# Airglow Observation and Statistical Analysis of Plasma Bubbles over China
## 3 Analysis of the Main Characteristics of Plasma Bubbles  
### Morphology and Spatial Scale of Plasma Bubbles  
Based on the image processing method of airglow data, as introduced above, we performed image processing on the original airglow images and obtained the clear and recognizable image products of equatorial plasma bubbles (EPBs). Then, the morphological characteristics and temporal-spatial variations of EPBs were analyzed and studied. For the optical images on the evening of 2 September 2014, image processing was performed on these data, and the corrected and projected images of EPBs can be obtained. Figure 4 presents the corrected airglow images of EPB from 18:32:36 UT to 20:27:51 UT, which shows the evolution of EPB from its beginning to its disappearance; the time interval of each subgraph is 6 min. From Figure 4a, EPB exhibits a smaller scale when EPB starts to appear, and EPB shows the shape of a long stripe. With the increase in local time, the spatial scale of the EPB gradually becomes larger. When EPB evolves to a larger scale, EPB is observed as plume-shaped structures and develops into plume irregularities. Moreover, it is visible from Figure 4 that EPB tilts westward as a whole, compared with its north-south distribution, and EPB extends to a long distance along the geomagnetic field lines. As the EPB scale increases gradually, it can be seen from Figure 4 that a bifurcation phenomenon occurs at the top of the EPB, for example, at 19:15:04 UT in Figure 4h. With the change in time, it is found that the degree of EPB bifurcation increases gradually, and the bifurcation of EPB becomes more pronounced with time. The east wall of the EPB gradually expands outwards over time, and the bifurcation point gradually extends from the top to the bottom of the EPB. Moreover, a small branch appears at the bottom of the EPB, starting from about 19:02:56 UT in Figure 4f, then the small branch gradually separates from the EPB, and becomes more apparent on the east wall of EPB. From Figure 4, the west wall of EPB is relatively stable during this period, while there are some bifurcations on the east wall of EPB. From these observations, it can be inferred that the east wall of EPB is more unstable than the west wall, and we speculate about the possible causes of this phenomenon. The ionospheric irregularities and the neutral wind move eastward together at night, and the plasma density in the EPB is lower than that in the surrounding ionosphere. The interaction between the ions in the EPB will be weakened, so that the velocity of neutral wind in the EPB will be slightly increased, and the extra momentum will affect the east wall of the EPB,  
Figure 3: Image correction and projection for 19:39:20 UT on 2 September 2014. (**a**) the corrected airglow image; (**b**) the projected airglow image. which will be correspondingly expanded. For the EPB occurrence on 2 September 2014, it is found that EPB lasts more than 2 h. When the EPB is about to disappear, the intensity gradually weakens over 30 min until the bubble finally disappears. Figure 5 shows the projected images of EPB changing with time on 2 September 2014; these are the results of airglow images after image enhancement, azimuth correction, and image projection. According to the projected images of EPB, the east-west scale of the EPB was analyzed. The EPB scale can be calculated through the longitude range of the EPB in the zenith direction, that is, the horizontal axis at the center of the projected image. In Figure 5b, the EPB spans a longitude of 1.4\({}^{\circ}\), and the east-west scale of EPB is about 165 km at 18:38:40 UT. In Figure 5h, the longitude span of the EPB is 2\({}^{\circ}\); thus, the width of EPB is calculated to be about 240 km at 19:15:04 UT. With the increase in local time, the longitude span of the EPB changes to 2.6\({}^{\circ}\) at 19:51:28 UT, and the EPB scale increases to 307 km, as shown in Figure 5n. It can be concluded that the spatial scale of EPB gradually increases with time.