# The Comparison of Electron Density between CSES In Situ and Ground-Based Observations in China
## 4 Comparing with Ionosonde Data  
Based on the two groups of ionosonde data, the continuous-time observations of three stations from IEF were used to carry out time series analysis, and the data of 10 ionosonde stations from CRIRP were applied to analyze the correlation at different latitudes.  
In the ionosphere, the electron density above the F2 region is determined by chemical production and loss, transport due to neutral wind, \(\mathbf{E}\times\mathbf{B}\) drift, ambipolar diffusion and so on. In the daytime, the production, loss and transportation of electrons are mainly affected by the sun. The ionospheric electron density is mainly produced by the photoionization of solar EUV and X-ray radiation, which is represented by the fact that NmF2, TEC and Ne at the topside of the ionosphere linearly change with the solar proxies at low and medium solar activity levels [26; 27; 28]. Solar activity and photochemical equilibrium play a leading role in the variability of Ne in the daytime, and the electron density at different altitudes may synchronously variate with the solar effect. In the nighttime, the compound and transport actions induced by chemical reaction, dynamic and electrodynamic processes are the main effect factors to make the ionosphere express local characteristics at the latitude, longitude and altitude [29; 30]. The linear dependence between electron density and altitude may be broken down, which means some biases will be brought when the NmF2 are inferred to Ne at satellite altitude based on the empirical model. Therefore, in the Ne comparison of the CSES and ionosonde, we just paid attention to the data analysis in the daytime.  
The detailed data process of the selected comparison dataset is as follows. On the one hand, the mean value (M) and standard deviation (std) of the CSES Ne falling in the region of \(\pm 10^{\circ}\) around the ionosonde station were calculated to represent the Ne data above the ionosonde station. As the CSES is a circular sun-synchronous satellite, the longitudinal distance of each orbit in a day is about \(24^{\circ}\). The reason for selecting \(\pm 10^{\circ}\) as the range is to provide a sufficient number of observations and temporally continuous data to make the comparison statistically meaningful, and to avoid the ionospheric variation on a large scale. On the other hand, the IRI model was used to infer the Ne data at satellite altitude based on the ionosonde observations. The topside density profile can be obtained by fitting data to a Chapman-\(\alpha\) layer [31], while the selection of ionospheric scale height (\(H\)) can also affect the fitting result [32; 33]. As the electron density is assumed to be mostly linear-dependent at altitudes above the hmF2 [34; 35], the ratio outputted by the IRI model was applied to obtain the Ne at satellite orbit in this study. The IRI model is an empirical standard model of the ionosphere, which includes the main ionospheric functions and available data sources [36], and is updated yearly according to the analysis of the IRI working group ([[http://irimodel.org/](http://irimodel.org/)]([http://irimodel.org/](http://irimodel.org/)), accessed on 1 September 2021). There are three steps to obtaining the comparison dataset for ionosonde data. Firstly, the ratio of Ne at 507 km and NmF2 at the ionosonde location was outputted with a 15-min time resolution based on the IRI 2016 model. Secondly, the NmF2 observed by the ionosonde was multiplied by the above ratio to obtain the Ne data at the satellite altitude. Finally, the mean value and standard deviation of the calculated Ne from 13:00 to 15:00 (LT) were acquired to represent the ionosonde data in the daytime.