Research Progress on Morphological Characteristics and Scintillation Effects of Equatorial Plasma Bubbles

Equatorial Plasma Bubbles (EPBs) are among the most prominent plasma irregularities in the low-latitude ionosphere. Their multiscale morphological evolution and modulation of trans-ionospheric radio-wave propagation represent major scientific and practical concerns in space weather research. This review aims to summarize recent advances in the morphology, magnetic signatures, and scintillation effects of EPBs, and to identify key issues that require further investigation. Long-term in situ observations from low-Earth-orbit satellites, including CHAMP, GRACE, and Swarm, are reviewed together with complementary measurements from COSMIC radio occultation, ground-based Global Navigation Satellite System (GNSS) receiver networks, and all-sky airglow imagers. The occurrence characteristics of EPBs are first examined in relation to solar activity, geomagnetic activity, longitude, season, altitude, and local time. Previous observations demonstrate that EPB occurrence exhibits pronounced longitudinal and seasonal variability and is strongly controlled by the prereversal enhancement of the eastward electric field, background ionospheric conditions, and lower-atmospheric forcing. The spatial scales and three-dimensional morphology of EPBs are then discussed. EPBs are characterized by strong field-aligned extension, considerable zonal variability, and hierarchical structures ranging from large-scale plasma depletions to kilometer- and subkilometer-scale irregularities. Their morphology may evolve through upward growth, bifurcation, merging, and secondary instability development. Magnetic perturbations associated with EPBs are also reviewed. Available measurements suggest that their electromagnetic signatures may result from plasma diamagnetic effects, field-aligned currents, polarization electric fields, and Alfvénic perturbations. However, the relative contributions of these processes remain uncertain and require coordinated plasma and magnetic-field observations. Finally, the effects of EPBs on L-band signals are assessed, including amplitude and phase scintillation, signal fading, loss of lock, and positioning degradation. These effects depend not only on the large-scale geometry of plasma depletions, but also on the intensity and spectral distribution of embedded small-scale irregularities. Overall, EPBs should be regarded as multiscale, three-dimensional, and electrodynamically coupled ionospheric structures. Future progress will rely on coordinated multisource observations, improved three-dimensional reconstruction, and physics-based modeling. Such efforts are essential for understanding EPB evolution and improving the reliability of satellite communication, navigation, and positioning systems in equatorial and low-latitude regions.