Physics of auroral phenomena : proceedings of the 33rd Annual seminar, Apatity, 02 - 05 March, 2010 / [ed.: A.G. Yahnin, A. A. Mochalov]. - Апатиты : Издательство Кольского научного центра РАН, 2011. - 206 с. : ил.

The role o f ion diffusion information o f 3-D spatial structure o f theplasmasphere The altitude axe is in logarithmic scale [ N(e>. a-3 J 00 Figure 4. The n(e) vertical cross-section along the midnight meridian after 5 days of modeling time. 2.1. Near-equatorial maximum An arched area o f high 0 +density appears during the first modeling day in the near-equatorial region and practically does not change further (Fig.3). On the horizontal section of the ionosphere (Fig.2) it looks similar to the equatorial anomaly, with peaks near latitude 10° and trough between them. But it was formed here exclusively by field-aligned diffusion process, without any participation of the electromagnetic drift and fountain effect. We explain the formation o f this maximum as follows. The tubes, in which it is formed, have a small length and small cross-section in the near-equatorial part. On the other hand, the "productivity" of the ionosphere source which "fills" this "reservoir" is greater than on other tubes, because big part of this near-horizontal tube is located in the active ionization area. Therefore, during the day a significant number of ions 0 +produces here and accumulates in the upper part o f tubes where the recombination is almost absent. The gravity force in this area does not "drag" the ions back to the dense atmosphere (and to the fast recombination) because tubes are horizontal. Efficiency o f 0 + losses in the charge exchange reaction is also lesser than in the longer tubes: H+ ions can not leave these tubes and recycled back to the 0 +. As a result, a significant concentration o f 0 + ions is formed here and remains through all the night. Shorter tubes are entirely in the more dense neutral atmosphere, and longer ones inclined stronger and the ions "roll" to the dense atmosphere under the force o f gravity. Therefore in the both shorter and longer tubes the night recombination goes faster, and the maximum appears between them. 2.2. Sub-auroral maximum Another noticeable feature of the forming structure is the sub-auroral night maximum and the "horns" of increased 0 + concentration above it (Fig.3). It appears in the first modeling day and later "melts" gradually from the low- latitude side as the "constant reserve" of H+accumulates in the upper part of corresponding tubes. Due to huge volume o f near-auroral tubes the plasma density remains very low even after several days of filling, and daytime outward flow o f 0 + doesn't meet any obstacles. Large quantities of 0 + still penetrate far into upper parts of the tubes even after 3-5 days, when at lower latitudes the accumulated H+ stops 0 + flows. As a result, the ion composition in the near-auroral tubes differs significantly. The H+ density is very low (the light-ion trough), but 0 + concentration is significantly increased as compared to the same heights on shorter tubes - the above described "horns". After sunset these heavy 0 + ions drop back into ionosphere and support increased plasma density in ionosphere (Fig.5). Downstream flow lasts whole night with weakening only before sunrise. The smaller tubes become empty faster (Fig.2). The efficiency o f 0 + returning into ionosphere from high-altitude "store" is more then for H+ ions: because of their mass they penetrate into dense atmosphere more easy. It results that thinner 0 + high-altitude plasma supports higher night F2-level density than more dense H+gas on shorter tubes of inner plasmasphere. In the real ionosphere this maximum maps on the area of convection, which erodes it and doesn't allow to reveal it. May be it's possible only in very quiet conditions, after severe storms. But the areas of high n (0+) at the outer edge of the plasmasphere which are genetically related with it are known. Figure 5. The vertical cross-section of ion fluxes along the midnight meridian (5th day of modeling). 131