Multi‐Scale Characterization of Ionospheric Disturbances Over East Africa During 17 March 2015 St. Patrick's Day Geomagnetic Storm

Abstract

We investigate the ionospheric response to the intense St. Patrick's Day geomagnetic storm of 17 March 2015 (minimum Dst ≈ −223 nT) using Global Navigation Satellite System (GNSS)–derived Total Electron Content (TEC) observations from 11 East African stations: Addis Ababa‐ADIS (9.04°N, 38.77°E), Arusha‐ARSH (3.38°S, 36.69°E), Dodoma‐DODM (6.19°S, 35.75°E), Eldoret‐MOIU (0.29°N, 35.29°E), Malindi‐MAL2 (2.99°S, 40.19°E), Mbarara‐MBAR (0.61°S, 30.65°E), and Mbeya‐MBEY (9.03°N, 38.74°E, Asab‐ASAB (13.06°N. 43.65°E), Ginr‐GINR (7.15°N, 40.71°E), Bahir Dar‐BDMT (11.60°N, 37.36°E), Debarek‐DEBK (13.15°N, 37.89°E), Assosa‐ASOS (10.05°N, 34.55°E). Instrumental biases were removed, data were screened for satellite elevation angles ≥40°, and analyzed using detrended TEC (dTEC) and Morlet continuous wavelet transform to identify storm‐time ionospheric disturbances across multiple temporal scales. The storm main phase was characterized by prolonged southward z‐component of interplanetary magnetic field (IMF‐Bz) (<−20 nT), enhanced solar wind speeds (>600 kms ⁻¹ ), and elevated y‐component of the interplanetary electric field (IEF‐Ey), indicating strong solar wind–magnetosphere coupling. These conditions drove significant ionospheric variability, with the largest dTEC amplitudes (>±4 TECU) observed near the Equatorial Ionization Anomaly (EIA), particularly at ADIS and MAL2. Wavelet analysis revealed the coexistence of medium‐scale traveling ionospheric disturbances (MSTIDs; ∼0.25–1 hr), large‐scale travelling ionospheric disturbances (LSTIDs; ∼1–4 hr), and short‐period fluctuations (<0.25 hr). Disturbances were most pronounced during the storm main and early recovery phases and exhibited clear latitudinal dependence, with enhanced activity near the EIA crest. Post‐sunset ionospheric irregularities, interpreted as equatorial plasma bubbles (EPBs), were identified through sharp dTEC depletions and enhanced short‐period wavelet power. EPBs occurred on most days but showed increased frequency, earlier onset, and wider latitudinal extent during the storm, consistent with intensified pre‐reversal enhancement and storm‐time electric field penetration. The temporal association between MSTIDs and EPB onset suggests a potential seeding role of atmospheric wave activity. These results demonstrate that ionospheric disturbances over East Africa are strongly controlled by the interplay between storm‐time electrodynamics and equatorial processes. The study highlights the effectiveness of combined dTEC and wavelet analysis for resolving multiscale ionospheric variability and provides important insights into space weather impacts on GNSS performance in equatorial regions.