## 4 Discussion  
We matched NmF2/hmF2 of FY3C/GNOS and COSMIC into data pairs, the correlation coefficients of both NmF2 and hmF2 data pairs are above 0.9, and the stds of NmF2 and hmF2 are 17.48% and 18.01%, respectively, which are comparable and even better than the statistical error between FY3C/GNOS and ionosondes, thus the precision of NmF2/hmF2 of FY3C/GNOS are verified successfully by comparison with COSMIC. However, at nighttime, the stds of both NmF2 and hmF2 data pairs are worse than that at daytime and during the whole day, indicating the deteriorated consistency of NmF2/hmF2 at nighttime. This may be caused by the lack of the strong daytime photochemical control in F2 layer at nighttime. Under the strong photochemical control at daytime, the carrier phase observation is not affected by the irregularities too much. However, at nighttime, the absence of strong photochemical control gives an opportunity to the daytime hold-over plasma bubbles [66], thus RO signals in different directions are randomly modulated when passing through the irregularities, resulting in discrepancies of NmF2/hmF2 between FY3C/GNOS and COSMIC at nighttime. From the ionospheric climatological characteristics of NmF2 in four seasons we can see that the ionospheric morphology of FY3C/GNOS and COSMIC are highly agreeable, both showing the typical ionospheric features such as EIA, annual anomaly, semiannual anomaly, winter anomaly, WSA, and so on. The occurrences of these phenomena are closely related to ionospheric dynamics, neutral composition, diurnal tides, solar activity, thermospheric winds, geomagnetic field, and other factors [67; 68; 69; 70; 71; 72; 73; 74]. For example, EIA is mainly caused by the equatorial fountain effect driven by E\(\times\)B drift, which lifts the plasma along the magnetic equator upward to higher latitudes, depleting the plasma in the ionization concentration region, namely, the magnetic equatorial region, thus forming  
Figure 13: Ionospheric climatological characteristics of season median hmF2 observed by FY3C/GNOS and COSMIC and their differences at 0800–1100 LT and 2000–2300 LT during DS-month (2016.311–2017.035). The regions denoted by red and black arrows indicate the diurnal variation of hmF2, where the general WSA and special WSA phenomenon can be observed at nighttime, respectively. Refer to Figure 10 for more detailed descriptions. two EIA crests on both hemispheres [67; 68; 69]. Meanwhile, the amplitude variation of EIA has a lot to do with the semiannual anomaly. The semiannual anomaly seems to abide by the ExB drift modulated by the semiannual variation of the diurnal tide in thermosphere [46], and the amplitude of the semiannual anomaly is positively correlated with solar activity [74]. In addition, [PERSON] [70] observed that the winter anomaly is limited at low-latitudes from 0800 LT to 1100 LT and is almost absent in mid-latitudes in the LSA period, which is consistent with what was revealed in our work. It is considered that the winter anomaly is directly related to the global change of [O]/[N2] ratio, which is the result of heating of the summer hemisphere causing some lighter neutral atmospheric compositions to form convection with the winter hemisphere [58; 70; 71]. Identical to general WSA observed in our work, the similar anomaly of TEC in the southeastern Pacific Ocean in summer hemisphere is also observed in work of [PERSON] et al. [72; 73], in which the TEC enhancement appears between 2200 LT and 2400 LT. The present work proposes that the mechanism of WSA phenomenon is most linked to thermospheric wind and regulation of the magnetic field [36]. The season pattern of hmF2 is pretty simple, regulated by an annual variation peaking in local summer [75]. The global ionospheric climatological characteristics of hmF2 measured by FY3C/GNOS are also highly consistent with that of COSMIC, such as hemispheric asymmetry, WSA.