Latitudinal Variation of Ionospheric Total Electron Content During the 17 March 2015 Geomagnetic Storm: A Multi-Station GNSS Analysis

Abstract. We investigated the latitude-dependent ionospheric response to the St. Patrick’s Day geomagnetic storm of 17 March 2015 (Dst =−234 nT, Kp = 8) using GNSS-derived vertical total electron content (VTEC) from four stations spanning mid-to auroral latitudes (34–70° N) in the European–African sector: RABT (Rabat, Morocco), MADR (Madrid, Spain), BRUX (Brussels, Belgium), and TRO1 (Tromsø, Norway). VTEC was derived from dual-frequency pseudorange measurements with differential code bias corrections. Storm-time VTEC responses showed clear latitude-dependent variability. Relative enhancements ranged from +46 % at TRO1 to +224 % at BRUX, while the absolute peak VTEC decreased systematically with latitude. Linear regression of peak storm-time VTEC against geographic latitude yielded a gradient of −1.61±0.06 TECU per degree (R² = 0.9968, p = 0.001578). Correlation analysis revealed statistically significant positive associations between hourly ∆VTEC and Kp at RABT and MADR, while BRUX showed a weaker positive but non-significant correlation under the adopted p < 0.01 criterion. These results quantify the latitudinal structuring of storm-time ionospheric variability in the European–African sector and highlight stronger positive responses at the mid-latitude stations than at auroral latitude during this event.