Multi‐Scale Variation Analysis of Low‐Latitude Ionospheric Disturbances During the Ascending Phase of Solar Cycle 25 Based on Multiple GNSS‐Derived Indices: A Case Study Over Hong Kong

Abstract

During the ascending phase of Solar Cycle 25, low‐latitude ionospheric irregularities occur frequently and can induce scintillation in Global Navigation Satellite System signals, degrading positioning, navigation, and timing services. To characterize low‐latitude ionospheric disturbances across multiple physical processes and time scales, this study uses regional observations from Hong Kong and data from 110 globally distributed stations from December 2019 to March 2025 to derive multiple disturbance indices, including the Rate of TEC Index (ROTI), Instantaneous Along‐Arc TEC Rate (IAATR), Along‐Arc TEC Rate (AATR), and the spatial‐gradient‐based Disturbance Ionosphere Index (DIXSG). Linear trend fitting, Fourier spectral analysis, and hourly statistics are applied to investigate interannual, seasonal, and diurnal variations. The results show that ROTI, IAATR, and AATR over Hong Kong, together with the global DIXSG index, increase significantly with enhanced solar activity, indicating more pronounced short‐term TEC fluctuations, stronger instantaneous along‐arc TEC responses, enhanced regional‐scale ionospheric activity, and a more disturbed global ionospheric background, respectively. Spectral analysis reveals a dominant semiannual variation in the Hong Kong low‐latitude region, with enhancements near the vernal and autumnal equinoxes, whereas the global DIXSG index exhibits a smoother annual variation superimposed with semiannual features. Seasonal and diurnal statistics further show stronger disturbances in spring and autumn than in summer and winter, with activity mainly concentrated from local afternoon to early nighttime. Overall, the multi‐index analysis provides complementary evidence for the multiscale evolution of low‐latitude ionospheric disturbances during the ascending phase of Solar Cycle 25 and supports ionospheric monitoring, space‐weather impact assessment, and GNSS scintillation modeling.