## 4 Results
### Multi-Orbits Analysis  
After the rough analysis using the Level 3 data products, the second step is to compare the relative long-term observations with the multi-orbits and their revisiting orbits. The physical values over the EQ influential zone from each orbit are extracted and rebuilt as time sequence data. The running quartile method [13; 30] is applied to examine the long-term trend. The running medians and the interquartile ranges (i.e., IQR, the difference between the third and first quartiles) were computed using the previous six revisiting orbits (30 days) and the current one (7 orbits in total). The running median values of the time sequence are used to build the background trend, while the median values of the current orbit are regarded as the current observation. The upper and lower bounds are computed by the running median values of physical values \(\pm\) n*IQR values; n is set from 1 to 5, depending on the different physical quantities. The disturbance amplitude (the difference values between the current observation and the background trend) is normalized as percentages, and if current observation values are within the upper and lower thresholds, the disturbance amplitude is then set to zero. Figure 3 shows an example of the revisiting orbit analysis, which shows the Ne and Te variation from 25 July to 24 August over the M 7.0 Sumatra epicenter (18 August 2020). From top to bottom are the space weather index (Kp and Dst), the variation in Ne, the disturbance amplitude of Ne, the variation in Te, and the disturbance amplitude of Te. The physical values during the current orbit are plotted as blue curves, and the background median values are plotted as black curves; the upper and lower boundaries are plotted as pink curves. It can be seen that an abnormal increase in Ne appeared on 8 August (10 days); according to the Kp and Dst index, we suggest that it is mainly related to the space weather conditions. The slight increase of 13 August (5 days before the mainshock) appeared under quiet space weather conditions, suggesting that it is possibly linked to the EQ. Figure 4 shows another example of the electric field, which shows the PSD (power spectral density) values of the Ex, Ey, and Ez components in the frequency range of 1000 Hz \(\pm\) 100 Hz during the 18 August 2020, M 7.0 Sumatra EQ, in the same format as Figure 3. It can be seen that there was a slight enhancement in the three components of the electric field under the quiet space weather conditions on 13 August, 6 days before the mainshock. Figure 3: The electron density disturbances over the 18 August 2020, M 7.0 Sumatra EQ influential area. The space weather index Kp is presented by blue bars, and the Dst index is displayed by the cyan curve. The red horizontal dashed line represents the values of Kp = 3. Alternatively, they are plotted in the format of Figure 5, which shows the ULF wave activities in the frequency band from DC to 15 Hz recorded by the revisiting orbits during two Alaska EQs, which occurred on 29 July 2021 (M 8.0) and August 14 (M 7.0) (see Table 2). It is noted that only orbits with abnormal ULF emissions and their revisiting orbits are plotted in Figure 5. Observations on July 28, and August 4 and 7 are removed as there is an apparent AE index enhancement due to the disturbed space weather condition. The variation in the magnetic field intensity along the latitude (\(-65^{\circ}\) to \(65^{\circ}\)) obtained by the revisiting orbits shows that on 19 July (10 days) and 27 July (2 days) before the 29 July M 8.1 mainshock, the eastward magnetic field in a frequency range from DC to 15 Hz appeared with apparent disturbances. On 10 August (4 days), 12 August (2 days), and 14 August (on the mainshock day) before the 14 August M 7.0 EQ, there were still similar abnormal ULF wave activities. On the other days (some revisiting orbits shown in Figure 4), the strange ULF wave activities were not discovered in the non-EQ activity times over the Alaska epicenter area.