Speaker
Description
Sub-ionospheric Very Low Frequency (VLF) signal propagation serves as a powerful diagnostic tool for probing the lower ionosphere, particularly the D-region, during various space weather phenomena. In this study, we present an unified, autonomous modeling framework to investigate VLF signal perturbations induced by two distinct solar events: the 2024 Total Solar Eclipse (TSE-2024) and selected weak (C-class) solar flares. The modeling framework integrates numerical simulations based on the D-region electron continuity equation (ECE) and the Long Wave Propagation Capability (LWPC) code to simulate spatio-temporal variations in VLF signal amplitudes across multiple propagation paths. For the TSE-2024 event, we analyze VLF signal propagation from four transmitters—NAA (24.0 kHz), NML (25.2 kHz), WWVB (60.0 kHz), and NLK (24.8 kHz)—received at the VRG station in North America (37°26′24″N, 79°16′16″W). A numerical model incorporating solar disk obscuration ($p$) and solar zenith angle ($\chi$) is applied to estimate variations in the ionospheric reflection parameters, $h^{\prime}$ and $\beta$, via a path-segmentation approach. A correction factor ($w$) is introduced to modulate the influence of $p$ on $h^{\prime}$ and $\beta$. Resultant variations in electron density ($N_e$) and modal attenuation coefficient ($\mu$) are computed, and simulated VLF amplitudes ($A_{sim}$) are compared with observations ($A_{obs}$).
In the case of weak solar flares, we analyze VLF data along three short propagation paths affected by three C-class events. The electron density profiles $N_e(t,h)$ are derived using ECE-based simulations without reliance on VLF observational input. These profiles are fitted to Wait’s model to extract $h^{\prime}$ and $\beta$, which are then used in LWPC to simulate $A_{sim}$. The results demonstrate good agreement with $A_{obs}$, particularly during flare-induced enhancements, validating the robustness of the modeling framework across distinct ionospheric disturbance scenarios.
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