## 3 Results
### Validation of NmF2/hmF2 observed by FY3C/GNOS with COSMIC
#### 3.1.2 Error Analysis of hmF2 between FY3C/GNOS and COSMIC  
Figure 4a,b depicts the linear fit and statistical error distribution of hmF2 between FY3C/GNOS and COSMIC, respectively. As can be seen in Figure 4b, the correlation coefficient of hmF2 data pairs, bias, and std of absolute errors of hmF2 data pairs are 0.91, \(-\)3.29 km, and 18.01 km, respectively, and the absolute errors of hmF2 are most focused within \(\pm\)20 km, showing a symmetric normal distribution with an average value around 0. Linear fits of hmF2 at daytime(0600 LT-1800 LT) and nighttime(1800 LT-0600 LT) are depicted in Figure 5a,b, respectively. During daytime, the correlation coefficient, bias, and std of hmF2 are 0.93, \(-\)2.48 km, and 15.59 km, respectively, during nighttime, the correlation coefficient, bias, and std of hmF2 are 0.81, \(-\)5.33 km, and 22.82 km, respectively.  
As can be seen in Figures 4 and 5, the correlation coefficients of hmF2 are all above 0.9 though exceptions do exist at nighttime, which shows a good correlation of hmF2 between FY3C/GNOS and COSMIC. In addition, the bias in range of \(-\)6 to \(-\)2 km with the std between 15 and 23 km indicates the little significance of systematic offset between FY3C/GNOS and COSMIC. It can be seen that the overall std and bias are 18.01 and \(-\)3.29 km, respectively, showing that the hmF2 between FY3C/GNOS and COSMIC are highly agreeable. Similar to NmF2, the precision agreement of hmF2 also deteriorated severely at nighttime.  
Figure 3: Linear fits of NmF2 between FY3C/GNOS and COSMIC at daytime and nighttime during 2016.035 and 2017.035: (**a**) the linear fit of NmF2 between FY3C/GNOS (horizontal axis) and COSMIC (vertical axis) at 0600–1800 LT, corresponding to daytime; (**b**) the linear fit of NmF2 at 1800–0600 LT, corresponding to nighttime. The correlation coefficient of NmF2 data pairs, the bias, and std of relative errors of matched NmF2 data pairs at daytime and nighttime are presented in the bottom right corner of (**a**) and (**b**), respectively.  
Figure 4: Linear fit and statistical error distribution of hmF2 between FY3C/GNOS and COSMIC over a whole day during 2016.035 and 2017.035: (**a**) the linear fit of hmF2 between FY3C/GNOS (horizontal axis) and COSMIC (vertical axis) over a whole day, in which the blue dots represent the matched data pairs of hmF2 and the red line depicts the linear regression fit of hmF2 data pairs; (**b**) the number (vertical axis) of hmF2 absolute errors (horizontal axis) between FY3C/GNOS and COSMIC, in which the red curve represents the distribution fit of the absolute errors. The correlation coefficient of hmF2 data pairs, bias, and std of absolute errors of matched hmF2 data pairs are also labelled in the right side of (**b**).  
Figure 5: Linear fits of hmF2 between FY3C/GNOS and COSMIC at daytime and nighttime during 2016.035 and 2017.035: (**a**) the linear fit of hmF2 between FY3C/GNOS (horizontal axis) and COSMIC (vertical axis) at 0600–1800 LT, corresponding to daytime; (**b**) the linear fit of hmF2 at 1800–0600 LT, corresponding to nighttime. The correlation coefficient of hmF2 data pairs, the bias, and std of absolute errors of matched hmF2 data pairs at daytime and nighttime are presented in the bottom right corner of (**a**) and (**b**), respectively.