# Observational Analysis of Variation Characteristics of GPS-Based TEC Fluctuation over China
## 3 Results and Discussion
### Daily Occurrence Ratio of TEC Fluctuations from 2002 to 2012  
Typically, TEC fluctuations satisfying ROTI \(\geq\) 0.5 indicate the occurrence of irregular ionospheric activities relevant to ionospheric scintillation [22]. In this study, we wanted to conduct a more in-depth quantitative analysis of the intensity of ionospheric disturbances and their occurrence ratio over China. As such, we based our analysis on the original ROTI value, which requires that the standard level of TEC fluctuations be preliminarily divided into three groups: weak if 0.25 \(\leq\) ROTI \(<\) 0.5; moderate if 0.5 \(\leq\) ROTI \(<\) 1; and strong if ROTI \(\geq\) 1. The daily occurrence ratio is the ratio of the TEC fluctuation epoch numbers to all of the observed epoch numbers within one day. Figure 8: The time series of ROTI in UT for all of the observed GPS satellites at the WUHN site during the four seasons of 2012. Figure 7: The time series of ROTI by day of year (DOY) for all of the observed GPS satellites at the WUHN site during 2012. The PRN numbers of each subfigure are as follows: (**a**) 1, 5, 9, 13, 17, 21, 25, 29; (**b**) 2, 6, 10, 14, 18, 22, 26, 30;(**c**) 3, 7, 11, 15, 19, 23, 27, 31; (**d**) 4, 8, 12, 16, 20, 28, 32. Figure 9: _Cont._  
Figure 9 shows the daily occurrence ratio of different-level TEC fluctuations day by day during a solar cycle (from 2002 to 2012) for each of the selected sites. All of the ROTI values satisfy the conditions detailed above. As shown in Figure 9, the long-periodic variation characteristics of ionospheric TEC fluctuation changes can be observed during a solar cycle, which is closely related to the years of the solar activity. Generally, the intensity of TEC fluctuations is observed to be much stronger during high solar activity than during low solar activity. Of the 11 years from 2002 to 2012, the occurrence frequencies and the intensity of TEC fluctuations reached the highest levels during 2002. Additionally, the intensity of TEC fluctuations was slightly weaker than that of 2002 during years of solar activity, such as 2011 and 2012, followed by 2003 and 2004. In other years, the intensity of TEC fluctuations became much smaller during periods of lower solar activity, such as from 2005 to 2010. Moreover, Figure 9 clearly indicates that strong TEC fluctuations that satisfy ROTI \(\geq\) 1 occurred mainly during the spring and autumn over most of the lower-latitude regions in southern China. Of those sites satisfying the strong ROTI requirements, the equinox characteristics in ionospheric disturbances were most significant at the YONG and QION sites. Equinox characteristics in ionospheric disturbances became slightly weaker at the GUAN, KMIN, XIAM, and XIAG sites, which is consistent with typical ionospheric scintillation characteristics over the north crest of the equatorial anomaly region in southern China. Significantly, the weakening of equinox characteristics in TEC fluctuations is related to not only the location but also the duration of solar activity during periods of high and low intensity. However, concurrent with the gradual weakening of equinox characteristics of TEC fluctuations, summer characteristics of TEC fluctuations gradually emerged at sites such as LUZH, WUHN, ZHNZ, and XIAA during the years of lower solar activity. The above phenomenon indicates that the weakening of equinox characteristics corresponds to the emergence of summer characteristics over southern China. The reason for the occurrence of TEC fluctuations during the June solstice over most of central China might be closely related to sporadic-E (Es) activity and medium-scale traveling ionospheric disturbances (MSTIDs) [45; 46; 47]. Additionally, the strong TEC fluctuation satisfying ROTI \(\geq\) 1 became increasingly rare with increasing latitude for all of the sites north of the LUZH site. Moreover, sites located in the higher mid-latitude areas, such as SUIY and HLAR, demonstrated weak or nonexistent summer characteristics in TEC fluctuations.