## 7 Discussion  
The change of thermospheric composition is often regarded as one of the sources causing the global ionospheric disturbances, and the O/N\({}_{2}\) ratio is used as an important index of thermospheric composition to investigate the ionospheric disturbance responses  
Figure 7: The electron density Ne from Swarm satellites A and C on 25–27 August 2018.  
during geomagnetic storms [56]. [PERSON] et al. [57] used the TEC data derived from the GPS receivers to investigate the latitudinal ionospheric response of three geomagnetic storms, and found that the enhancement of O/N\({}_{2}\) ratio possibly contributed to the positive disturbance signatures of the storm during the recovery phase. [PERSON] et al. [58] proposed that the production or enhancement of ionospheric plasma at F region heights is attributed to the enhancement in atomic oxygen, which can explain the enhancement in O/N\({}_{2}\) ratio at low-latitude regions. To observe the variation of thermosphere neutral composition during 24-29 August 2018, the O/N\({}_{2}\) ratio variation is collected from Global Ultraviolet Imager (GUVI) instrument onboard the TIMED satellite. The variation of O/N\({}_{2}\) ratio is presented in Figure 8. The O/N\({}_{2}\) ratio is about 0.4-0.6 at quiet time on 24-25 August 2018. Compared with the value of O/N\({}_{2}\) ratio at quiet time, the O/N\({}_{2}\) ratio shows a prominent enhancement to nearly 1.0 at equatorial and low latitudes at 0-10 UT on 26 August 2018. During the recovery phase of the storm on 27-28 August, the O/N\({}_{2}\) ratio also remains at high values compared with that on 24 August. Compared with other low latitudes, the areas that have a higher enhancement of O/N\({}_{2}\) ratio are close to the locations of CIBG station and SANYA and LEARMONTH ionosondes in the longitude direction, where a persistent positive disturbance response is observed during the recovery phase of the storm. We further think that the TEC enhancement over low-latitude areas with a higher O/N\({}_{2}\) ratio might contribute to the EIA anomaly, disturbed neutral composition, and equatorward winds [37].  
The variation of interplanetary electric field during 24-28 August 2018 is presented in Figure 9, which can be controlled by the combination of high-speed solar wind and IMF Bz. As shown in Figure 9, the interplanetary electric field displays the long-duration positive phases during the recovery phase of the storm. Previous studies have demonstrated that the persistent prompt penetration of interplanetary electric field (PPEF) into the low latitudes can be attributed to the combined effects of high-speed solar wind and IMF Bz fluctuations [39]. Therefore, the enhancement of electric field caused by solar wind and IMF Bz in the low altitudes during the recovery phase of the storm possibly contributes the positive ionospheric response. The neutral winds and PPEF both result in the positive ionospheric response due to the expansion of EIA crest to higher latitudes from Figure 7 [55; 59]. Additionally, the solar wind variation also can facilitate the response of O/N\({}_{2}\) ratios during the geomagnetic storms [60]. From these results, the uplift of F layer height and the enhanced O/N\({}_{2}\) ratio are possibly the main factors causing the local-area positive disturbance signatures over low latitudes during the recovery phase of the storm in August 2018.  
Figure 8: Variation of O/N\({}_{2}\) ratio from TIMED/GUVI during 24–29 August 2018.