# The Comparison of Electron Density between CSES In Situ and Ground-Based Observations in China
## 2 Data  
The CSES is a circular sun-synchronous satellite, with the descending and ascending nodes at 14:00 LT (local time) and 02:00 LT, respectively. The satellite is three-axes stable to set the altitude at about 507 km with an orbital inclination of 97.4\({}^{\circ}\). As the orbit of the CSES is strictly revisited every five days, the global electromagnetic environment of the ionosphere can be obtained every five days [17].  
In this study, we pay attention to the in situ electron density data observed by LAP. The LAP payload is installed on the front of the satellite with two spherical sensors and one electronics box. Sensor 1 is a larger one with a diameter of 5 cm, and the diameter of sensor 2 is 1 cm, designed as a backup. There is a long stub 50 cm in length for each sensor to reduce the interference from the satellite surface [18]. The data we used in this study were observed by sensor 1. The plasma parameters can be obtained by fitting the \(I\)-\(V\) characteristic curve of LAP based on the sweep voltage and sensor current, which is introduced in detail by [PERSON] et al. [19]. The Ne measurement range of CSES LAP is \(5\times 10^{2}\)-\(1\times 10^{7}\) cm\({}^{-3}\) with a relative accuracy of 10% [20]. For the LAP payload, the time resolutions of the burst and survey modes are 1.5 and 3 s, which means the latitudinal interval of each sample is about 0.1\({}^{\circ}\) and 0.2\({}^{\circ}\), respectively. The electron density data observed by the CSES can be downloaded from the mission website ([[http://www.leos.ac.cn/](http://www.leos.ac.cn/)]([http://www.leos.ac.cn/](http://www.leos.ac.cn/)), accessed on 10 August 2021) after registration. Currently, the CSES mission does not stamp the quality flag to LAP Ne. In our analysis, Ne samples with a value higher than 1 \(\times\) 10\({}^{7}\) cm\({}^{-3}\) or lower than 5 \(\times\) 10\({}^{2}\) cm\({}^{-3}\) were omitted, as it is considered that there may be bias in the fitting process if the data exceed the measurement range.  
The accuracy of the ionosonde and ISR observations is widely accepted, and these data are usually applied to validate the satellite observations [11; 21; 22]. In China, there is an ISR in operation at the Qujing site and some ionosondes at different latitudes, the data of which can be applied to compare with the electron density of the CSES in the China region. The Qujing ISR (QJISR) was constructed by the China Research Institute of Radio Wave Propagation (CRIRP) at the beginning of 2012 [23], and the station is situated in Yunnan province with the latitude 25.6\({}^{\circ}\)N and longitude 103.7\({}^{\circ}\)E, which is plotted in Figure 1 with a green star. There are two groups of ionosonde data to compare with the CSES Ne. One is from the Institute of Earthquake Forecasting (IEF), China Earthquake Administration, including three ionosonde stations in the southwest China, which is marked by a blue point in Figure 1. Leshan station is located in Sichuan province, with the longitude 103.8\({}^{\circ}\)E and latitude 30.0\({}^{\circ}\)N, and Yunnan province has two stations, located in Tengchong (98.5\({}^{\circ}\)E, 25.0\({}^{\circ}\)N) and Puer (101.1\({}^{\circ}\)E, 22.7\({}^{\circ}\)N) [24]. The other group is from CRIRP. Since the 1940s, CRIRP has established several ionosondes from lower (20.0\({}^{\circ}\)N) to middle latitudes (49.6\({}^{\circ}\)N) in the mainland of China [25], and the locations of the data we used are marked by red triangles in Figure 1.