## 1 Introduction  
Strong earthquakes (EQs) are one of the most destructive natural hazards, claiming countless deaths and economic losses in human history. The diversity and scale of EQ activity are daunting: however, the earthquake monitoring capabilities of human society still lack development. Promoting earthquake disaster prevention and reduction capabilities is a common issue faced by all countries worldwide. The abnormal geophysical and geochemical variation around the EQ preparation zone is interpreted as the seismic precursors [1; 2; 3]. Electromagnetic signals are regarded as the most promising candidate for short-term earthquake prediction [4; 5; 6]. With the development of space technology, by the early 1980s, scientists found abnormal electromagnetic emissions, plasma parameter irregularities, and energetic particle precipitations over seismic fault zones from electromagnetic satellite observations [7; 8; 3; 9]. For example, [PERSON] et al. [7] reported an abnormal electromagnetic field intensity enhancement over the seismic zones from the Intercosmos 19 satellite data. [PERSON] and [PERSON] [10] systemically summarized previous studies and concluded that the seismic signals can propagate upward from the lithosphere to the atmosphere and ionosphere, in particular circumstances, even up to the inner magnetosphere. These new scientific findings bring hope for a breakthrough in earthquake science. Several countries have started building dedicated electromagnetism satellites to monitor these earthquake precursors from space. In the early 21 st century, France launched the DEMETER (Detection of Electromagnetic Emissions Transmitted from Earthquake Regions) satellite mission, which successfully operated from December 2004 to December 2010, for studying ionospheric perturbations caused by earthquakes, volcanic eruptions, and human activities [11]. Since then, increasing attention has been given to the seismo-ionospheric disturbances phenomena. Much evidence on possible seismo-ionospheric precursor signatures has been accumulated after extensive studies have been carried out. Abnormal electromagnetic emissions in a broad frequency range were observed over the epicenter zones or their conjugated points during the impending earthquake time [11; 12; 13; 14]. [PERSON] et al. [14] statistically evaluated the abnormal electromagnetic field before M 4.8+ shallow-depth EQs by using 3.5 years of DEMETER data and confirmed the existence of a very small but statistically significant decrease in the electromagnetic wave intensity before the mainshock time. The ionospheric parameter irregularity at different altitudes over the EQ preparation zones is most commonly reported [9; 15; 16; 17]. For example, [PERSON] et al. [15] reported an unusual large-scale enhancement of electron density during the 2008 great Wenchuan earthquake. [PERSON] et al. [18] statistically analyzed the variation in the magnetic field and electron density recorded by Swarm satellites and reported anomalies starting about 40 days before the beginning of the seismic sequence, coinciding and following surface and atmospheric alterations. Energetic particle precipitation was also observed during the seismic time [11]. Driven by the objective of earthquake disaster prevention and mitigation, China has been devoted to building a stereoscopic earthquake monitoring system from ground to space, planning to launch a series of Low-Earth-Orbit (LEO) electromagnetism micro-satellites in the following decades step by step [19]. This mission is called the Zhangheng mission, named after the ancient Chinese scientist [PERSON] who invented the world's first seismo-scope in the second-century CE. The first electromagnetism microsatellite of the Zhangheng mission, known as the China Seismo-Electromagnetic Satellite (CSES or ZH-1), was successfully launched into a sun-synchronous circular orbit at an altitude of 507 km in the upper ionosphere [19]. TheCSES flies 15.2 orbits around the Earth per day at the local time around 02:00 a.m. (nightside) and 2:00 p.m. (dayside).