An Ionospheric Storm Scale Index Based on TEC: Comparative Analysis for China Region

Abstract

Defining a standardized ionospheric storm scale is challenging given the ionosphere's inherent complexity and susceptibility to multiple driving factors. In this study, a novel ionospheric storm scale (ISS) index is developed based on statistical analysis of total electron content (TEC) data from 257 ground‐based GNSS stations across China between 2008 and 2021. Designed for broad applicability, the index is independent of season, local time, and geographic location. We first derive an ionospheric activity index in percentage by comparing observed TEC values with a quiet‐time reference. To account for seasonal, diurnal, and latitudinal variations in TEC fluctuations, each percentage deviation is normalized using the robust Z‐score method. The ISS is then established by defining thresholds for the normalized data, categorizing activity into seven levels: ISS0, ISS _P 1, ISS _P 2, ISS _P 3, ISS _N 1, ISS _N 2, and ISS _N 3. Results reveal that the new index exhibits no diurnal or seasonal dependence, a key requirement for a homogeneous ionospheric activity time series. Correlation analysis between the ionospheric activity index and the residuals of the ionospheric TEC model we developed previously, along with variations in the index during geomagnetic storms, demonstrates that the ISS can serve as a reliable indicator of the expected performance of ionospheric models and well represents ionospheric activity. Finally, analysis of the relationship between standard point positioning accuracy and the ionospheric activity index under different solar activity levels reveals that higher ionospheric activity corresponds to increased positioning errors during periods of high solar activity.