Impact of F10.7 Flux and Proton Intensity on Delayed Ionospheric TEC Responses at Low Latitudes during Geomagnetic Storms across Solar Cycles 23–25

Abstract There has been a growing interest in ionospheric delay as a crucial element in comprehending the impact of solar phenomena and geomagnetic storms on the ionosphere. This study investigates Total Electron Content (TEC) data collected from two International GNSS Service (IGS) stations: CUSV, Thailand (Geographic 13.74° N, 100.53° E), and COCO, Keeling Island (Geographic 12.19° S, 96.839° E). The investigation focuses on the geomagnetic storms during Solar Cycles (SC) 23, 24 and the early phase of Cycle 25. The ionospheric delay time is defined as the duration from the start of a geomagnetic storm to its peak effect on the deviation of TEC (DTEC) from the quiet time reference level. We found a relationship between the ionospheric delay time and the Disturbance Storm Time (Dst) index, with specific selection criteria applied to storm events to enhance the accuracy of this association. The lowest correlation is for COCO station during the declining phase of SC 23 with value 0.27 due to system errors and gaps. The Highest value is CUSV station during the Ascending phase SC 25 which is 0.89. High and statistically significant correlation values were observed across most cases and time periods, with the F10.7 flux imposing a direct influence on these relationships, which contradicts some prior research. Our research highlights the significant part that proton flux plays during these periods, as variations in logarithmic proton intensity across four distinct energy ranges (6.95, 16.40, 32.95, and 46 MeV) have influenced the correlation’s linearity and its association with F10.7 flux at different stages of the Solar Cycle. Linear correlation equations were developed for each time period, revealing a linear relationship between the DTEC and the logarithm of the proton intensity across the four specified energy ranges. We report that the Lyman-alpha irradiance affects the DTEC linearly, characterized by a good correlation coefficient. The average ionosphere delay time varied by time period; the shortest delay time was observed during the declining phase of Solar Cycle 24, and the longest was during the early part of the declining phase of Solar Cycle 23.