| 17 Jun 2026
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.
The manuscript presents a analysis of the latitude-dependent ionospheric VTEC response to the 17 March 2015 geomagnetic storm, using publicly available GNSS and geomagnetic datasets together. The results are generally consistent with previous studies of the St. Patrick’s Day storm and provide a quantitative assessment of the European–African sector. However, I recommend moderate revision to improve the clarity of the manuscript, and strengthen the discussion of novelty, data reproducibility, and the limitations of the analysis.
Novelty of the study: Could the authors clarify more explicitly what the principal novelty of the study is in comparison with previous investigations of this storm. Particularly regarding the integrated multi-station analysis of the European-African sector and the quantitative assessment of the latitudinal gradient of peak storm-time VTEC?
Data sources and Data processing: The study relies on publicly available GNSS observations. Could the authors provide additional details on the quality-control procedures applied to these datasets. How missing or low-quality GNSS observations were handled, how temporal consistency among the different datasets was ensured. Are there any limitations of using publicly available data may. Did this affect the robustness of the derived VTEC estimates and subsequent statistical analyses?
Latitudinal gradient robustness: The study reports a strong linear relationship between peak storm-time VTEC and geographic latitude based on four GNSS stations. Could the authors discuss this in details.
Choice of geomagnetic indices: The correlation analysis shows that Kp is more strongly associated with ΔVTEC than −Dst. Could the authors elaborate on the physical reasons for this difference and discuss whether other geomagnetic or solar-wind parameters, or interplanetary electric field components, might provide additional insight into the regional ionospheric response?
Code & Data availability: The authors states that the data processing and statistical analyses were performed using Python scripts that are available from the corresponding author upon reasonable request. To enhance transparency and reproducibility, could the authors provide the analysis scripts, processing workflow, and relevant documentation in a publicly accessible repository, e.g. GitHub or Zenodo?
Conclusions & limitations: The present study focuses on the ionospheric response to a single geomagnetic storm event in the European - African sector. Could the authors discuss how the proposed analytical framework might be extended in future investigations. For example, by examining additional geomagnetic storms, including a denser network of GNSS stations, or integrating physics-based ionospheric models?