## 4 Spatial and Temporal Variation Characteristics of TEC in the Polar Regions
### Spatial Distribution Characteristics of Ionosphere TEC in the Polar Regions
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. The TEC values at different latitudes varied widely at the same time and generally increased with the decrease of latitude. However, during Arctic polar days, the TEC decreased with decreasing latitude at night (Figures 6 and 8). This was due to the fact that there was more solar radiation in high-latitude areas within a day during polar days. At night, the solar radiation weakened as the latitude decreased during polar days, and the generation of free electrons diminished. Similarly, during polar nights, the ionospheric TEC significantly increased over the Arctic and Antarctic when the latitude became lower in the daytime (Figures 5, 7, 10 and 12). The polar ionospheric TEC exhibits significant diurnal variations, which are clearly shown during polar days (Figures 6 and 8) and polar nights in the Arctic (Figures 5 and 7), as well as during polar nights in the Antarctic (Figures 10 and 12). At a given time, the ionospheric TEC value generally reached its maximum from 12:00 to 14:00 LT. However, in the Antarctic, during polar days (Figures 9 and 11), the TEC at UTC (Universal Time Coordinated) from 1:00 to 11:00 did not reach the maximum value near the time window 12:00-14:00 LT. During this period, the maximum value of TEC mostly appeared in the areas of the Antarctic Peninsula and the Weddell Sea. This was mainly caused by the WSA, which is characterized by a greater electronic density at nighttime with respect to daytime in summer over the Antarctic Peninsula and Weddell Sea areas [23]. As a consequence, during the polar days in the Antarctic, the most active ionospheric TEC occurred at night in the Antarctic Peninsula and the Weddell Sea. of GNSS tracking stations with uneven distribution in the Antarctic before 2010, the ionospheric TEC cannot be effectively modeled. Thus, the available TEC model in the Antarctic starts from 2010. As seen in Figure 13, the mean diurnal variation of C0,0 coefficient was very close to that of the GIM model. The correlation coefficients between them were 0.97 in the Arctic, and 0.92 in the Antarctic, respectively. During about one solar cycle, the ionospheric TEC variation was strongly correlated with the solar activity level. From 2006 to 2018, the solar activity reached its highest level in 2014, and the maximum values of the ionospheric TEC also occurred in that particular year over the Antarctic and Arctic. The ionospheric TEC of the Antarctic had a larger amplitude of annual variation than that of the TEC in the Arctic. Moreover, the TEC variation of the Arctic during one year was opposite to that of the Antarctic. In the Arctic, the ionospheric TEC reached the maximum in June and the minimum in December; on the contrary, the maximum appeared in December and the minimum occurred in June over the Antarctic. This was directly connected to the seasonal variation of the ionosphere. In order to analyze the diurnal TEC variation in different seasons, the four months March, June, September, and December were selected for representing spring, summer, autumn, and winter in the Arctic, while in the Antarctic representing autumn, winter, spring, and summer, respectively. Figures 14 and 15 show the diurnal change of the C0,0 with the LT in four seasons under low (2010) and high (2014) solar activity conditions, respectively. The TEC was monthly averaged with a 2 h resolution. In the same season, the ionospheric TEC in 2014 was significantly higher than that in 2010 due to the influence of solar radiation, which is consistent with the spatial distributions of TEC shown in Figures 5-12. The diurnal variation of the polar ionospheric TEC had lower amplitude than that in the middle and low latitude regions. In the Arctic, the minimum of the ionospheric TEC occurred in winter, and the maximum values appeared in spring and summer under active and calm ionosphere conditions, respectively.