On the fresh development of equatorial plasma bubbles around the midnight hours of June solstice

Using the 47MHz Equatorial Atmosphere Radar (EAR) at Kototabang, Indonesia, the nocturnal evolution of equatorial plasma bubbles (EPBs) was examined during the moderate solar activity years 2011-2012. While the evolution of EPBs was mostly (86%) confined to post sunset hours (1900-2100LT) during equinoxes, in contrast, the majority of EPBs (similar to 71%) in June solstice found evolve around the midnight hours (2200-0300LT). The mechanisms behind the fresh evolution of summer time midnight EPBs were investigated, for the first time, through SAMI2 model simulations with a realistic input of background ExB drift variation derived from CINDI IVM on board C/NOFS satellite. The term-by-term analysis of linear growth rate of RT instability indicates that the formation of high flux tube electron content height gradient (K-F) (steep vertical gradient) region at higher altitudes is the key factor for the enhanced growth rate of RT instability. The responsible factors are discussed in light of relatively weak westward zonal electric field in the presence of equatorward neutral wind and bottomside recombination around the midnight hours of June solstice. The effects of neutral winds and weak westward electric fields on the uplift of equatorial F layer were examined separately using controlled SAMI2 simulations. The results indicate that relatively larger linear growth rate is more likely to occur around midnight during June solstice because of relatively weak westward electric field than other local times in the presence of equatorward meridional wind.