Transition from westward to eastward tilts of equatorial plasma bubbles observed by an all-sky airglow imager

Equatorial plasma bubbles (EPBs) typically form after sunset at the bottom of the F layer in the equatorial region and then grow upward and elongate along magnetic flux tubes. According to optical observations, in many cases, the poleward ends of EPBs tilt westward with respect to the magnetic north, but in a few cases, eastward tilts are observed. In this work, the EPB tilt directions, drift velocities, and morphology are investigated from an all-sky airglow imager over South China during the geomagnetically quiet night of April 3, 2022. During this event, the poleward ends of the EPBs in the off-equatorial region initially tilted westward after sunset (20:04–23:27 LT), but gradually turned to tilt eastward around and after midnight (23:27–03:38 LT). Successive airglow images revealed different groups of EPBs with westward tilts, westward-eastward tilts, and eastward tilts throughout a single night. The changes in morphology, tilt angles, and drift velocities of EPBs are analyzed according to the airglow images. The tilt angles ranged from −12° to 25°, which were calculated by Principal Component Analysis (PCA) through Singular Value Decomposition (SVD). The westward-to-eastward tilt direction changed in less than 17 min, and the eastward tilt of the EPBs lasted 4 h. When the EPBs drifted eastward over the observation region, the ionogram displayed distinct spread F signatures, and satellite signals manifested significant scintillation phenomena. The drift velocities of the EPBs at relatively high latitudes clearly increased and were greater than those at lower latitudes when the EPBs turned to tilt eastward at approximately 23:27 LT (15:53 UT), with a velocity differential of 20 m/s. The changes in tilt direction may result from latitudinal variations in ion drift velocities, which might be related to the latitudinal/altitudinal variations in F-region Pedersen conductivity and the presence of greater zonal winds or electric fields along the geomagnetic field line.