## 2 Background Information  
CSES was launched successfully on 2 February 2018, with a circular Sun-synchronous orbit, at an altitude of about 507 km, inclination of about 97.4\({}^{\circ}\), descending node at LT14:00 in daytime and ascending node at LT02:00 in the nighttime, a strictly revisited time period of 5 days, and the observing scale within a latitude of \(\pm 65^{\circ}\)[26]. Its scientific objectives are to detect the electromagnetic environment and earthquake-related perturbations in the topside ionosphere [27]. There are eight scientific payloads installed onboard, includinghigh precision magnetometer (HPM) to detect the scalar and vector of geomagnetic field in DC-15 Hz [28; 29]; search coil magnetometer (SCM) to observe the induced magnetic field within 10-20 kHz [30; 31]; electric field detector (EFD) to obtain the three-component electric field in the DC-3.5 MHz frequency band [32]; Langmuir probe (LAP) for electron density (Ne) and temperature (Te) [33]; plasma analyzer package (PAP) for ion-related parameters, such as ion density, temperature, drift velocity, and so on; high energetic particle package (HEPP) to get particle flux and pitch angle for 0.3-3 MeV electrons and 2-20 MeV protons [34; 35]; and high energetic particle detector (HEPD) produced in Italy for 2-50 MeV electrons and 15-200 MeV protons [36]; GNSS Occultation Receiver (GOR) and tri-band beacon (TBB) to inverse total electron content (TEC) and Ne profiles [37; 38; 39]. To detect the weak ionospheric perturbations induced by earthquakes or other disasters on ground, CSES is designed to operate during low solar activity years with less disturbances from outer space. In the 23 rd solar cycle, 2008 was under the extremely low solar activity with the lowest electron density during the DEMETER satellite operation time in 2004-2010 [40], when Ne exhibited different temporal and spatial features with that in high solar activity years [41; 42]. Ten years after, 2018 began into another quiet solar year, when CSES may show similar electromagnetic features in the topside ionosphere. The obvious differences between two satellites are, (1) the altitude, for DEMETER at about 710-660 km, and CSES at a lower one about 507 km, much closer to the peak F2 layer in ionosphere; (2) the local time, for DEMETER being LT10:30, with CSES at LT14:00, which is always the occurrence time for maximum electron density each day. Unfortunately DEMETER has ended its operation in 2010, so we do not have chance to compare both similar observations about a same event. However, Swarm satellite is still in-orbit since its launch in November 2013, with the observations of geomagnetic field, Ne, Te, etc., [43]. Swarm consists of three satellites, A, B, and C, in which B was at a higher altitude of 550 km in the beginning and now descends to about 510 km similar with CSES, while paralleled A and C decrease from 450 km to 440 km. Unlike DEMETER and CSES, Swarm does not have a stable local time and it is designed to cover the entire 24 local time for the construction of global geomagnetic field model. These differences among satellites can provide more information for understanding the ionospheric background and its responses to kinds of space events at different times and altitudes. With the unprecedented spatial and temporal coverage of the global navigation satellite system (GNSS) and the widely distributed ground receivers, the total electron content (TEC) data have been extensively used in ionospheric studies. On the basis of TEC data from hundreds of GNSS stations worldwide, global ionosphere mapping (GIM) was developed to produce the global ionospheric TEC by different institutions, and CODE (Center for Orbit Determination in Europe, Astronomical Institute, University of Bern, Switzerland) is one of them. The GIM TEC, published by CODE, is modeled in a solar-geomagnetic reference frame using a spherical harmonic expansion up to a degree of order 15.