There are the indications that plasma bubbles/flux tube aligned plasma density depletions, produced by Rayleigh–Taylor instability at the bottomside of ionosphere, could rise up to the topside ionosphere and plasmasphere. Maruyama and Matuura [Maruyama, T., Matuura, N. Longitudinal variability of annual changes in activity of equatorial spread-F and plasma bubbles. J. Geophys. Res. 89(A12), 10903–10912, 1984.], using ISS-b satellite data for the high solar activity period, 1978–1979, have seen the plasma bubbles over equator at 1100 km altitudes in 46 cases in 1700 passes. That is ∼3% only. However, there is distinctly another picture in He+ density depletions (subtroughs) according to the ISS-b data for the same period. He+ density subtroughs were observed in the topside ionosphere over equatorial and low-latitudinal regions (L ∼ 1.3–3) in 11% of the cases [Karpachev, A.T., Sidorova, L.N. Occurrence probability of the light ion trough and subtrough in He+ density on season and local time. Adv. Space Res. 29, 999–108, 2002; Sidorova, L.N., He+ density topside modeling based on ISS-b satellite data. Adv. Space Res. 33, 850–854, 2004.]. We have carried out a statistical study of the He+ density subtrough characteristics. The subtrough depth (depletion value) as function of local time (evening–night hours) was compared with the vertical plasma drift velocity variations, obtained for the same periods from the AE-E satellite and IS radar (Jicamarca) data. Striking similarity in development dynamics is revealed for the different seasons. It is noted also that the He+ density subtroughs are mostly observed in the evening–night sector (18–05 LT) from October till May, which is very similar to the peculiarities of the equatorial spread-F (ESF), usually associated with plasma bubbles. The monthly mean He+ density subtrough occurrence probability, plotted in local time versus month, was compared with the similar plots for ESF occurrence probability derived by Abdu et al. [Abdu, M.A., Sobral, J.H.A., Batista, I.S. Equatorial spread-F statistics in the american longitudes: some problems relevant to ESF description in the IRI scheme. Adv. Space Res. 25, 113–124, 2000.] from ground-based ionograms obtained over Brazilian region for the same years. The comparison shows good enough correlation (R ∼ 0.67). It is concluded that: (a) He+ density subtroughs like ESF are controlled by pre-reversal enhancement electric field (vertical drift); (b) He+ density subtroughs and ESF/bubble irregularities may be considered as phenomena of the same plasma bubble origin; (c) it seems, plasma bubbles, reaching the topside ionosphere altitudes, are most easily observable in He+ density as depletions.
