Longitudinal statistics of plasma bubbles observed as He+ density depletions at altitudes of the topside ionosphere

This work presents a new examination of the hypothesis regarding the equatorial origin of low He+ density plasma depletions (or subtroughs). For this purpose, we have conducted a detailed comparative analysis of longitudinal variations in the occurrence probabilities of subtroughs in both hemispheres and variations in the occurrence probabilities of equatorial F-region irregularities (EFIs), equatorial spread F (RFS and ESF), and equatorial plasma bubbles (EPBs). Taking into consideration the seasonal dependence and some peculiarities of magnetic field variations in different hemispheres, a conclusion has been reached regarding the similarity between longitudinal statistical occurrences of subtroughs and equatorial ionospheric F-region irregularities. In addition, another piece of evidence in favor of the similarity of the nature of the above-mentioned phenomena has been obtained. We have got a confirmation once again that low He+ density depletions (or subtroughs) can be rightfully considered as equatorial plasma “bubbles,” which can be observed at altitudes of the topside ionosphere as depletions in the He+ density.