# Observational Analysis of Variation Characteristics of GPS-Based TEC Fluctuation over China
###### Abstract  
In this study, the characteristics of the total electron content (TEC) fluctuations and their regional differences over China were analyzed by utilizing the rate of the TEC index (ROTI) based on GPS data from 21 reference stations across China during a solar cycle. The results show that there were significant regional differences at different latitudes. Strong ionospheric TEC fluctuations were usually observed at lower latitudes in southern China, where the occurrence of TEC fluctuations demonstrated typical nighttime- and season-dependent (equinox months) features. This phenomenon was consistent with the ionospheric scintillation characteristics of this region. Additionally, compared to low-latitude China, the intensity of TEC fluctuations over mid-latitude China was significantly weaker, and the occurrence of TEC fluctuations was not a nighttime-dependent phenomenon. Moreover, the intensity of TEC fluctuations was much stronger during high solar activity than during low solar activity. Furthermore, the summer-dependent characteristics of TEC fluctuations gradually emerged over lower mid-latitude areas as equinox characteristics weakened. Similar to the equinox characteristics, the summer-dependent characteristics gradually weakened or even disappeared with the increasing latitude. Relevant discussions of this phenomenon are still relatively rare, and it requires further study and analysis.  
The TEC fluctuation index ROTI (which is derived from ordinary non-scintillation GNSS receivers sampled at low frequencies) was first proposed by [PERSON] et al. [7,11] in 1997. Since then, it has been widely used to detect irregular ionospheric activities, which are defined as the standard deviation of the rate of TEC (ROT) in a given 5 min interval. Previous studies have demonstrated close correlations between ROTI and scintillation indices [7,15,21,22]. Over the past two decades, ROTI has been widely used to detect regional and global TEC fluctuations, especially with the growing number of GNSS tracking sites [7,11,23,24,25,23,26,1,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25].  
China, which is situated at low and middle latitudes, is a suitable choice for conducting studies on irregular ionospheric activities, owing to the complexity and variability in ionospheric structures over this country. During the past two decades, many scholars have investigated and studied the ionospheric scintillations relevant to irregularities over the north crest of the equatorial anomaly region in southern China [15,22,26,27,28,29,30,31,32,33,34,35,36,37,38,39]. These studies revealed variations in temporal and spatial characteristics in ionospheric scintillations. [PERSON] et al. [26,27] investigated the statistical features of ionospheric scintillations derived from GPS satellites over south China and discussed the close correlations among TEC depletion, scintillation, and ROTI. [PERSON] et al. [33] analyzed the equinoctial behaviors of ionospheric plasma density during low solar activity and revealed the phenomenon of equinoctial asymmetry in ionospheric plasma density. [PERSON] et al. [37] analyzed the close relationship between strong range spread F and ionospheric scintillation over Hainan; their correlation coefficient was as high as 0.93. [PERSON] et al. [22] discussed the close relationship between ROTI and the scintillation indices over Hong Kong. [PERSON] et al. [15] investigated the ratio of ROTI to the amplitude scintillation indices over Sanya and showed that it varied between 0.3 and 6.0.  
However, previous investigations on the characteristics of irregular ionospheric activities over China have been insufficient because of the restrictions over the lower latitudes in China. Thus, in this study, ROTI indices were used to investigate the characteristics of ionospheric TEC fluctuations, varying with local time (LT) and season, and their regional differences over China during a solar cycle (the 11-year pattern from 2002 to 2012).