## 4 Spatial and Temporal Variation Characteristics of TEC in the Polar Regions
### Spatial Distribution Characteristics of Ionosphere TEC in the Polar Regions  
In order to analyze the spatiotemporal characteristics of the polar TEC under disturbed conditions, including different phases of solar activity, the polar days and nights, and the WSA, days of year (DOYs) 001 and 185 (corresponding to January 1 and July 4) in 2010 (low solar activity year) and 2014 (high solar activity year) were selected in this paper. The solar activity index of F10.7 on DOYs 001 and 185 in 2010 were 75.2 and 71.6 sfu, respectively, and in 2014 were 159.6 and 116.8 sfu, respectively. DOYs 001 and 185 were polar night and polar day in the Arctic, while in the Antarctic they were polar day and polar night, respectively. Moreover, the polar day on DOY 001 in the Antarctic was accompanied by the WSA. Figures 5-12 show the spatial distributions of the ionospheric TEC on DOYs 001 and 185 in 2010 and 2014 over the Arctic and Antarctic. As shown in the figures, the solar activity significantly affected the changes of the ionospheric TEC. For the same DOY and region, the overall ionospheric TEC level during the high solar activity year was generally three times higher than that during the low solar activity year (Figure 5 vs. Figure 7; Figure 6 vs. Figure 8; Figure 9 vs. Figure 11; Figure 10 vs. Figure 12). In addition, the ionosphere TEC had different characteristics during polar days and nights. In the same year and region, the ionospheric TEC underwent a more active condition in polar days than in polar nights. For example, the solar activity levels were similar on DOYs 001 and 185 in 2010, but the TEC in the polar day was about twice as high as that in polar night in the same region. The main reason is that during days, there is more solar radiation within one day in most of the areas, and the amount of neutral gas ionization in the ionosphere increases. On the contrary, during polar nights, most of the region has no solar radiation in a single day at all, the neutral gas cannot be ionized and the free electrons are constantly depleted. The main source of ionization in the polar nights is from the high-energy particles which impinge on the ionosphere due to the extended magnetic field lines in the magnetosphere [34]. Figure 5: Spatial distribution of mapping TEC of 12 sections under low solar activity year, polar night (DOY 001 in 2010) over the Arctic. The magenta lines in each subplot represent the longitudes at 12:00 and 14:00 at the middle of the 2 h section of the corresponding subplot. Figure 6: Spatial distribution of mapping TEC of 12 sections under low solar activity year, polar day (DOY 185 in 2010) over the Arctic. The magenta lines in each subplot represent the longitudes at 12:00 and 14:00 at the middle of the 2 h section of the corresponding subplot. Figure 7: Spatial distribution of mapping TEC of 12 sections under high solar activity year, polar night (DOY 001 in 2014) over the Arctic. The magenta lines in each subplot represent the longitudes at 12:00 and 14:00 at the middle of the 2 h section of the corresponding subplot. Figure 8: Spatial distribution of mapping TEC of 12 sections under high solar activity year, polar day (DOY 185 in 2014) over the Arctic. The magenta lines in each subplot represent the longitudes at 12:00 and 14:00 at the middle of the 2 h section of the corresponding subplot. Figure 9: Spatial distribution of mapping TEC of 12 sections under low solar activity year, polar day (DOY 001 in 2010) over the Antarctic. The magenta lines in each subplot represent the longitudes at 12:00 and 14:00 at the middle of the 2 h section of the corresponding subplot. Figure 11: Spatial distribution of mapping TEC of 12 sections under high solar activity year, polar day (DOY 001 in 2014) over the Antarctic. The magenta lines in each subplot represent the longitudes at 12:00 and 14:00 at the middle of the 2 h section of the corresponding subplot. Figure 10: Spatial distribution of mapping TEC of 12 sections under low solar activity year, polar night (DOY 185 in 2010) over the Antarctic.