Physics of auroral phenomena : proceedings of the 34th Annual seminar, Apatity, 01 - 04 March, 2011 / [ed.: A. G. Yahnin, A. A. Mochalov]. - Апатиты : Издательство Кольского научного центра РАН, 2011. - 231 с. : ил.

"Physics o fAuroral Phenomena", Proc. XXXIVAnnual Seminar, Apatity, pp. 137 - 1422011 © Kola Science Centre, Russian Academy of Science, 2011 Polar Geophysical Institute TOPSIDE IONOSPHERE He+ D EN S ITY DEPLETIONS : LONGITUD INAL OCCURRENCE PRO BAB IL IT Y FOR V ERN A L AND W INTER SEASONS L.N. Sidorova L.N., S.F. Filippov (Pushkov Institute o f Terrestrial Magnetism, Ionosphere and Radiowave Propagation, IZMIRAN, 142190 Troitsk, Moscow region, Russia; E-mail: lsid(cbizmiran. ru) Abstract. The idea of an equatorial origin of the He" density depletions was validated once again. For this aim their longitudinal occurrence distributions (Рнг+) were obtained for vernal equinox and winter solstice in the different hemispheres. The detailed comparative analysis with the same occurrence distributions of the equatorial F region irregularities (EFI, PO>o.5%, P<y>o. 5 i%> Р<т>о.з%)> equatorial spread F (EFS, P rsf ) and equatorial plasma bubbles (EPB, Pb65o) was made. The best conformity was obtained for vernal equinox. Rather different longitudinal PHe+pictures were revealed in the different hemispheres during winter solstice. It was found, however, that the PHe+variations of the Northern Hemisphere are the most similar to PrsFi Pa>0.5i% and Po>0. 5 %variations. Other part of the equatorial plots (P<j>o.3%, Рвб5о) has surprisingly good similarity with PHe+ in the Southern Hemisphere. The obtained results may be considered as new evidence supported the idea about plasma bubble origin of the He+density depletions. 1. Introduction He" density depletions were repeatedly observed by many investigators from the first decade of spacecraft observations. They were observed in the low- and mid-latitude regions (L=1.3-3.5, ±20°-58°A) of the topside ionosphere. Firstly these structures were mentioned by Taylor et. al. (1971) based on the OGO-4 data. Taylor and Cordier (1974) called them as subtroughs and identified as plasma depletions distinctly observed equatorward of the light ion troughs. Subtroughs were seen by (Ershova et al., 1977; Sivtseva et al., 1982) based on the Oreol-1,-2 data. They were detected either under disturbed conditions (or several hours after storm) in the external plasmasphere or under completely quiet geomagnetic conditions deeply inside of plasmasphere (L~l.3-3.5). Worthy note that until 1996 the subtroughs were observed in case-to-case basis only (-100 cases). The huge subtrough dataset was presented by Sidorova and Ruzhin (1996) on the ISS-b observations (RRL, 1983, 1985). Subtroughs were detected in the latitude interval from -20° up to 58°A (L-l.3-3.5) during high solar activity (1978-1979, F10.7~200) at the topside ionosphere altitudes (-1100 km). There were -700 cases in -4000 satellite passes. It was shown that the He+ density subtroughs are not so unique and seldom structure as it was supposed earlier. Later the subtrough statistics in respect to season, LT and longitude was obtained by Karpachev and Sidorova (1999, 2000a, 2000b, 2003). It was pointed (Karpachev and Sidorova, 2002) that the subtroughs can be divided into two groups. One group (-256 cases) is usually detected during the magnetic disturbances (or immediately after them) within ±45-58°Л. Another group (-400 cases) appears during the prolonged magnetically quiet periods within ±20-50°A. It was suggested that these groups are of different origin (Karpachev and Sidorova, 2002). An examination of the theoretical works reveals that as usually the subtroughs were interpreted as "plasmatails" resulting from earlier storm time depletions. According to the magnetospheric convection model (Chen et al., 1975) the “plasmatails” would tend to be frozen in the outer layers of the external plasmasphere. Apparently, only one part of the subtroughs observed during storms could be interpreted in this way. As to the subtroughs, observed deeply inside the plasmasphere without any connection with disturbances, it was proposed by Sidorova (2004) to consider them of an equatorial origin. This idea was put forward after comparative analyses of He+ density depletion (subtrough) and equatorial spread-F (ESF), plasma bubble, vertical plasma drift characteristics. It was suggested that ESF/plasma bubbles and He+density depletions may be considered as phenomena of the same plasma bubble origin (Sidorova, 2007). It means that plasma bubbles, reaching the topside ionosphere altitudes, are mostly seen not in Ne density but in He+density. Why? Plasma bubbles are not "visible" in Ne (0 +) since surrounding Ne density becomes comparable with the same bubble density. Meanwhile, the plasma bubbles become "visible" in minor species (i.e. in He*) since the background in He+ density strongly increases in the topside ionosphere and shows contrast with insignificant small He* density content inside of plasma bubble (Sidorova, 2007,2008). Statement o f the problems. According to (Sidorova, 2007) there is a good enough correlation (R-0.67) between ESF and He+ density depletion occurrence rates plotted as functions of a season and local time. ESF statistics were derived from ground-based ionograms obtained over Brazilian regions (Abdu et al., 2000) during similar solar activity (1980-81, Fi07~230). It is reasonable to ask: why the regional map (ESF statistics) and the global map (He+ density depletion statistics) are so good correlated? Probably, the cause is hidden in the features of the longitudinal distributions of the both phenomena. Obviously, the validation of the obtained results is possible if we have the detailed longitudinal statistics of the He+ density depletions and can compare with the same statistics of other equatorial plasma irregularities. 137