# Observational Analysis of Variation Characteristics of GPS-Based TEC Fluctuation over China
## 4 Conclusions  
In this study, we investigated and analyzed the features of TEC fluctuations and their regional differences over China during a solar cycle, the 11-year pattern from 2002 to 2012. We utilized ROTI for the measurement of TEC fluctuations and correlated the measurements with irregular ionospheric activities (based on dual-frequency GPS data from 21 tracking stations provided by CMONC).  
Our main conclusions are as follows.  
(1) Strong TEC fluctuations were usually observed over low-latitude southern China and took on typical variations in temporal characteristics. Moreover, strong TEC fluctuations mainly occurred at night in the equinox months, especially during the years of higher solar activity. This phenomenon is consistent with the ionospheric scintillation characteristics over southern China.  
(2) We observed that the intensity of equinox characteristics of TEC fluctuations weakened or disappeared entirely as the summer characteristics of TEC fluctuations gradually emerged. This is especially evident with increases in latitude, such as at the LUZH and WUHN sites, particularly during the years of lower solar activity. Under the above conditions, the occurrence of TEC fluctuations over China's mid-latitude regions is no longer restricted to nighttime but might also occur during the daytime.  
(3) With respect to the lower mid-latitude regions of China, ionospheric TEC fluctuations mainly occur during the summer, such as at the ZHNZ site. Nevertheless, season-dependent variations in the characteristics of TEC fluctuations were rarely observed at the higher mid-latitude regions of northern China, such as at the SUIY and HLAR sites. Additionally, our results revealed that the season-dependent characteristics of TEC fluctuations weaken or disappear with increases in latitude. Relevant discussions of this phenomenon are still relatively rare, so our results contribute to the development of a more in-depth understanding of irregular ionospheric activities, specifically the characteristics and features that occur over China.  
This work was partially supported by the Collaborative Precision Positioning Project funded by the Ministry of Science and Technology of the People's Republic of China (No. 2016 YFB0501900) and the National Nature Science Foundation of China (Grant No. 41674022, 41231064, 41304034 and 41574033). We also acknowledge the funding support provided by the State Key Laboratory of Geodesy and Earth's Dynamics (the Institute of Geodesy and Geophysics, CAS) (No. SKLGED2014-3-1-E). The authors would like to extend their sincere gratitude to the Crustal Movement Observation Network of China for providing us with observational data. The authors are grateful to the Chinese Meridian Project for supplying relevant information concerning ionospheric scintillations. The authors are thankful to the IGS Multi-GNSS Experiment ([[http://mgex.igs.org](http://mgex.igs.org)]([http://mgex.igs.org](http://mgex.igs.org))) and International GNSS Monitoring and Assessment Service (iGMAS) for supplying the related produtions and services.  
[PERSON] and [PERSON] provided the initial idea for this study; [PERSON] and [PERSON] conceived and designed the experiment; [PERSON] and [PERSON] analyzed the results of the experiment; [PERSON] and [PERSON] wrote the paper.  
The authors declare no conflicts of interest.  
The following abbreviations are used in this manuscript:  
\begin{tabular}{l l} CMONOC & Crustal Movement Observation Network of China \\ TEC & Total Electron Content \\ ROT & Rate of TEC \\ ROTI & Rate of TEC Index \\ \end{tabular}