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.

P lasm a convection in the high-latitude ionosphere dedu ced from Cluster EDI d ata a n d th e IMF Bx component a ) U0 = (Sect2()0) + Seet6«0))/2 b ) (Sect2()0) - Sect6((0))/9.75 Northern hem, BY, exl U0 = ( [sector 2 for Bx > 0] + [sector 6 for Bx> 0] )/2 ; U+y = ( [sector 2 for Bx > 0] - [sector 6 for Bx> 0]) /9.9 (for U we have the opposite sign of convection). U_x = ( [sector 6 for Bx< 0] - [U0+U_y x5.2]) /2.3. For the corresponding relations for the IMF Bx dependence at the Southern Hemisphere we obtain with analogous formulas of the two methods described above (not shown here). The characteristics of the potential pattern and the potential values of the BCP elements U0 and U±y (Figure. 1 and Figure 2, Table 2) show a good agreement for both methods as well as with the previously determined values according to Forster et al. (2009, Figure. 2) with U0 = 31.0 kV and U+y = -3.2 kV. Such a good correspondence can serve as an additional valid argument in favour of the legitimacy of the presumtions adopted for the calculation of the BCPs. The characteristics and the intensity of the convection pattern deduced for U±x is not that clear. There is no regular, systematic convection pattern, but rather some small randomly distributed vortices. The isolines are very irregular and the vortices have peak values, which are several times smaller than those related to the IMF By and Bz components. There didn’t appear specific distributions, which could be related to the sign of IMF Bx or to hemispheric differences. We couldn’t therefore reveal any conclusive evidences for the presence of the IMF Bx dependence in the convection pattern, its characteristics or intensity. The analysis of the high-latitude c ) (Seet6()0) - (U d +B.,*5.36)/3.6 g 'j 0 I 3.5kV Figure 1. Basic Convection Pattern (BCP) derived from Cluster EDI observations of the Northern Hemisphere for various orientations of the IMF, using the first method of our Bx trial: a) basic convection part U0 for vanishing IMF contribution, b) U+y part of the BCPs, describing the dependence on IMF By, normalized to 1 nT, c) and d) patterns resulting from the first method for IMF B+x and B_x dependence, respectively, normalized to 1 nT. daytime convection cells, on the other hand, evaluating the potential difference between the foci, which appear for northward IMF orientation, reveal some indications for an IMF Bx dependence. For an anti-parallel orientation of Bx and the geomagnetic force lines along Table 2. Potential difference A U (in kV) for two BCPs (see text), based on Cluster EDI data for 2001-2009._______ BCP element __________ Northern Hemisphere______________________Southern Hemisphere__________ Method 1 Method 2 Method 1 Method 2 Un U *y 30.4 -2.9 29.5 -3.0 33.3 2.8 30.9 2.5 the magnetospheric tail of the same hemisphere, the potential difference between the day side vortices appears to be slightly enhanced compared with the parallel orientation (Table 3). The potential difference is greater in that hemisphere where geomagnetic field lines and the IMF are oriented anti-parallel to each other. Table 3. Potential differences AC/between the high-latitude day side convection foci (in kV) of sector 0 deduced from Cluster EDI observations of 2001-2009, sorted both for IMF toward ( Bx > 0) and away from the sun (Bx< 0). Northern Hemisphere fix> 0 # ,< 0 Southern Hemisphere Bx> 0 Bx< 0 Sector 0 11.4 15.8 13.1 7.4 The significance of this effect and its statistical reliability has to proven yet in further studies. Acknowledgments. This study is supported by the Deutsche Forschungsgemeinschaft (DFG), the RFBR grant 11- 05-00306., and the Norwegian Research Council. 45