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.

L.N. Sidorova, S.F. Filippov It should remind that He+ density depletions are considered as originating from equatorial middle-/large-scale plasma bubbles or as possible fossil bubble signatures (Sidorova, 2007, 2008). It was supposed that plasma bubbles, produced by collision Rayleigh-Taylor instability at the bottomside of the F-layer, could rise up to the topside ionosphere and plasmasphere altitudes and seen as He+ density depletions. Namely, equatorial plasma and separate irregularities uplift (due to vertical plasma drift) to the topside ionosphere. Large irregularities like plasma bubbles spread (due to diffusion processes) along the magnetic tubes. Their spreading becomes more and more significant in process of their uplifting. So extended bubbles look like ‘bananas’ with the extremities reaching the ionosphere heights (may be) in both the hemispheres (e.g., Abdu et a l, 2000). Such structures could be detected by satellites as subtroughs or depletions not only above the equator but also in the low-latitudinal region. In the other hand, ESF registered as spread of traces of F region echoes on ionograms results from small-scale (small-amplitude) equatorial ionosphere irregularities. There are indications (McClure et a l, 1998) that the detection of the small-amplitude irregularities (as ESF) usually means a bubble is nearby. According to Aggson et al. (1992) a ~100 km halo of such irregularities typically surrounds each plasma bubble. These irregularities could exist not only around and within the bubbles but also along the wake that plasma bubble produces (Woodman and La Hoz, 1976). Tsunoda (1980) point that ESF observed at equatorial latitudes represent bottomside irregularities that may be or may not be associated with plasma bubbles that grow into the topside ionosphere, whereas the spread F observed in low latitude ionograms can be indicative of equatorial topside plasma bubble irregularities. (ESF observations used in this study are registered within ±20° DIPLAT). So, if we suppose that the observed He+density depletions are of equatorial origin, it means that the main features of the equatorial F-region irregularities should be reflected in the features of the plasma bubbles, seen as the He+ density depletions. Hence it seems reasonable to assume that the seasonal/longitudinal (s/1) variations of the He+density depletion statistical plots will be similar enough to the same statistics of the typical equatorial phenomena as ESF, equatorial F region irregularities (EFI) and equatorial F region plasma bubbles (EPB). Let’s verify this assumption. He+ density depletions. Some s/1 statistical characteristics of the He density depletions were obtained earlier (Karpachev and Sidorova, 2000b; Sidorova, 2004). ISS-b spacecraft (35° orbital inclination) observations were used (RRL, 1983-85). The available statistics has been revised and added. The data taken within ±20-50° INVLAT and gathered in each geomagnetic longitudinal interval of 15° were considered for Northern and Southern Hemispheres separately. Each season has been presented by four-month intervals, which were centered around a solstice/equinox accordingly. For example, spring season was considered for the period of two months before and after spring equinox day (March 22), i.e. from January 22 to May 22. Evidently, such season fractioning implies partial data overlapping. However this approach has been caused by peculiarities of the available dataset: there were many gaps in the ISS-b measurement dataset. Note that the He+ density subtroughs with density depletions from two times to two orders of magnitude of the background plasma density (Karpachev and Sidorova, 2002) were taken into account. The obtained PHe+ histograms with spline smoothing curves (factor 2.2) are presented in Fig. l(top and bottom). Equatorial F region irregularities (EFI)._EFl statistics was taken from the different satellite observations (Tables 1). The most valuable patterns were taken from (McClure et al, 1998), where all results were presented in very convenient form. (It was favorable case to find this study!). Note that the most o f these plots have been obtained for 1978-80 or for the years of the same solar activity. The brief data characteristics are shown below. a) S/1 occurrence patterns were derived by McClure et al. (1998) from ion number density (N;) data from ion drift meter on AE-E (19.76° orbital incl.). N( data from 19 to 06 LT and within ±20° DIPLAT, where most bubbles are found, were under consideration. Only values with ct >0.5% were taken into account, (a is normalized irregularity index; o=AN,/Ni, where N, is the average of ion density, ANj is a standard deviation of the background density.) The device characteristics and used experimental technique allowed to detect the irregularities space-sized from 50 to 1000 km. EFI statistics (Pa>0,5%) has been calculated as average for three-month intervals. (These intervals entirely coincide with the intervals used in present work.) S/1 variations of EFI statistics are shown in Fig. lby thin black curves. b) S/1 variations of EFI statistics were obtained by Su et al. (2006) on the base of Nf data from ROCSAT (circular orbit, 35° orbital incl.). Calculations were made within 1 ±15° DIPLAT for one month o f each season (e.g., December, March). The statistical occurrence distribution of EFI has been studied for many years (1999-2004) The data were obtained during high (1999-2003, F[ 0 . 7 = 140-И90, 23'rd cycle) and moderate (2003-2004, F i0 7 = 1 10+130) solar activity. High solar activity period was dominated that is why it was possible to use the obtained EFI statistical results for comparison. Ion irregularity values with cr>0.3% were taken into account. Due to high spatial resolution it was possible to detect the ion irregularities in characteristic scale length between 7. 5 km to 75 km. However P for the ROCSAT observation is noted to be about half of that derived from (e. g.) the AE-E data (McClure 1998) because of different selection criteria. The s/1 variations of the EFI statistics are shown in Fig. 1 by dotted curves The scale of the plots is on the right. Comparable data 138