## 4 Case Analysis of Concepcion Earthquake
### Ionospheric Structure Anomaly before Earthquake
The intense Nmf2 disturbance is distinguished by the depth of color, especially for the dark grey or black patches indicating severe traveling ionosphere disturbances (TIDs) in the F layer. The subgraph on the upper left is the reaction amplification near the epicenter. The triangles and dots in this map represent the occultation event distribution of Period 1 (07–12 UT) and Period 2 (06–07 UT) near the epicenter, respectively. The focal sphere in both graphs represents the epicenter. Using the singular spectrum analysis filter (see [5] for details), the positive/negative high-frequency signals of the profiles remained. We calculated the double averaging value of the positive high-frequency data as the upper limitations. With the same definition, the double averaging value of the negative high-frequency signals was regarded as the lower limit. [PERSON] et al. [40] revealed that the acoustic gravity wave excited by the Rayleigh wave can cause prominent long wave fluctuations of 50-105 km in the vertical electron density profile. In panels (b) and (c), three profiles, #1-#3 (the strongest Nmf2 disturbance on February 21, see Figure 8), were analyzed with a similar feature. The periodic or quasi-periodic oscillation was quite prominent in Profiles #1 and #3, with long-wave atmospheric oscillations, even to a height >400 km. On the contrary, the fluctuation in Profile #2 failed to be regarded as an abnormal state. These fluctuations leave traces that are triggered by the bottom atmosphere, propagate upward to the F layer, and ultimately disappear in the upper ionosphere. These features imply potential evidence of acoustic transmission in the seismogenic area. Though restricted to sparse occultation events, the acoustic gravity wave's specific propagation mode was hard to determine. We are optimistic that, with dense RO events in the following 2-generation COSMIC, an enhanced performance is conducive to collecting more interesting signals. Two groups of COSMIC occultation events by the ionosphere maximum (\(\delta>\) 0.7, Prof#4/5/6) and minimum disturbances (\(\delta<\) 0.1, Prof#7/8/9) in Figure 9 are divided. The corresponding electron density profiles were extracted to judge the vertical structure change in the ionosphere on the second day before the case. During the nighttime (local time), the \"normal\" ionosphere structure information was captured by the Prof#7-9 (around 6 a.m.) COSMIC event group. A typical F layer and E layer were visible in this group. Yet such a  
Figure 10: Vertical ionosphere structure in the seismogenic area on February 13–27; (**a**) fluctuation statistics. The strongest Nmf2 disturbed profile (Prof#1-3) on February 21 was extracted; (**b**) the profile of the COSMIC electronic density; (**c**) the profile after removing the non-linear trend. familiar ionosphere structure was collapsed in the matinal time. At the Proff#4-6 (collected at 10-12 a.m.) group, the vague stratification of the F layer (F1/F2 layer) emerged. In the right panel of Figure 11, under the control of day-night transmission and the particle precipitation, the F2 layer peak electron density at Proff#4-6 recorded the largest value. However, in the adjacent space and time, the Nmf2 at Proff#7-9 is significantly lower than the former group, consistent with the ionospheric abnormal enhancement at two points. In [41], it was found that the ionosphere peak layer height (hmF2) enhanced abnormally before the earthquake. Based on the hmF2 the of both event groups in the right panel, the extraordinary feature before the Concepcion earthquake was not prominent. The study of the variations of the ionosphere's hierarchical structure was continually carried out. For the 06-12 UT period on February 25 (two days before the earthquake), analysis was done on the electron density data for five heights from 100 to 500 km (interval: 100 km). Following the GNM construction, the same strategy was used to construct each height's electronic density maps to study the ionosphere disturbance at a different elevation.