Wavelet analysis of nighttime excess ionospheric TEC during a geomagnetic storm over Indian low latitudes

Understanding nighttime ionospheric variability is essential for reliable ionospheric modelling for improved GNSS performance over low latitudes. In this study, we examine the mature phase of the March 17, 2015 geomagnetic storm (St. Patrick's Day storm) using normalized nighttime excess vertical TEC (XTEC) derived from four isomeridian low-latitude grid points in the Indian sector (10°N–25°N, 75°E). Continuous wavelet transforms with a Morlet basis are employed to decompose XTEC into spectro-temporal modes, enabling isolation of process-dependent signatures. The results suggest a diminishing influence of prompt penetration electric fields (PPEFs) toward the equator, while reverse-fountain-type equatorward transport and recombination losses appear to coexist near the magnetic equator, alternating in their relative dominance. Mesoscale modulations are strongly suppressed at the equator, consistent with enhanced conductivity and rapid recombination, yielding a pronounced low-pass response. Synchronous modal peaks across relatively higher-latitude scalograms point to isocausal PPEF forcing, whereas systematic delays at lower latitudes indicate subsequent equatorward plasma transport and redistribution. Monte-Carlo-based significance testing supports the robustness and reliability of the identified modal features. Overall, the spectro-temporal framework effectively disentangles multiple drivers of nighttime XTEC variability during an extreme storm.