# Observational Analysis of Variation Characteristics of GPS-Based TEC Fluctuation over China
## 3 Results and Discussion
### Temporal Variation Analysis of ROTI at the QION and WUHN Sites  
Figure 5 shows the time series of ROTI by day of year (DOY) at the QION site during 2012. The results show that most of the TEC fluctuations occurred during spring and autumn (equinox months) and that TEC fluctuations rarely occurred during summer and winter. Additionally, the daily occurrence of TEC fluctuations during the equinox months demonstrated obvious randomness, which makes it difficult to forecast irregular ionospheric activities (as described in Figure 5). Figure 6 shows the time series of ROTI in UT for all of the observed GPS satellites at the QION site in each season in 2012. Our results show that the TEC fluctuations mainly occurred between 7 p.m. LT (pre-midnight) and 3 a.m. LT (post-midnight) during the spring and autumn, primarily in the pre-midnight hours. The TEC fluctuations were much weaker during summer and winter than they were during spring and autumn. The weak TEC fluctuations in the summer mainly occurred during the nighttime. Interestingly, during the winter, the occurrence of weak TEC fluctuations could be observed not only at night but also during the day. The variations in the temporal characteristics of the TEC fluctuations over the Hainan region coincide with those of previous investigations of season- and nighttime-dependent variations in ionospheric scintillation characteristics over the north crest of the equatorial anomaly region in China's lower latitudes.  
Figure 4: The spatial distribution of IPP according to the ROTI value for all of the observed GPS satellites from 20 sites across China (8 October 2003). The red point line refers to the IPP locations satisfying extremely strong TEC fluctuations ROTI \(\geq 2.5\); the pink point line refers to the IPP locations satisfying sub-strong TEC fluctuations \(1.0\leq\text{ROTI}<2.5\); the yellow point line refers to the IPP locations satisfying moderate TEC fluctuations \(0.5\leq\text{ROTI}<1.0\); the green point line refers to the IPP locations satisfying weak TEC fluctuations \(0.25\leq\text{ROTI}<0.5\); and the light blue point line refers to the IPP locations satisfying no TEC fluctuations ROTI \(<0.25\).  
Similar to Figure 5, Figure 7 shows the time series of ROTI by DOY at the WHUN site. The intensity of TEC fluctuations was significantly weaker at WHUN than at QION. Unlike QION, most of the TEC fluctuations over WUHN were observed in the spring, summer, and autumn. However, TEC fluctuations were only rarely observed during the winter. Thus, in contrast to QION, the season-dependent characteristics of TEC fluctuations over WUHN were significantly weaker. Moreover, the daily occurrence of TEC fluctuations demonstrated obvious randomness over WUHN. Figure 8 shows the distribution of TEC fluctuations (in UT) derived from all of the observed GPS satellites at the WHUN site during each season of 2012. Compared with the QION site, the LT time-dependent characteristics of the TEC fluctuations were weak and occasionally disappeared entirely during each season. This indicates that the occurrence of TEC fluctuations ceased to be a nighttime phenomenon.  
Figure 5: The time series of ROTI by day of year (DOY) for all of the observed GPS satellites at the QION 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 6: The time series of ROTI (in UT) for all of the observed GPS satellites at the QION site during the four seasons of 2012. The observation periods of each subfigure are as follows: spring: 1 March to 31 May; summer: 1 June to 31 August; autumn: 1 September to 30 November; winter: 1 December to 28 February.