Interesting Equatorial Plasma Bubbles Observed by All-Sky Imagers in the Equatorial Region of China

This paper reports some interesting equatorial plasma bubbles (EPBs) on 4 and 5 October 2013 and 29 and 30 September 2013 observed by two all-sky imagers located in the magnetically equatorial region of Hainan (19.5 degrees N; 9.5 degrees geomagnetic latitude) and Guiping (23.8 degrees N; 13.9 degrees geomagnetic latitude), China. The case of 4 and 5 October 2013 shows the following: (1) EPBs had planar wave-like structures and their wavefronts were parallel to each other before midnight. (2) The angle between the wavefronts of those EPBs and the geomagnetic meridian were similar to 30 degrees. (3) Near midnight, the higher-latitude part of one EPB suddenly began to rotate with a large eastward zonal drift, resulting in an unusual C-shaped EPB that was observed for the first time by an all-sky imager. (4) After midnight, one EPB merged into another EPB and formed an integrated EPB. The other EPB case on 29 and 30 September 2013 shows the following: (1) The images captured the full evolution processes of an "I" shaped EPB with an angle of similar to 30 degrees with respect to the geomagnetic meridian. The imagers also observed "S" and "Y" shaped EPBs. (2) The change of EPBs' shape was directly related to the change of EPBs' zonal drift velocities. (3) The angle between the I-shaped EPBs and the geomagnetic meridian was likely caused by the change of the latitudinal gradient of zonal neutral wind velocities and ionospheric conductivity. (4) The zonal velocity of each branch of the "Y" shape EPB was different, which could be related to the polarization electric fields within each branch.