Evolution of post-sunset equatorial plasma bubbles: relationships to the equatorial ionospheric anomaly induced by pre-reversal enhancement electric fields

This paper presents GNSS radio occultation (RO) observational analyses on deducing the relationships and dependences between post-sunset EPB occurrences and EIA strength variability. The RO data were acquired from the FS7/COSMIC2 Program from 2020 to 2025. In this study, we incorporate both effects from crest peak electron density (N_emax) and crest-to-trough N_emax ratio and propose a new EIA strength parameter defined as the mean of northern and southern crest-to-trough N_emax differences to recognize and characterize the post-sunset EIA features. Both seasonal–longitudinal appearances of intense post-sunset EPB occurrences and strong EIA events occurred on more or less 30 days expanded from when and where magnetic flux tubes align with the sunset terminator at the magnetic equator but have more intense EPB and/or strong EIA days during southern (northern) hemispheric summers in the South American area (the Central Pacific area and the Africa area). It is well consistent with Tsunoda’s hypothesis during the evening pre-reversal enhancement (PRE) and reveals more informationt on day-to-day variability, intensities and extents of post-sunset EPB occurrences and EIAs subject to seasonal, longitudinal, and solar cycle variability. Moreover, the local-time evolutions of peak post-sunset EIAs occurred during 19~20 LT which is earlier than that of the obtained experimental peak (i.e., 20:20 LT) of post-sunset EPB occurrences. We expect that the post-sunset EIA detection could be a potential precursor for post-sunset EPB occurrence.