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.

Topside ionosphere He+ density depletions: Longitudinal occurrence probabilityfor vernal and winter seasons Table 1. Comparable studies, data sources, geophysical parameters and space/time coverage. Study Data Parameter LT interval Altitudes Latitudes Years F 10.7 Present study ISS-b Р не+ 20-04 LT -1100 km ±20+50° INVLAT 1978-80 150+220 Maryama and Matuura, 1980 ISS-b P rsf 18-06 LT -1100 km ±20° DIPLAT 1978-80 150+220 McClure et al, 1998 AE-E P ct >0.5% 19-06 LT 300-475 km ±20° DIPLAT 1978-80 150+220 Watanabe, Oya, 1986 Hinotori PB650 19-06 LT 650 km ±20° DIPLAT 1981 -200 Basu et a l, 1976 OGO-6 P< j >0.51% 18-06 LT 400-500 km ±20° DIPLAT 1969-70 -150 Su et a l, 2006 ROCSAT P d>0.3% 18-06 LT -600 km ±15° DIPLAT 1999-04 110+190 Study Data Parameter LT interval Altitudes Latitudes Years F r 10.7 c) The results derived by Basu et al. (1976) from N, data from the OGO-6 spacecraft (90° orbital incl.) were also used for comparison. Longitudinal variations of EFI statistics were calculated for one winter season (November- December, 1969-70) in the interval ±20 DIPLAT. Only ion irregularities with ct>0.51% from 400+500 km altitudes were taken into account. EFI statistics (Pa>o. 5 i%) are shown in Fig. 1 by thick dark grey curve. Equatorial spread F._As it is well known the small irregularities ( 1 + 1 0 km ) or so called “noise” are simply detected as equatorial spread-F (ESF). ESF statistics as function of longitude was found by Maryama and Matuura ( 1 9 8 0 ) . The authors have used the data from ISS-b spacecraft. They have used RSF parameter defined as range-spreading parameter: an occurrence probability P R S f of one and more range gates darkened on single ionogram by topside bubbles and irregularities. P r s f values, obtained in the interval of ± 2 0 DIPLAT, have been divided into three 4 - month seasonal intervals. Unfortunately, these intervals have not been centered around a solstice/equinox, i.e. there is - 8 0 % overlapping intervals used before. S / 1 variations of the RSF statistics ( P r s f ) are shown in Fig.l by thin grey curves. Equatorial plasma bubbles o f F region (EPB)._ Since the present work is devoted to the problem of plasma bubble evolution at the topside ionosphere heights, it is extremely desirable to have an information about the equatorial plasma bubbles (EPB) in F region. These information has been found in (Watanabe and Oya, 1986) based on N; measurement data obtained from Hinotori spacecraft (31° orbital inclination) during high solar activity year (1981, F i o . 7 ~ 2 0 0 ) . Longitudinal EPB statistics were calculated in the interval ± 2 0 DIPLAT. Each season is presented as average for three-month interval. Seasonal intervals fall within our 4-moths intervals and approximately centered around a solstice/equinox. S/1 variations of the EPB probability (Рвб 5 о) are shown in Fig. 1 by thick light grey curve. 3 The comparative analysis It is necessary to note that all results used for comparison were received under the same solar and geophysical conditions (local time, season, solar activity level). F region observations were made during the same LT interval 18(19)-06 LT. However LT interval of the He+ density depletion observations has been intentionally chosen more narrow (20-04 LT). To our mind, this choice is more adequate for sunset-night time at the topside ionosphere heights (-1100 km). The longitudinal occurrence rates for two different seasons (winter and vernal) were under consideration. Firstly, let’s focus on the equinox period, when there are approximately equal conditions for the different hemispheres. Vernal conditions. As it was expected, studied the Рщ- variations appear to be the most similar to the PrsFvariations (Maryama and Matuura, 1980), derived from ISS-b data (Fig.l). Really, if the hypothesis about an equatorial origin of He+ density depletions is true, the results received from the data of one satellite should be similar as much as possible. In other words, an equatorial picture (±20° DIPLAT) of the longitudinal occurrence probability (PrsF) should be as much as possible similarly projected in the latitudinal intervals ±20+50° INVLAT of the different hemispheres. (Certainly, it is fair if we bear in mind the magnetic field differences in the different hemispheres.) And really, this assumption is validated (Fig.l). So, the P H e+ variations of the both hemispheres are similar to the equatorial statistical plots. Moreover, note that the PHc+peaks appear approximately at the same longitudes as PRSF. Comparison with Po>0. 5 % (McClure et a l, 1998) and Ро>0.з%- (Su et al, 2006) also reveals good similarity in the shape of the longitudinal variations. The similarity becomes the most convincing if we examine minor and “thin” details of the plots. So, according to the EFI data the local maxima (longitudes -180° and -270°) are well visible. Moreover, they are distinctly pronounced in the Рнг* plots. 139