Analysis of Ionospheric Disturbances in China During the December 2023 Geomagnetic Storm Using Multi-Instrument Data

Abstract

This study investigates the ionospheric response over China during the geomagnetic storm that occurred on 1–2 December 2023. The data used include GPS measurements from the Crustal Movement Observation Network of China, BDS-GEO satellite data from IGS MEGX stations, [O]/[N₂] ratio information obtained by the TIMED/GUVI, and electron density (Ne) observations from Swarm satellites. The Prophet time series forecasting model is employed to detect ionospheric anomalies. VTEC variations reveal significant daytime increases in GNSS stations such as GAMG, URUM, and CMUM after the onset of the geomagnetic storm on 1 December, indicating a dayside positive ionospheric response primarily driven by prompt penetration electric fields (PPEF). In contrast, the stations JFNG and CKSV show negative responses, reflecting regional differences. The [O]/[N₂] ratio increased significantly in the southern region between 25°N and 40°N, suggesting that atmospheric gravity waves (AGWs) induced thermospheric compositional changes, which played a crucial role in the ionospheric disturbances. Ne observations from Swarm A and C satellites further confirmed that the intense ionospheric perturbations were consistent with changes in VTEC and [O]/[N₂], indicating the medium-scale traveling ionospheric disturbance was driven by atmospheric gravity waves. Precise point positioning (PPP) analysis reveals that ionospheric variations during the geomagnetic storm significantly impact GNSS positioning precision, with various effects across different stations. Station GAMG experienced disturbances in the U direction (vertical positioning error) at the onset of the storm but quickly stabilized; station JFNG showed significant fluctuations in the U direction around 13:00 UT; and station CKSV experienced similar fluctuations during the same period; station CMUM suffered minor disturbances in the U direction; while station URUM maintained relatively stable positioning throughout the storm, corresponding to steady VTEC variations. These findings demonstrate the substantial impact of ionospheric disturbances on GNSS positioning accuracy in southern and central China during the geomagnetic storm. This study reveals the complex and dynamic processes of ionospheric disturbances over China during the 1–2 December 2023 storm, highlighting the importance of ionospheric monitoring and high-precision positioning corrections during geomagnetic storms. The results provide scientific implications for improving GNSS positioning stability in mid- and low-latitude regions.

1. Introduction

During geomagnetic storms, the sudden influx of increased solar wind energy into the magnetosphere-ionosphere-thermosphere system triggers intense ionospheric disturbances [1,2,3]. A wide range of studies in the field of space weather have been conducted to investigate the ionospheric response during geomagnetic storms. The response is affected by factors including solar flares, regional time, solar illumination, storm duration, seasonal patterns, and meteorological conditions on both global and regional scales [4,5,6]. Both positive and negative responses have been observed through the changes in ionospheric total electron content (TEC) or electron density (Ne), and they vary widely [7,8]. A recent study has shown that fluctuations in ionospheric TEC and Ne exhibit notable spatiotemporal variations, characterized by intricate propagation patterns and intensities that vary across different space weather conditions [9]. During geomagnetic storms, significant fluctuations in ionospheric TEC and Ne are common responses. The fluctuations are categorized into two types: large-scale traveling ionospheric disturbance (LSTID) and medium-scale traveling ionospheric disturbance (MSTID), based on propagation periods and wavelengths [10,11]. It is important to better understand the spatiotemporal dynamics of the ionosphere by detecting various ionospheric responses to geomagnetic storms and further investigating the underlying mechanisms of the disturbances.

The geomagnetic storm that occurred from 1 to 2 December 2023 was triggered by interplanetary coronal mass ejections (ICMEs). Previous studies have shown that these ICME events often enhance interactions between the solar wind and the Earth’s magnetosphere, leading to significant geomagnetic fluctuations and severe effects on the ionosphere, particularly in mid- and low-latitude regions [12]. It is important to note that coronal mass ejections (CMEs) near Earth should be referred to as ICMEs because they undergo significant changes upon reaching 1 AU. Regarding the causes of geomagnetic storms, the initial decrease in the SYM-H index is often associated with the influence of the sheath region, which is not part of the ICME. While the main phase following the largest decline may be caused by the magnetic cloud component of the ICME, this relationship must indeed be confirmed through beta value analysis [13,14]. The positive response of the equatorial ionosphere to geomagnetic storms is primarily driven by penetration electric fields. Tsurutani et al. (2004) first proposed this mechanism through global magnetosphere-ionosphere coupling simulations [15]. Subsequent studies by Mannucci et al. (2005) and Tsurutani et al. (2008) further validated the role of PPEFs in storm-time plasma transport [16,17]. Building upon this foundation, Imtiaz et al. demonstrated that the combined effects of PPEFs and extreme ultraviolet (EUV) radiation dominate the equatorial ionospheric positive response, where EUV modulates the background ionization state for PPEF-driven disturbances [18]. These findings indicate that ionospheric disturbances are collectively affected by these space weather factors, especially in low-latitude regions. Cheng et al. also demonstrated that the propagation of MSTIDs in low-latitude regions is closely associated with atmospheric gravity waves and ionospheric irregularities [19]. Studies in recent years further investigated ionospheric responses and variations. Smith et al. studied ionospheric disturbances during the 2020 geomagnetic storm and found that TEC fluctuations were significantly correlated with solar wind intensity (Sw), accompanied by the propagation of MSTID [20]. Ye et al. analyzed a moderate geomagnetic storm on 16 July 2003, revealing that the impacts of PPEF and MSTID on the low-latitude ionosphere varied significantly, especially under different meteorological conditions [21]. Similarly to these studies, the geomagnetic storm during 1–2 December 2023 is likely to disturb the ionosphere through PPEF and equatorial electrojet (EEJ), particularly in the East and South Asian regions [22,23]. It should be noted that the ionospheric response during the main and recovery phases of the storm can be different [24]. During the main phase, a positive ionospheric response is often observed mainly because of the PPEF effect [15]; meanwhile, equatorward neutral winds push more plasma into the upper ionosphere, leading to an increase in electron density. In contrast, poleward neutral winds tend to push plasma downward due to the inclination of magnetic field lines at higher latitudes. However, the ionospheric response during the recovery phase becomes more complex, potentially exhibiting both positive [25] and negative responses [26,27]. Positive responses are typically associated with the restructuring of the ionosphere, particularly at night and in low-latitude regions, where the ionosphere shows increased density as the energy input gradually rebalances [28]. While negative responses usually show in the afternoon and evening, mainly due to the changes in thermospheric composition, such as a decrease in the [O]/[N₂] ratio [29], MSTID, poleward-moving neutral winds, and other factors also affect Ne during the recovery phase [30,31]. These studies reveal the complexity of positive and negative ionospheric responses to geomagnetic storms, especially during their recovery phases, indicating that different ionospheric response mechanisms are driven by multiple physical processes at different stages.

In this study, the ionospheric response to the geomagnetic storm over mid- and low-latitude regions of China during the 1–2 December 2023 geomagnetic storm is analyzed. Both observations and predictions from the Prophet model, which is notable for its strong time series forecasting capability, are employed. Furthermore, the impact of ionospheric disturbances on the positioning precision of PPP is examined, providing insights into improving PPP performance and ionospheric modeling accuracy during future geomagnetic storms.

2. Data and Methods

This study utilizes observations from about 250 GNSS stations of the Crustal Movement Observation Network of China (CMONOC) network and 5 International GNSS Service (IGS) Multi-GNSS Experiment (MGEX) stations in China and nearby regions. The analysis focuses on ionospheric disturbances over China, utilizing data from Beidou System (BDS) geostationary orbit (GEO) satellites and [O]/[N₂] ratio maps from the TIMED/GUVI. Figure 1 shows the distribution of CMONOC stations (blue squares) and MEGX stations in China and surrounding regions (red triangles), with the magenta dashed line indicating magnetic latitude (mlat). Ionospheric delays are calculated using dual-frequency GNSS observations from CMONOC with an undifferenced and uncombined method. The STECs are then projected to VTEC at the ionospheric piercing points (IPPs). According to previous studies, the background trend of TEC is filtered out using a second-order function of Universal Time and latitude to obtain the actual TEC disturbances, defined as detrended TEC (DTEC) [32].

To effectively analyze TEC variations and disturbances during geomagnetic storms, this study employs the Prophet model, which is capable of handling time series with trends and is robust even when data are missing or have outliers. The Prophet model is applied to perform sliding window predictions on BDS-GEO VTECs from the MEGX stations. The predicted TEC data are then used to calculate the residuals between the predictions and measurements. Two times the standard deviation (STD) of the residuals is used to define the upper and lower bounds of the reference background TEC as in Equation (1):

$$TE{C}_{\pm }=TE{C}_{pre}\pm 2std$$

(1)

where TEC_± represents the predicted error threshold, TEC_pre denotes the background value obtained from the prediction, and std refers to the STD calculated from the residuals between the predictions and measurements [33].

The horizontal component of the geomagnetic field measured by magnetometers reflects the geomagnetic disturbance during the storms. This study examines its variations across different latitudes to investigate the geomagnetic conditions. Ne measurements from the Swarm A and C satellites are utilized to observe changes in ionospheric electron density with latitude over China, and then to confirm the ionospheric response during the storm event. The Swarm mission, launched by the ESA, comprises three satellites: two in lower orbit (A and C) operating in tandem at the altitude of 470 km, and one in a higher orbit (B) at 520 km [34].

3. Ionospheric TEC Fluctuations

3.1. Geomagnetic Conditions During the December 2023 Geomagnetic Storm

This section primarily discusses the evolution of the geomagnetic storm and the main factors related to the geomagnetic disturbances. Figure 2 illustrates the time series of the solar wind speed (Sw), southward interplanetary magnetic field component (IMF-Bz), and the geomagnetic indices (SYM-H and Kp) during the period from 29 November to 3 December 2023. As shown in the figure, the Sw experienced two sharp increases during the storm, attributed to the formation of fast forward shocks related to the rapid interplanetary coronal mass ejections [35]. The solar wind speed increased from 330 km/s to 480 km/s between 23:00 UT on 30 November and 1:00 UT on 1 December. Subsequently, it rose sharply to 541.8 km/s just after the onset of the geomagnetic storm at 0:00 UT on 1 December. It then gradually decreased and rebounded to 551.6 km/s after the storm during the night of 3 December. The IMF-Bz and SYM-H parameters increased at the onset time and then rapidly declined after the onset, reaching, respectively, relative minimums of −11.21 nT and −108 nT at 10:00 UT on 1 December. Subsequently, they exhibited significant fluctuations before the storm ended, during which the Kp index peaked at 6.7.

3.2. Ionospheric TEC Variations

Figure 3 presents the variations in VTECs measured at about 250 stations of the CMONOC on 1 December 2023, with a 2 h temporal resolution. In the mid-latitude region (around 35°N to 45°N), VTECs reached a maximum of 113 TECU at 07:00 UT, while in the low-latitude region (around 25°N to 35°N), they peaked at 129 TECU at 11:00 UT, as shown in Figure 3. After 15:00 UT on 1 December, VTECs showed a significant downward trend, decreasing to the lowest values in the subsequent hours across both mid- and low-latitudes. The rapidly increased VTECs over China were consistent with the changes in geomagnetic conditions illustrated in Figure 2 during the storm on 1 December. Subsequently, VTECs declined quickly around the end of the storm, aligning with the simultaneous recovery of the geomagnetic indices.

Figure 4 shows the longitudinal sections of VTEC values at geographic latitudes 25°N and 30°N from 29 November to 3 December 2023. The bottom panel presents the time series of the SYM-H index, with dashed lines indicating the onset of the geomagnetic storm and the recovery phase lasting until 3 December 2023. Figure 5 displays the latitudinal variations in VTECs with time at geographic longitudes 100°E and 110°E during the identical period. As shown in Figure 4, the longitudinal variations in VTEC values exhibited both similarities and differences between the two latitudes (25°N and 30°N). Compared to the geomagnetically quiet periods (29–30 November), VTEC values at 25°N displayed a distinct upward trend after the onset of the geomagnetic storm at 01:00 UT on 1 December, increasing from an average of 80 TECU during the calm period to a peak of 110 TECU at 07:00 UT on 1 December (Figure 4). In contrast, VTECs at 30°N showed a more moderate increase, rising from an average of 70 TECU during the calm period to a peak of 90 TECU at 07:00 UT on 1 December. Following the end of the storm on 2 December, VTECs at both 25°N and 30°N showed significant decreases, dropping to 60 TECU and 50 TECU, respectively. From 2 to 3 December, VTECs at 25°N exhibited a more sustained enhancement, increasing to 75 TECU, while VTECs at 30°N remained relatively stable at around 55 TECU. These variations highlight the regional differences in ionospheric responses to the geomagnetic storm, with low-latitude regions (25°N) showing more pronounced and sustained enhancements compared to mid-latitude regions (30°N). These findings confirm different ionospheric responses at different latitudes during the geomagnetic storm. Figure 5 shows that VTECs decreased with increasing latitude, and the VTEC variations at 100°E and 110°E had similar patterns from 29 November to 3 December. During the geomagnetic storm on 1 December, mid-latitude VTECs showed a significant increase compared to those on 29–30 November. In contrast, low-latitude VTECs displayed an unusual pattern, with a longer increase during the daytime on 1 December and 2 December throughout the storm. The SYM-H index indicates a significant enhancement of geomagnetic activity on 1 December, characterized by a rapid decline and notable fluctuations, suggesting that the intensity of geomagnetic disturbances was higher than usual.

Figure 6 shows the VTEC variations measured by BDS-GEO satellites at the five MEGX stations in China and nearby regions from 29 November to 3 December 2023. The dark blue and light blue dashed lines, respectively, represent the upper and lower bounds of predictions with thresholds calculated by Equation (1). The values above the red dashed lines, shown as red areas in Figure 6, are defined as positive responses, and those below the blue dashed line (blue areas) are defined as negative responses. As shown in the figure, VTECs at URUM and GAMG increased to varying degrees after the onset of the storm at 1:00 UT on 1 December, indicating moderate positive responses during the storm. Subsequently, slight negative responses were observed at the two stations during the night of the recovery phase from 2 to 3 December. VTECs at the low-latitude station CMUM exhibited generally similar variations, but a more significant decrease during 2–3 December, meaning a much stronger negative response. Stations JFNG and CKSV experienced moderate negative anomalies during the storm, but different patterns: JFNG reached its minimum VTEC on 1 December, while CKSV reached its minimum on 2 December. Both stations exhibited significant VTEC increases on 3 December, consistent with the results shown in Figure 5. VTECs at JFNG showed a distinct positive response of 15–20 TECU, while exhibiting a moderate positive response at CKSV. The different ionospheric responses during the storm and the recovery phase suggest that the responses to the geomagnetic storm had regional differences.

The analysis of VTEC variations during the geomagnetic storm on 1–2 December 2023 reveals significant spatial and temporal changes in the ionosphere over southern and central China. These variations are closely linked to geomagnetic activity, as indicated by the SYM-H index (Figure 4 and Figure 5), and exhibit distinct regional characteristics driven by different physical mechanisms. At mid-latitudes (35°N–45°N), the rapid increase in VTEC during the initial phase of the storm (01:00–07:00 UT on 1 December) is likely driven by the effect of prompt penetration electric fields from high latitudes, facilitating the transport of plasma from the equatorial region to mid-latitudes, which culminates in a peak VTEC value of 113 TECU at 07:00 UT (Figure 3). However, during the storm’s peak, particularly under the influence of storm-induced prompt penetration electric fields, the electron density in the day-side equatorial region becomes suppressed, resulting in a temporary decrease in TEC values. This decline is more pronounced in the low-latitude region (25°N–30°N), where VTEC peaks at 129 TECU at 11:00 UT, followed by a noticeable decrease, indicating the significant impact of penetrating electric fields on equatorial electron density. This stronger response can be attributed to the expansion of the equatorial ionization anomaly (EIA) during the storm, supported by enhanced equatorial electric fields and equatorward neutral winds [27]. The regional differences in VTEC response, particularly the sustained enhancement at 25°N during the recovery phase (2–3 December), further underscores the influence of the EIA, suggesting that the increase in electron density in low-latitude regions, influenced by penetrating electric fields, may persist longer, allowing for the prolongation of the EIA state.

The findings of this study are consistent with previous observations of ionospheric responses to geomagnetic storms, such as the 2020 event reported by Smith et al. (2023) [20], where similar enhancements in VTEC at low latitudes were observed. However, the timing of the VTEC peak in this study (07:00 UT on 1 December) differs from that observed in other events, which may reflect the unique characteristics of the December 2023 storm. These variations have important implications for GNSS positioning accuracy, particularly for precise point positioning. The rapid fluctuations in VTEC during the storm, especially at low latitudes, can introduce significant errors in signal propagation delays, leading to degraded positioning precision (Figure 6). The regional differences in VTEC response also suggest that GNSS users in low-latitude regions may experience more severe positioning errors during geomagnetic storms compared to those in mid-latitude regions. These insights highlight the importance of developing region-specific models to improve GNSS positioning accuracy during geomagnetic disturbances.

4. Analysis of Ionospheric Disturbance Characteristics and Multi-Data Source Verification

MSTIDs are a common phenomenon in the ionosphere, typically driven by atmospheric gravity waves (AGWs). AGWs induce plasma density fluctuations in the ionosphere through disturbances in neutral wind fields and electric fields, leading to the formation of MSTIDs. The propagation characteristics of MSTIDs are closely related to plasma density fluctuations in the ionosphere. During geomagnetic storms, these fluctuations are significantly enhanced, leading to increased propagation range and intensity of MSTIDs.

4.1. Analysis of MSTID Propagation Characteristics Based on TEC Data

To investigate the propagation characteristics of MSTIDs, we calculated detrended TEC (DTEC) variations using GNSS data from the Crustal Movement Observation Network of China (CMONOC). Figure 7 presents a series of 2D DTEC maps over China and surrounding regions at 15 min intervals from 15:00 to 17:00 UT on 1 December 2023. These maps illustrate the characteristics of nighttime ionospheric disturbances. At 15:00 UT, ionospheric TEC disturbances first appeared in the region between 100°E and 110°E and 35°N to 45°N, characterized by alternating positive and negative TEC deviations (represented by red and blue dots, respectively). Over time, the disturbances gradually expanded southward, with their intensity and range significantly increasing between 15:45 UT and 16:30 UT. Negative TEC deviations extended down to 25°N, while the alternation between positive and negative deviations became more pronounced, indicating enhanced periodic fluctuations and typical MSTID propagation characteristics.

Figure 8 further illustrates the propagation of VTEC disturbances at specific latitudes (25°N and 35°N) and longitudes (100°E and 115°E). Figure 8a shows the temporal variations in TEC disturbances at 25°N and 35°N latitudes. At 25°N, TEC remained stable before 09:00 UT, after which negative deviations gradually appeared within the band between 98°E and 110°E, intensifying from 12:00 UT to 16:00 UT, particularly around 100°E. The alternation between positive and negative deviations indicates enhanced ionospheric disturbances, consistent with the typical propagation characteristics of MSTIDs. The disturbances propagated westward from 122°E to 94°E at a speed of approximately 80 m/s. At 35°N, TEC variations exhibited more pronounced wave-like patterns, with negative deviations gradually intensifying from 09:00 UT and reaching a maximum around 110°E between 12:00 UT and 15:00 UT. During this period, disturbances propagated westward from 126°E to 98°E at a speed of approximately 90 m/s.

Figure 8b shows the latitudinal variations in TEC disturbances at 100°E and 115°E longitudes. At 100°E, negative deviations gradually extended northward from 45°N to 25°N starting at 09:00 UT, peaking between 12:00 UT and 17:00 UT. The propagation speed of this disturbance was approximately 81 m/s, demonstrating typical MSTID propagation from higher to lower latitudes. At 115°E, disturbances first appeared at 11:00 UT and propagated from 42°N to 22°N. Although the disturbances in this region were less pronounced, alternating positive and negative deviations were still observed. The propagation speed of ionospheric disturbances along the 115°E profile was approximately 88 m/s, further confirming the typical poleward propagation characteristics of MSTIDs.

4.2. Validation of MSTIDs and Their Ionospheric Impact Through Electron Density Observations

To further investigate the driving mechanisms of MSTIDs, we analyzed changes in thermospheric composition. Figure 9 presents the thermospheric [O]/[N₂] ratio maps obtained by the GUVI/TIMED satellite from 29 November to 3 December 2023. On 1 December, a significant increase in the [O]/[N₂] ratio was observed over southern and central China (25°N to 40°N, 100°E to 120°E), indicating an increase in atomic oxygen (O) concentration during the geomagnetic storm. This increase in atomic oxygen contributed to higher electron density, corresponding to the positive deviations in VTEC (red regions). The enhanced [O]/[N₂] ratio exacerbated MSTID, further suggesting a strong correlation between ionospheric disturbances and atmospheric gravity waves. It is also important to emphasize that penetrating electric fields can simultaneously trigger both positive and negative ionospheric storms, indicating that during the formation of MSTIDs, there may be both positive ionospheric enhancements and accompanying negative ionospheric depletions.

Meanwhile, Figure 9 shows the spatio-temporal variations in the [O]/[N₂] ratio from 30 November to 3 December 2023. Notably, a significant increase in the [O]/[N₂] ratio was observed over southern and central China (25°N to 40°N, 100°E to 120°E) on 1 December compared to the values on 30 November. This indicates that higher levels of atomic oxygen (O) during the geomagnetic storm led to increased electron density, corresponding to the positive deviations in VTEC (red regions). The increased [O]/[N₂] ratio contributed to the intensification of MSTID, further highlighting the strong correlation between ionospheric disturbances and atmospheric gravity waves. Since 16:00 UT, the alternating structure of positive and negative TEC deviations showed wave-like patterns propagating from the northwest to the southeast, aligning with the classic nighttime MSTID propagation mode. At 17:00 UT, the TEC disturbances reached their furthest geographic extent, expanding between 100°E and 120°E and latitudes from 25°N to 40°N, with maximum intensity. These findings indicate that MSTIDs continued to intensify during this period, displaying significant wave amplitudes. The spatial distribution and spatio-temporal evolution of the TEC disturbances suggest that the propagation pattern of this event aligns with typical mid- and low-latitude nighttime MSTID events. The alternating positive and negative TEC deviations underscore the crucial role of atmospheric gravity waves in these disturbances. From 15:00 UT to 17:00 UT, the disturbances propagated from the northwest to the southeast, consistent with the typical characteristics of nighttime MSTIDs [36]. These findings suggest that the event was driven by strong ionospheric disturbances induced by atmospheric gravity waves.

4.3. Relationship Between Thermospheric Composition Changes and MSTIDs

To validate the presence of MSTIDs and their impact on the ionosphere, we analyzed electron density (Ne) observations from the Swarm A and C satellites. Figure 10 shows the Ne variations observed by Swarm A (top) and Swarm C (bottom) at different times on 30 November and 1 December. On 1 December, significant electron density disturbances were observed at mid- and low-latitudes, particularly within the 25°N to 35°N range. These disturbances were highly consistent with the propagation range and duration of MSTIDs shown in Figure 7 and Figure 8, confirming the presence of MSTIDs during the geomagnetic storm. Notably, Ne significantly increased from 11:52 UT to 13:33 UT on 1 December, aligning with the propagation of MSTIDs driven by atmospheric gravity waves.

4.4. Multi-Source Validation of MSTID Characteristics

Figure 11 displays the multiple system PPP residuals at five MGEX stations across China and its surrounding regions on 1 December 2023. This section highlights the diverse impacts of ionospheric disturbance observed during this period.

By verifying data from multiple sources, including TEC, thermospheric composition, and electron density observations, this study explores the propagation characteristics of MSTIDs over southern and central China during the geomagnetic storm on 1–2 December 2023. MSTIDs demonstrated typical northwest-southeast propagation with speeds ranging from 80 to 90 m/s, which aligns with the driving mechanisms of atmospheric gravity waves. Additionally, an increase in the thermospheric [O]/[N₂] ratio exacerbated ionospheric disturbances, resulting in significant electron density increases. The electron density observations from Swarm satellites closely matched TEC variations, confirming the presence of MSTIDs and their substantial effects on the ionosphere.

5. Analysis of Multi-Instrument Data on the Impact of Disturbances on PPP Performance

Figure 11 displays multi-GNSS precise point positioning residuals at five MGEX stations distributed across China and adjacent regions on 1 December 2023. This section analyzes the station-dependent impacts of ionospheric disturbances on PPP accuracy during the geomagnetic storm event, with cross-validation from multi-instrument observations.

At URUM, the U-direction PPP residuals increased sharply by approximately 0.5 m (RMSE = 0.3 m) following the onset of the geomagnetic storm at 1:00 UT, coinciding with a significant increase in VTEC. This indicates that the rapid rise in electron density during the initial phase of the storm significantly degraded vertical positioning precision. In contrast, PPP results at GAMG showed a distinct pattern: although positioning precision decreased after the storm onset, it quickly converged within a short period, with only mild subsequent disruptions. The faster recovery at GAMG suggests that the ionospheric conditions at this station were more stable, or the multi-system observing geometry provided greater resilience to disturbances. For the low-latitude station CMUM, slightly lowered precision was observed on 1 December, with U-direction residuals remaining relatively small (RMSE = 0.2 m). This indicates that low-latitude regions experienced smaller disturbances during the early phase of the storm. In contrast, mid-latitude stations JFNG and CKSV exhibited significant increases in N- and U-direction residuals (up to 0.8 m and 1.2 m, respectively) during the afternoon of 1 December (15:00 to 17:00 UT). These increases coincided with the westward propagation of MSTIDs observed in Figure 7 and Figure 8, suggesting that periodic ionospheric fluctuations in mid-latitude regions had a greater impact on PPP precision. The electron density (Ne) observations from the Swarm A and C satellites further validated these findings. A substantial increase in Ne over southern China around 10:30 UT on 1 December aligned closely with the rise in U-direction PPP residuals at URUM. However, PPP at GAMG station was able to converge rapidly during this period, highlighting the varying sensitivities of different stations to ionospheric changes under similar geomagnetic storm conditions.

The comprehensive analysis of PPP precision, combined with cross-validation using multi-instrument data, reveals the varying influences of ionospheric disturbances at different GNSS stations during the geomagnetic storm. The rapid convergence at GAMG station implies that, under multi-system conditions, certain stations possess stronger resilience to ionospheric disturbances for PPP.

6. Conclusions

The primary objective of this study was to analyze the characteristics of ionospheric disturbances over southern and central China during the geomagnetic storm on 1–2 December 2023, using multi-instrument data from GNSS, TIMED/GUVI, and Swarm satellites. By analyzing TEC data and ionospheric responses, the study draws the following conclusions:

(1)

Significant increases in VTEC were observed during the storm, particularly in low-latitude regions (25°N–30°N), where VTEC peaked at 129 TECU at 11:00 UT on 1 December. In contrast, mid-latitude regions (35°N–45°N) exhibited a more moderate increase, with VTEC reaching 113 TECU at 07:00 UT. These regional differences are attributed to the combined effects of prompt penetration of electric fields, neutral wind dynamics, and the expansion of the equatorial ionization anomaly.

(2)

The rapid fluctuations in VTEC during the storm, especially at low latitudes, introduced significant errors in GNSS signal propagation delays, leading to degraded positioning precision. This highlights the vulnerability of GNSS systems to ionospheric disturbances during geomagnetic storms, particularly in low-latitude regions.

(3)

The findings of this study are consistent with previous observations of ionospheric responses to geomagnetic storms, such as the 2020 event reported by Smith et al. (2023), where similar enhancements in VTEC at low latitudes were observed. However, the timing of the VTEC peak in this study (07:00 UT on 1 December) differs from that observed in other events, which may reflect the unique characteristics of the December 2023 storm.

(4)

The study also reveals that the thermospheric [O]/[N₂] ratio significantly increased over southern and central China during the storm, particularly within the range of 25°N–40°N. This increase in atomic oxygen concentration contributed to the enhancement of electron density, further exacerbating ionospheric disturbances.

(5)

The comprehensive analysis of VTEC variations, combined with multi-instrument data validation, demonstrates the importance of considering regional differences in ionospheric response when assessing the impact of geomagnetic storms on GNSS positioning accuracy. These findings provide a scientific basis for improving ionospheric modeling and GNSS positioning stability, particularly in mid- and low-latitude regions.

In conclusion, this study enhances our understanding of ionospheric disturbances during geomagnetic storms and their impact on GNSS positioning accuracy. The results highlight the need for developing region-specific models to improve GNSS positioning stability during geomagnetic disturbances. Future research should focus on investigating the ionospheric response under different solar and geomagnetic conditions, as well as exploring the impact of ionospheric disturbances on emerging GNSS technologies, such as multi-frequency and multi-constellation systems.