## 2 Data Processing and Analysis Procedure  
### Data Sources  
In the study of solar storm events, selecting station data with high sensitivity and long-term stability is crucial. For this purpose, this study utilized signal strength data from the Xi'an monitoring station, which records low-frequency time-code signals transmitted from the Shangqiu station. The Xi'an station has been in continuous operation for over two years, and this study specifically selected one year of accumulated signal strength data following its formal commissioning. Located in the terrestrial wave interference zone of the Shangqiu transmitting station, the Xi'an station provides signal strength data that is particularly valuable for investigating ionospheric response characteristics.  
During the daytime, the ionosphere is directly influenced by solar radiation, leading to a significant increase in electron density and more pronounced disturbances. Consequently, the ionosphere exerts a greater impact on the reflection and propagation of long-wave signals during the daytime. In contrast, at night, the Earth's rotation shields the ionospheric region above the receiving station from direct solar radiation, resulting in a substantial decrease in electron density and reduced ionospheric disturbances. Therefore, this study specifically selected solar storm events that occurred during daytime periods at the receiving station to accurately evaluate the impact of solar storms on low-frequency signal strength while minimizing interference from nocturnal ionospheric activity.  
Additionally, geomagnetic activity is an important factor influencing the propagation of low-frequency signals, particularly in mid-latitude regions. Enhanced geomagnetic activity can induce severe ionospheric disturbances, significantly altering the signal propagation path and strength. This study utilizes XRA data obtained from NOAA's satellite monitoring systems [20]. Moreover, geomagnetic activity may simultaneously affect both X-ray flux and signal strength, thereby acting as a confounding variable that could obscure the direct impact of solar flares. To address this issue, events coinciding with significant geomagnetic activity (Kp index > 4) were excluded from the analysis to minimize potential interference.  
In general, to ensure data reliability and consistency, the following selection criteria were applied in this study: (1) Data Integrity: Only events with complete and continuous data for both X-ray flux and low-frequency signal strength were retained; any events with missing or incomplete data were discarded. (2) Daytime Impact: Only events that occurred during daytime periods at the monitoring station were included to avoid the influence of nocturnal ionospheric activity. (3) Geomagnetic Activity: Events coinciding with significant geomagnetic activity (Kp index > 4) were excluded to reduce the impact of geomagnetic disturbances as a potential confounding variable.  
Regarding the input data, this study utilizes a systematically organized dataset to analyze C-class solar flare events, as depicted in Figure 1. The dataset is structured into two primary categories: XRA Data and Signal Strength Data, with each category containing multiple event entries. For each event, the XRA Data and Signal Strength Data are synchronized to the same UTC time period, ensuring consistency and reliability in capturing the ionospheric response to solar activity. The focus of this study is on events recorded during the daytime by the Xi'an monitoring station between May 2023 and June 2024. After applying rigorous selection criteria--such as excluding nighttime events, ensuring data integrity, and filtering out events with Kp index > 4--over 150 valid events were retained for analysis.  
Figure 1: Hierarchical Structure of C-Class Solar Flare Event Dataset.