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.

M. Forster et al. geomagnetic field model. The methodology of determining the electric field potential pattern in dependence of the IMF orientation were described in detail by the companion papers of Haaland et al. [2007] and Forster et al. [2007]. The relatively large EDI data set allows to perform statistical studies about the influence of the IMF Bx component on the high-latitude plasma convection pattern. For this purpose, the data set was divided into two approximately equal parts with Bx > 0 (IMF toward the Sun) and Bx < 0 (IMF away from the sun) at the magnetopause. Then each part was sorted with respect to IMF orientation in the GSM j-z-plane into 8 sectors of 45° width. The average amplitude of the IMF Bx component varies between 2 nT and 4 nT, with ~2 nT in those regions, where Bx and By are со-aligned, and ~4 nT within the usual sector structures with opposite signs of these components. The IMF Bx component and the high-latitude ionospheric convection Table 1 lists the potential differences Д U between the night side (main) convection foci for all 8 sectors of the Northern and Southern Hemispheres and the two subsets with Bx > 0 and Bx < 0. The data show that not only the characteristic pattern are kept for both subsets, but also the regularity of cross-polar potential changes with the IMF: AU increases monotonically from sector 0 (B, > 0, northward IMF) via positive IMF By values to sector 4 (5Z< 0, southward IMF) and then decreases again for negative IMF By from sector 4 to 0. This regularity is kept at the Southern Hemisphere without exception, but on the Northern Hemisphere there is an exception for sector 5 and Bx< 0. We suppose that this exceptional case is due to an insufficient data coverage: firstly, because of the increasing Cluster orbit tilt toward South, so that there are more data at the Southern compared with the Northern Hemisphere and, secondly, due to the much smaller amount of data within sectors of со-aligned IMF Bx and By components. The survey of AU at both hemispheres for different IMF Bx orientations (Table 1) does not reveal any distinct influence of Bx on night time the cross-polar cap potentials. We made further an attempt to find out parts of the high-latitude convection, which might be controlled by IMF Bx. As described below, this attempt considered two different methods which are based on different assumptions. The first method draws upon the presumption that the IMF Bx component does not have an effect for the geomagnetic variations recorded at the Earth’s surface and for the convection in case of equally directed Bx and By, i.e., at the Northern Hemisphere and Bx > 0 this is the case for sectors 1, 2, and 3, while for Bx < 0 this is valid for sectors 5, 6, and 7. In the further calculations we consider only sectors 2 and 6, where the influence of IMF B- is practically vanishing and the convection pattern becomes therefore more simple. The Bx dependent part of convection, if it ever exists, we are going to isolate in two steps. First we calculate the BCP elements, which are free o f the Bx influence, as Table 1. Cross-polar cap potential differences AU between the night side (main) convection foci (in kV) for all the 8 sectors of different IMF orientation deduced from Cluster EDI measurements within the years 2001-2009, sorted for both IMF toward the Sun (Bx > O') and awav from the Sun (B,< O'!. Sector IMF orientation AU (kV) Northern Hemisphere AU (kV) Southern Hemisphere вх> 0 Bx< 0 Bx> o Bx< 0 0 в : 15.8 14.4 14.2 12.4 1 в ; / в ; 21.8 24.6 22.9 23.4 2 в* " У 35.4 34.0 37.9 37.9 3 в ; / в ; 50.8 44.5 47.2 48.2 4 К 58.5 54.6 52.5 57.4 5 в ; / в ; 44.2 59.4 49.5 49.7 6 в; 32.9 35.9 33.4 34.8 7 в ; / в ; 17.7 24.2 21.9 20.6 follows: U0 =( [sector 2 for Bx > 0] + [sector 6 for Bx< 0] )/2 ; U+y = ( [sector 2 for Bx> 0] - [sector 6 for Bx< 0 ]) /9.75 (for U_ we have the opposite sign of convection). In the second step we calculate the possible IMF independent part of the convection using: U+x = ([sector 6 for Bx > 0] - [U0 + U_y >=5.36]) /3.6 U+x = ([sector 2 for Bx< 0] - [U0 + U x5.30]) /3.9 The second method draws upon the circumstance, that the Earth’s dipole magnetic field lines over the northern polar cap region have an opposite direction to the IMF field lines for Bx< 0, but over the southern for Bx> 0. An opposite direction of the IMF and the magnetospheric tail magnetic field constitutes a favourable condition for the penetration of solar wind plasma into the polar cap region [ Newell et al., 2009]. An enhanced flow of soft energetic particles can likewise affect the convection due to an increased level of ionisation of the upper atmosphere. The possible IMF Bx dependence of convection over the Northern Hemisphere we also get in two steps: 44