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.

PB650 statistics are more narrow and less intensive; their tops are of simple form like the PHe+ curve in the SH. Moreover, the main maximum of PB 65 o near 360° longitude appears in PHe+ at the same longitudes. And again the local P<j>o.5i%maximum (Basu et a l, 1976) near 180° longitude is still clearly visible in PHe+curve of the SH. Thus we should conclude that there are the most significant differences in the PHe+ distributions of the different hemispheres for solstice periods. The PHe+ variations in one hemisphere are strongly deformed in comparison with the PHe+ variations of other hemisphere. Moreover some PHt+ peaks appear with longitudinal shifts. However, even so different PHt+pictures have the common features with the equatorial irregularity occurrence distributions. Note that the comparison was done not for one dataset but for several datasets (EFI, ESF, EPB) obtained by the different investigators on the base of the different observations. It seems this fact is the most valuable one for this study. Availability of the sufficient statistics supplies not only reliability but also the “repeatable” results. Moreover, this fact allows to see a tendency in the EFI dynamics subsequently developed in the PHe+ curves of the SH and NH. We should conclude once again that all data involved in comparison show that the main geophysical characteristics of the equatorial F region irregularities really repeat in new results. The obtained results have all main features repeated even in the details. Apparently, the seasonal factor should be considered as a factor of the first approximation. Due to seasonal factor only the more favorable conditions for plasma bubble "survival" can appear in one hemisphere and, on the contrary, less favorable conditions can appear in other hemisphere. The thermosphere meridional winds, depending on a season, play the important role in these processes. They can suppress the irregularity development in F region during irregularity generation due to inhibiting the growth of Reyleigh-Taylor instability and flux tube integrated conductivity (e. g , Abdu, 2001). As result they can "lock" the plasma bubble development in one hemisphere and, on the contrary, favor them in another hemisphere (e.g., Abdu, 2001; Sidorova, 2008). The magnetic declination effect also makes significant amendments to the processes above equator and low latitudes. In particular, there are indications that the magnetic declination effect control the seasonal occurrence probability of EFI and ESF (Tsunoda, 1985; Abdu et al, 1992). In other words, the occurrence probability of EFI and ESF essentially increases, when there is a small angle between the sunset terminator and magnetic meridian. It seems the combination of two main factors (seasonal peculiarities and the magnetic declination effect) generates the preference/suppression mode, which "works" for some longitudes in one hemisphere and for other longitudes in other hemisphere. As a result, the plasma bubble occurrence distribution can be minimally deformed and shifted in one hemisphere in relation to primary equatorial picture and more strongly deformed (or absolutely "suppressed") in other hemisphere under the "bad" conditions. 2. Conclusions The longitudinal occurrence distributions of the He+density depletions were obtained for vernal and winter seasons in the different hemispheres. The obtained characteristics were compared with the longitudinal occurrence distributions of the equatorial F region irregularities (EFI), equatorial spread F (EFS) and equatorial plasma bubbles (EPB). 1. It was found for vernal equinox _that: (a) The PHe+ plots of the different hemispheres are of almost the same intensity and practically equally developed. The light shape difference of the PHe+plots of the different hemispheres can be explained by superposition of some factors. First of all, it is caused by magnetic declination effect, (b) The main longitudinal occurrence probability maxima of the equatorial F region irregularities (EFI, ESF, EPB) are well enough reflected in the PHe+plots of the both hemispheres. 2. It was found for winter solstice that: (a) Rather different longitudinal PHe+ pictures appear in the different hemispheres. It was suggested that at least two major factors are responsible for discrepancy of the PHe+plots. The deviations are caused by seasonal peculiarities of the different hemispheres (insolation and meridional winds) and by magnetic declination effect, (b) The PHe+ variations have the common features with the equatorial irregularity occurrence distributions. PRSF (Maryama and Matuura, 1980), Po>o. 5 i%(Basu et al, 1976) and Pa>0. 5 %(McClure et al, 1998) are the most similar to the PHe+plot in the Northern Hemisphere. Other part of the equatorial plots (P0xu% (Su et a l, 2006), P B650 (Watanabe, Oya, 1986)) has surprisingly good similarity with the PHe+ plot in the Southern Hemisphere. 3. All mentioned results obtained in this study may be considered as new evidence supported the idea about the plasma bubble origin of the He density depletions. Acknowledgements. We express our gratitude to ISS Research and Operation Committee (Japan) for providing the ISS-b data, M.A. Abdu (Brazil), T. Maruyama (Japan) for the useful advices and discussions. This work was partly supported by Russian Foundation for Basic Research (RFBR) under grant № 09-05-01071-a. Topside ionosphere He+ density depletions: Longitudinal occurrenceprobabilityfor vernal and winter seasons 141