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    Impact of CIR/HSS-Driven Geomagnetic Storm and HILDCAAs* Events on Brazilian Equatorial and Low Latitude Ionosphere
    (2025-10-20) Oliveira, Virgínia Klausner de; Pillat, Valdir Gil; Prestes, Alan; Picanço, Giorgio Arlan da Silva; Echer, Ezequiel; Silva, Marlos Rockenbach da; Essien, Patrick; Abaidoo, Samuel; Cândido, Claudia Maria Nicoli; São José dos Campos
    This research investigates the impact of CIRs/HSS-driven geomagnetic storms and High- Intensity, Long-Duration Continuous Auroral Electrojet Activity (HILDCAA) events on the Brazilian equatorial and low-latitude ionosphere during the early descending phase of solar cycle 23, with a focus on the geomagnetic disturbance from October 12–23, 2003. Ionospheric variability was analyzed using Digisonde data from São Luís and Cachoeira Paulista, representing the equatorial region and the southern crest of the Equatorial Ionization Anomaly (EIA), respectively, along with GNSS-derived Total Electron Content (TEC) data from a latitudinal chain of stations spanning Fortaleza to Uruguaiana, Brazil. To assess ionospheric perturbations, disturbed-day values were compared with a five quiet-day (5QD) average, serving as a baseline for normal conditions. Plasma drift disturbances were examined using the Fejer-Scherliess empirical model, employing the auroral electrojet index (AE) as a key parameter. The CIR/HSS events were characterized by prolonged geomagnetic forcing, with solar wind speeds reaching ˜ 619 km/s and IMF-Bz components facilitating enhanced magnetosphere-ionosphere coupling. These storms exhibited an intense main phase (Sym-H ˜ -100 nT) and a notably extended recovery phase (˜ 8 days), during which disturbance dynamo electric fields (DDEFs) played a crucial role in ionospheric restructuring. Results indicate a strong coupling between the auroral and equatorial ionospheres, with equatorward propagating disturbance winds driving F-region uplifts and nighttime spread- F occurrences. TEC variations exhibited distinct temporal and latitudinal patterns, with enhancements predominantly observed during daytime and post-sunset hours, influenced by the interplay of penetration electric fields (PPEFs), DDEFs, and auroral-driven disturbance winds. Notably, TEC increases were prominent at the Equatorial Ionization Anomaly (EIA) crest during equinoctial months, particularly in March–April, whereas winter months exhibited minima. Analysis of thermospheric O/N2 ratios confirmed that neutral composition changes were not the primary driver of TEC variability during HILDCAA events, whereas they played a significant role during CIR/HSS-driven geomagnetic storms. Additionally, large-scale ionospheric irregularities, examined through the Rate of TEC Index (ROTI), were most pronounced from nighttime to post-midnight, peaking at the equator during equinoxes and shifting toward lower latitudes from May to August. Empirical modeling validated that disturbance-induced vertical plasma drifts modulate F-region dynamics, with significant uplifts in h’F and hmF2, alongside foF2 reductions. Comparative statistical analysis of geomagnetic, solar wind, and ionospheric parameters demonstrated that CIR/HSS-driven HILDCAA events induce persistent ionospheric perturbations, comparable to intense ICME- driven storms. These findings underscore the prolonged influence of CIR/HSS-induced geomagnetic activity on equatorial and low-latitude ionospheric behavior, contributing to a deeper understanding of space weather impacts on radio propagation and other technological systems in the near-Earth environment.