in the E- and F-regions. To quantify D-region variability, VLF signals were parameterised into three metrics: mean amplitude before sunrise (MBSR), daytime mean amplitude (DTMA), and mean amplitude after sunset (MASS). Relative to pre-storm conditions, storm-day observations revealed significant reductions in all three metrics, with the strongest response consistently observed in DTMA, indicating enhanced daytime D-region ionisation and absorption. Concurrently, ionosonde-derived virtual heights (h’E, h’F1, h’F2) and critical frequencies (foE, foF1, foF2) exhibited pronounced increases and fluctuations during storm periods, reflecting substantial restructuring of the F-region plasma environment. The largest enhancements in virtual heights were observed near the DHO transmitter, suggesting strong regional coupling between geomagnetic forcing and upper ionospheric dynamics. The combined VLF-HF diagnostics demonstrate coherent storm-time perturbations across multiple ionospheric layers, highlighting the effectiveness of integrated radio techniques for probing vertical ionospheric coupling during geomagnetic disturbances.