## 4 Discussion
They observed that there were disturbance anomalies in the atmospheric electric field near mid-latitudes during the magnetic storms, and the disturbance range was approximately \(-\)100-150 V/m. However, disturbances also occurred on days without magnetic storms (20 July, 27 July, and 28 July). The reason for this situation may be that the atmospheric electric field is also affected by weather conditions such as thunderstorms [66], rainfall [67], and cloud cover [68]. Previous research conducted by [PERSON] et al. [69] analyzed atmospheric electric field data obtained from a station near the epicenter prior to the Luanzhou earthquake on 16 April 2021. The authors found that the electric field observations from multiple stations showed continuous negative abnormal signals before the earthquake, and they proposed that it may be associated with impending earthquakes. [PERSON] et al. [70] observed that prior to the earthquake, a bay-like variation in the magnitude of kilovolts existed in the vertical electric field. More studies have observed that the persistent negative anomalies in the magnitude of kilovolts in the atmospheric electric field occurred before earthquakes, and the authors of these studies claimed that they may be potential precursor signals of earthquakes [71; 72]. Based on existing research into atmospheric electric field anomalies, we try to explain the generation mechanism of this negative abnormal signal. It has been supposed that under normal space weather conditions, a positive electric field gradient is observed due to the Earth's surface carrying negative charges and the ionosphere carrying positive charges [71]. During the pre-seismic period, a substantial release of energy from the lithosphere to the atmosphere induces ionization, leading to the accumulation of positive ions at the Earth's surface and subsequently resulting in negative anomalies in the electric field. Consequently, to extract an abnormality with a certain duration, the time range where the values remain below \(-\)1 kv/m for 2 h was highlighted, as shown in Figure 6. It can be seen that the continuous and negative phase disturbance in the atmospheric electric field only occurred on 7 August, one day prior to the Jiuzhaigou earthquake, which suggests a possible correlation between them. The ionospheric disturbance resulting from the earthquake may also be linked to the acoustic-gravity waves triggered by surface motion [30; 73; 74; 75; 76]. The coupled modeling theory proposed by [PERSON] implies that the pressure and density changes induced by acoustic-gravity waves could cause local ionization, resulting in anomalies in the electric field and coupling with the ionosphere [11; 12]. Additionally, there exists resonance coupling among the lithosphere, atmosphere, and ionosphere, indicating that acoustic-gravity waves can influence charged particles through fluctuations, leading to changes in ionospheric TEC [30]. [PERSON] et al. [77] investigated the interaction between the ionosphere and acoustic-gravity waves and observed that acoustic-gravity waves have the potential to cause ion accumulation in localized areas, creating plasma density gradients and ionospheric disturbances. [PERSON] et al. [30] spectrally analyzed TEC data and noted that TEC enhancement prior to earthquakes predominantly occurs within the frequency band of 0.004-0.006 Hz. They also identified that the anomalous frequency band of acoustic-gravity waves during propagation closely aligns with the anomalous frequency band of TEC in the ionosphere, with frequencies within the range of 0.004-0.005 Hz. A spectral analysis of TEC data is conducted in this study to investigate the potential influence of the acoustic-gravity wave coupling pathway on ionospheric TEC changes.