Topside Ionophere Plasma Bubbles Seen as He+ Density Depletions: Study Results and Problems
Sidorova, Larisa; Philippov, Sergey
Pushkov Institute of Terrestrial Magnetism, Ionosphere and Radiowave Propagation, IZMIRAN, RUSSIAN FEDERATION

Ionosphere plasma density irregularities (e.g. plasma bubbles) are important to understand and predict because they can cause radio scintillation that affects communications and navigation systems. He+ density depletions considered as originating from equatorial plasma bubbles were involved in this study. They are usually detected in the topside ionosphere (~1000 km) deeply inside the plasmasphere (L~1.3 - 3) [1, 2, 3, 4].
a) Plasma bubbles are produced by Rayleigh-Taylor instability at the bottomside of ionosphere and transported up to the topside ionosphere. Since there are some questions about the survival possibilities of the topside plasma bubbles, the characteristic times of the main photochemical and electrodynamics processes, in which plasma bubbles are involved, were compared. He+ loss process, ambipolar and cross-field (Bohm) diffusion transport, vertical plasma drift and He photo ionization were under consideration. It was found that there is enough time (~40 hr) for the plasma bubbles to survive and to be detected.
(b) It was revealed that the topside plasma bubbles can be easily detected as He+ density depletions during high/maximal solar activity. The convenient conditions for observations appear because the strong depleted in He+ density bubbles most well contrast with the He+ density background layer very well developed in the topside ionosphere during high solar activity [5].
The results of this pioneer study suggest the new investigation questions based mainly on data lacking. There is need to understand better the role of electrodynamics in plasma bubble evolution/decay and the conditions appropriated for plasma bubble lifting to the topside ionosphere heights.

References
[1] L.N. Sidorova, Adv. Space Res. 33, 850 (2004).
[2] L.N. Sidorova, Adv. Space Res., 39, 1284 (2007).
[3] L.N. Sidorova, Geomag. and Aeronomy, 48, 56 (2008).
[4] L.N. Sidorova, S.V. Filippov, J. Atm. Solar-Terr. Phys. 86, 8391, doi: 10.1016/j.jastp.2012.06.013 (2012).
[5] C.R. Wilford et al., J. Geophys. Res., 108(A12), 1452, doi:10.1029/2003JA009940 (2003).