# The Comparison of Electron Density between CSES In Situ and Ground-Based Observations in China
## 3 Comparison with ISR Data  
QJISR works several hours in each day, almost all during the daytime, with a time resolution of about 5 min. Corresponding to the available data of the CSES Ne, 25 events were found for QJISR from 2018 to 2021. The ISR can give an Ne profile with an altitude resolution of about 5 km. The principles of the selected comparison dataset are as follows. (1) The mean value (M) and standard deviation (std) of QJISR Ne at the altitude 450-550 km from 13:00 to 15:00 (LT) were obtained to represent the observation at the satellite altitude in the daytime for each day. There are differences for the working time of QJISR each day; thus, the time range covers 2 h to ensure sufficient samples are obtained for the comparison  
Figure 1: Locations of ground-based observation stations in China. Blue points represent ionosonde stations of IEF with continuous-time data. Red triangles indicate ionosonde stations of CRIRP at different latitudes. Green star means the location of the ISR station. Green lines are the footprints of the CSES in one day. between the conjugate data. (2) Corresponding to the date of QJISR observations in 25 days, the M and std of the CSES Ne in the region between 93.8-113.8\({}^{\circ}\)E and 15.6-35.6\({}^{\circ}\)N were calculated for the daytime data. The purpose of the selected \(\pm 10^{\circ}\) region is to provide a sufficient number of observations. With the burst mode of CSES, the resolution of latitude is almost 0.1\({}^{\circ}\); thus, there are about 200 data points to obtain the M and std. Figure 2 gives the Ne time series for the CSES (red line) and QJISR (blue line) in the comparison days, which is not continuous and the detailed date is marked on the \(X\) axis. Every day, the circle and bar represent the mean value and standard deviation calculated according to the above principles. The two time series are displayed with different amplitude scales to better underline their own variations. The Ne variation pattern between the CSES and QJISR is almost similar, with a high value in 2018 and a relative low value in 2020 and 2021. Furthermore, some trends in a short time frame fit perfectly, for example, the data from 25 May to 7 June 2018. To check the difference in the value of electron density, the relative change of CSES Ne corresponding to QJISR data was calculated according to Equation (1). \[Rel=\frac{Ne_{-}CSES-Ne_{-}QJISR}{Ne_{-}QJISR}\times 100 \tag{1}\]  
where \(Ne_{-}CSES\) represents the data of CSES Ne, and \(Ne_{-}QJISR\) is the Ne at satellite altitude calculated based on the above principle. A histogram of the relative change value is shown in Figure 3. It can be seen that all of the ratios are negative with a median value \(-84.04\%\), which means the average value of CSES Ne is lower than that of QJISR Ne at the satellite altitude. Figure 2: Ne time series for CSES (red line) and QJISR (blue line). Each day, the circle means mean value, and the bar represents standard deviation. The two time series are displayed with different amplitude scales to better underline their own variations. The comparison days are not continuous and the detailed dates are marked on the \(X\) axis. To further analyze the correlation between the two datasets, panel a of Figure 4 gives the Ne scatters of CSES and QJISR with the detailed date for each point. It seems that these data can be separated into two groups, which are marked by the red oval. It can be seen that except four points in 2018, the time range of the group 1 data is in 2020 and 2021. The data of group 2 belong to 2018, except one point in 2020. Combined with the curves in Figure 2, the magnitude of the group 2 data is mostly higher than that of the group 1 data. Since the available dataset for the comparison of CSES and QJISR Ne is limited, the characteristics of ionospheric variation cannot be completely considered, and the magnitude of observation may play a leading role in the correlation analysis.