# Airglow Observation and Statistical Analysis of Plasma Bubbles over China
## 6 Conclusions  
Based on optical data acquired by the all-sky airglow imager in Fuke, Hainan (geomagnetic latitude = 9.9\({}^{\circ}\) N), we utilized an image processing method to process the original airglow data, including image enhancement, azimuth correction, and image projection, and obtained the clear and recognized image products of plasma bubbles. We selected the airglow data of plasma bubbles on the evening of 2 September 2014, and then analyzed the main optical characteristics of plasma bubbles, such as morphology, spatial scale, drift features, etc. Moreover, we employed nearly one year of airglow data (from 1 September 2014 to 31 August 2015) from the all-sky airglow imager in Fuke, and made a statistical analysis of the occurrence of plasma bubbles. In the statistics, we investigated the monthly and seasonal occurrence rates of plasma bubbles during one year, and obtained the statistical characteristics of the occurrence of plasma bubbles. Plasma bubbles generally have a significant impact on high-frequency (HF) communications, satellite-ground communications, and GPS navigation and positioning. Therefore, it is of great scientific significance to further investigate equatorial plasma bubbles (EPBs) at low latitudes, which can be beneficial to the forecasting of ionospheric scintillations. The main conclusions and findings in this study are presented as follows. 1. For the optical observation of EPB on September 2 in 2014, it is found that EPB usually appears as plume-shaped structures in morphology, EPB often tilts to the west as a whole, compared with its north-south distribution, and EPB extends to a long distance along the geomagnetic field lines. It is found that the spatial scale of the EPB gradually increases over time. As the EPB increases to a larger scale, EPB shows the features of a vertical bifurcation, the bifurcation of EPB becomes more pronounced with time, and the bifurcation point gradually extends from the top to the bottom of the EPB. The observation results show that the west wall of EPB is relatively more stable, while there are some bifurcations on the east wall of EPB. Moreover, when EPB is about to disappear, the intensity of EPB gradually weakens until EPB finally disappears;
2. According to the projected images of EPB, the spatial scale and drift velocity of EPB were analyzed and studied. During the early period of EPB occurrence, EPB appears on a smaller scale. With the increase in time, EPB gradually widens in the east-westdirection, the EPB scale gradually increases with time. It can be obtained that the east-west scale of EPB is about several hundred kilometers. In our observations, EPB exhibits the features of drifting eastward. The analysis indicates that the drift velocity of EPB increases gradually with time, and then reaches a relatively stable condition. It is concluded that the drift velocity of the EPB is in the range of 40-130 m/s (+ / - 20 m/s), and the average drift velocity of the EPB is about 76 m/s (+ / - 20 m/s);
3. Based on the airglow observation data for one year, we investigated the statistical features of the occurrence of EPBs. The statistical results show that EPBs mainly appear in the months of September-November and February-April. The occurrence rate of EPBs presents the highest in September, reaching 40%, and EPBs are less observed from June to July 2015, and the occurrence rate is less than 15% in the two months. Considering seasonal occurrence, EPBs in the ionospheric F-region tend to occur more frequently in spring and autumn, while rarely being observed in summer and winter. The statistical results show occurrence rates of 32 % in autumn, the highest of the year, followed by 24 % for spring, 11 % for winter, and the lowest of the year, 5 % in summer. Investigation, X.M. and M.W. ; Methodology, X.M. and P.G. ; writing--original draft preparation, X.M. and M.W. ; writing--review and editing, J.X. and P.G. ; supervision, P.G. and M.W. All authors have read and agreed to the published version of the manuscript.