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 hysics o fAuroral Phenomena", Proc. XXXTVAnnual Seminar, A patity, p p . 43 - 4 6 2011 Polar © Kola Science Centre, Russian Academy o f Science, 2011 \"/№l Geophysical \ V У Institute PLASMA CONVECTION IN THE HIGH-LATITUDE IONOSPHERE DEDUCED FROM CLUSTER EDI DATA AND THE IMF Bx COMPONENT M. Forster1, Y.I. Feldstein2, L.I. Gromova2, L.A. Dremukhina2, A.E. Levitin2, S.E. Haaland3’4 1GFZ German Research Centrefor Geosciences, Helmholtz Centre Potsdam, 14473 Potsdam, Germany ' IZMIRAN, Russian Academy o f Science, Troitsk, Moscow region, Russia 3Max Planck Institutefor Solar System Research, 37191 Katlenburg-Lindau, Germany 4Department o fPhysics and Technology, University o fBergen, Norway Abstract. The relatively large data set of the Cluster Electron Drift Instrument (EDI) allows to perform statistical studies about the influence of the /^-component of the Interplanetary Magnetic Field (IMF) on the high-latitude plasma convection pattern. 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 y-z-plane into 8 sectors of 45° width. The derivation of high-latitude convection parts, that might be controlled by the IMF Bx component, has been carried out with two different methods that make use of IMF sectors 2 and 6 only, i.e. IMF orientation with B2~ 0 and By> 0 and В y< 0, respectively. The first is based on the assumption of the absence of any Bx influence for equal signs of the IMF Bx and By components. The second makes use of the fact, that an opposite orientation of the IMF and the tail lobe field favours the entry of solar wind plasma into the polar cap region. The cross-polar cap potential difference at both hemispheres of the main night side convection cells of all 8 sectors does not reveal any distinct influence of Bx. On the other hand, there are indications of some influence of the IMF Bx component on the high-latitude day side circulation cells in sector 0 (for northward IMF). Introduction The ionospheric plasma convection at high latitudes appears as the characteristic feature of the interaction between solar wind, magnetosphere and ionosphere [Axford and Hines, 1961]. The decisive parameters for the control of this convection turned out to be the orientation and intensity of the Interplanetary Magnetic Field (IMF) [Dungey, 1961]. Various empirical coupling functions were proposed that can represent best the interaction between the solar wind and the magnetosphere [ Newell et al., 2007]. The vast majority o f these functions make use of the IMF B. and Bv components, and only Akasofu's e parameter applies the intensity of the full magnetic field vector, including all three IMF components [Perreault and Akasofu, 1978]. There are different positions in the public literature on the role of the IMF Bx component on geophysical processes at high latitudes, ranging from noticeable effects [Crooker, 1986\ Belenkaya, 1998; Bloomberg et al., 2005; Newell et al., 2009; Peng et a l, 2010] to unverifiable influences of this component on the high-latitude geophysical processes by Levitin et al. [1982], Holzworth and Meng [1984], and Newell et al. [1989]. The challenge to deduce any influence of the IMF Bx component on geophysical phenomena is due to the predominant sector structure of the IMF in the ecliptic plane, which consists of two different sectors, in each of which the IMF Bx and By components have opposite signs. Analysing geomagnetic field variations at near-polar regions in dependence on IMF Bx and By, Friis-Christensen et al. [1972] as well as Sumaruk and Feldstein [1973] concluded, that they are controlled solely by the IMF Bv component. This conclusion was based on analyses of the relations between geomagnetic field variations and the IMF components during periods, where this sector structure was disturbed and both components pointed in one and the same direction. This result obtained by them is now generally accepted and all known statistic high-latitude convection models of the ionospheric plasma describing the spatial-temporal convection pattern rely on the IMF Bz and By components only [cf., e.g., Haaland et al., 2007]. Investigations, on the other hand, which describe the connection between the plasma convection and the IMF Bx component, are as a rule global MHD simulation studies [e.g., Chapman et a l, 2004; Peng et al., 2010]. As far as we know, up to now there are no publications on direct observations, which provide an unchallengeable evidence for ionospheric convection contributions, that are controlled by the IMF Bx component. Data The European Space Agency (ESA) Cluster mission consists of four identical spacecraft flying in a tetrahedron-like formation. Cluster has a nearly 90° inclination elliptical orbit with perigee initially around 4 RE and apogee around 19 Re, and an orbital period of about 57 h. More than 10 millions individual “good” plasma drift measurements of the Electron Drift Instrument (EDI) on board Cluster within the region of data collection inside the magnetosphere were obtained from February 2001 up to now. They are assembled in about 20000 h of 1-min averages and mapped along the magnetic field lines into the upper ionosphere (~400 km) by use of the Tsyganenko-2001 (T01) 43