Statistical Characteristics and Correlation of Low-Latitude F Region Bottom-Type Irregularity Layers and Plasma Plumes Over Sanya

Ionospheric F region bottom-type irregularity layer has been frequently detected at equatorial latitude and regarded as a possible precursor of equatorial backscatter plumes producing scintillations. In this study, the VHF radar observations of ionospheric F region bottom-type irregularity layers and plasma plumes over the low-latitude station Sanya (18.4 degrees N, 109.6 degrees E, dip lat. 12.8 degrees N) during the equinoctial months (March, April, September, and October) of 2011-2018 are analyzed. The results show an equinoctial asymmetry in the occurrence of F region backscatter plumes, wherein the occurrence could be greater in spring or autumn months under different solar activities and background ionospheric conditions (F layer height and density). On the other hand, the bottom-type irregularity layer occurred almost exclusively in the descending phase of solar cycle (2015-2018). No clear correlation is found between the occurrences of bottom-type irregularity layer and plasma plume at low latitude. The F layer virtual height (hF) observations from the Sanya ionosonde show that the average hF is higher during the months when the plume occurrence is higher, suggesting that the asymmetric plasma vertical drift could play a role in the equinoctial asymmetry of plume occurrence. Larger vertical drift (and higher hF) could cause higher occurrence of equatorial plasma plumes which extend to low latitudes along magnetic field lines. In contrast, during the days when bottom-type irregularity layers occur, the hF is relatively lower. We surmise that only when the background F region is situated at low altitudes (with h ' F < similar to 250 km), the low-latitude bottom-type irregularity layer could be generated.