## 1 Introduction  
Earth observation from satellites has rapidly developed due to its advantages of fast-speed, large-scale, and high-resolution results, especially for areas with harsh natural conditions. More and more attention has been paid to ionospheric precursors associated with strong seismic activities since the modern satellite-borne receivers were developed, especially after the launch of the DEMETER satellite in 2004 in France, although a record of ionospheric irregularities prior to the 1964 Alaskan earthquake (EQ) was reported as early as 1965 [1]. [PERSON] et al. [2] investigated the seismo-ionospheric influence during three strong EQs: 29 September 2009 in the Samoa islands, an \(M_{\rm W}\) 8.1 EQ: 6 April 2009 in L'Aquila, an \(M_{\rm w}\) 6.3 EQ: and 12 May 2008 in Sichuan, an \(M_{\rm w}\) 7.9 EQ. Their results showed that positive and negative anomalies of the DEMETER ion density and electron density appeared 1 to 5 days before the studied events during quiet geomagnetic conditions, which indicates that these anomalous behaviors were highly regarded as seismo-ionospheric precursors. [PERSON] and [PERSON] [3] reported that ionospheric variations on DEMETER VLF (very low frequency) occurred two weeks before the L'Aquila event. [PERSON] et al. [4] summarized ionospheric fluctuations reported in 23 published papers written by 21 authorsworldwide on the 2008 Sichuan EQ. They found the anomalous starting time varied from 2 to 30 days prior to the main event, but most of the anomalies (22 among 23 publications) appeared two weeks before, with a magnitude from 20% to 123%. At the same time, satellite observation has also been characterized by the disadvantages of discontinuous measurement and a measurement depending on local time (LT) in a fixed area. For this reason, it is possible to miss some EQs or catch an incomplete ionospheric temporal-spatial evolution before some events. There has been much statistical work on ionospheric variations associated with strong seismic events using satellite measurements. Pulinets [5] presented obvious seismo-ionospheric disturbances that occurred within a few days in 73% of EQs with \(M>5.0\) and in 100% with \(M>6.0\). Statistical analyses have also been performed using DEMETER datasets. A statistically significant decrease in the electrical field during nighttime four hours before the occurrence of EQs has been reported by [PERSON] et al. [6,7] and [PERSON] et al. [8,9]. [PERSON] et al. [10] found that there were increases in the number of electron bursts prior to strong EQs with a magnitude over 7.0 during the entire operation period of the DEMETER satellite. Statistical analyses were also performed using a total ion density dataset during the DEMETER lifetime (6.5 years) in the epicenter areas of EQs as well as in their magnetically conjugate point areas [11, 12, 13, 14, 15, 16, 17]. These results demonstrated a significant statistical correlation between ionospheric anomalies and large events: I. the detection rate of EQs involved increased as magnitudes increased and hypercentral depths decreased; II. the number of perturbations was higher the day of the EQ and gradually decreased for the days before, and the appearance probability of ionospheric perturbations within one week in the northern hemisphere can be up to 80%; III. on average, the amplitude of the perturbations was related to the magnitude of the EQs, etc. As an attempt at EQ prediction, a positive result was attained by comparing the real position of the epicenter of an impending event with one determined by ionospheric perturbations searched automatically by software. [PERSON] et al. [18] conducted statistical work on 10 crustal EQs with a magnitude \(5.5\leq M<\)6.0 that occurred in Italy during 1979-2009 and reported that the ionospheric precursors spread from the epicenter towards the periphery during the EQ preparation process.