Characteristics of Zonal Differences in Equatorial Plasma Bubble Morphology Revealed by GOLD Observations

This study presents a climatological analysis of the systematic zonal differences in Equatorial Plasma Bubbles (EPBs) morphology, leveraging continuous observations from NASA's Global-scale Observations of the Limb and Disk (GOLD) mission between January 2023 and May 2025. Within the GOLD field of view, which continuously scans the nighttime ionosphere over the Americas and the Atlantic Ocean, a pronounced longitudinal asymmetry is uncovered: although backward C-shaped EPBs dominate overall, forward C-shaped EPBs occur more frequently in the western longitudinal sector than in the eastern sector. Statistical results demonstrate that this zonal morphological difference is a recurrent phenomenon, the occurrence of which follows the same seasonal pattern as the local EPB rates, being most frequent around equinoxes. Its occurrence rate increases with solar activity (F10.7 index) but shows no significant correlation with geomagnetic activity (Kp index). The magnetic longitudinal demarcation line between these morphological regions approximates a normal distribution, peaking around 25.9° in magnetic longitude—a value notably close to the region of maximum magnetic declination. Analysis incorporating ionospheric zonal drift velocities from ROCSAT-1 data suggests that this morphological dichotomy originates from longitudinal variations in the latitudinal distribution of the plasma zonal drift, which is itself governed by the geomagnetic configuration. These findings provide robust, long-term observational evidence for the control of large-scale geomagnetic field geometry on the morphology of ionospheric plasma depletion.