## 4 Discussion
Initially, the data were filtered based on disturbance and epicentral distance, leading to the selection of six IPPs with complete data, anomalies, and the closest proximity to the epicenter, as depicted in Figure 7. Significantly, areas exhibiting anomalies near the epicenter were predominantly situated in the southeast and southwest directions. Subsequently, Fourier transform was performed on the time domain data of the selected IPPs, mainly examining the time changes of frequency amplitude in the frequency range of 0.004-0.006 Hz, as shown in Figure 8. During two geomagnetic storm periods, TEC exhibited overall enhancement across the frequency band, which was particularly evident in the IPP2 and IPP4 sub-figures. In relatively calm spatial periods, significant increases in TEC within the ranges of 0.004-0.0045 Hz were experienced at IPP2, IPP3, IPP5, and IPP6 on 7 August, particularly at IPP3. Observations and numerical simulations indicate that ground vibrations and ionospheric TEC share a frequency of approximately 0.004 Hz [73; 74; 75]. [PERSON] et al. [41] proposed that the 0.004 Hz fluctuation in pre-earthquake TEC is likely associated with atmospheric acoustic-gravity waves generated by earthquakes. [PERSON] et al. [78] highlighted the correlation between the excitation of acoustic-gravity waves in the study area and terrain uplift, with the preferred propagation direction of acoustic-gravity waves varying based on factors such as terrain, season, and altitude. On 7 August, the locations of IPPs exhibiting amplified TEC amplitudes within the frequency range of 0.004 to 0.0045 Hz were situated in various directions from the epicenter, indicating a tendency for acoustic wave propagation outward from the epicenter. On 1 August, however, only the IPP point in the southwest direction from the epicenter demonstrated amplitude enhancement within the range of 0.004-0.0045 Hz, potentially suggesting that acoustic-gravity waves resulting from the earthquake did not propagate to other directions. From the above discussion, we preliminarily inferred that before the Ms7.0 Jiuzhaigou earthquake occurred, the radon release near the epicenter showed an increasing trend, which may potentially be the reason for the significant disturbance in the total TEC of the ionosphere on 28 and 29 July. Subsequently, acoustic-gravity waves near the epicenter propagated southwestward on 1 August, resulting in significant TEC disturbances in this direction on 1 and 2 August. Anomalies in the atmospheric electric field appeared near the epicenter on 7 August; meanwhile, the disturbances of ionospheric TEC at 0.004 to 0.0045 Hz were also detected. Under the combined influence of the anomalous electric field and the acoustic-gravity waves, the ionospheric TEC exhibited substantial superimposed anomalies from 7-8 August, as shown in Figure 5d. In the hours preceding the earthquake on 8 August, disturbances displayed a tendency to aggregate and spread to distant areas near the epicenter. This propagation trend is supposed to be associated with the stress  
Figure 7: Spatial distribution of the six IPPs with the closest and anomalous data. (The red star represents the position of the epicentre.) accumulation of the fault and the amplification of EIA effected by the abnormal electric field around the epicenter. The phenomena uncovered in this investigation offer valuable insights for advancing our comprehension of the earthquake precursor phenomena, as well as the LAIC mechanism.