Mesoscale Wave-like Structure of the Post-sunset Equatorial Ionization Anomaly Observed by the GOLD Mission

Abstract:

Both ground- and satellite-based airglow imaging have importantly contributed to our understanding of the low-latitude ionosphere, especially concerning the morphology of the equatorial ionization anomaly (EIA) and the evolution of post-sunset equatorial plasma depletions (EPDs). The GOLD satellite mission focusses on ultraviolet airglow images within about 120°W and 15°E. At this location, previous studies based on Swarm measurements have shown distinct electromagnetic characteristics of EPDs such as changes in the orientation of their related field-aligned currents and Poynting flux, as well as the highest occurrence of high-plasma pressure EPDs. In this paper, we focus on nighttime airglow images to assess spatiotemporal characteristics of a mesoscale structure of the post-sunset EIA and its possible connection with the occurrence of EPDs over the Atlantic and South American sectors. GOLD airglow images often show a wave-like pattern in the post-sunset EIA crests with typical zonal wavelengths of about 6.7x10³ km and 3.3x10³ km. It is found that the mesoscale structure is not local time-dependent, which means it is observed as a steady-wave centered at a specific longitude throughout the night. In quasi-dipole coordinates, its maxima are generally found around 20° and 60° of longitude and ±12° of latitude. However, some day-to-day variability is noticed with no apparent evidence of temporal or geomagnetic activity dependence. We further discuss the possible causes of this wave-like feature of the EIA as well as its potential relation to the occurrence of EPDs.