Physics of auroral phenomena : proceedings of the 36th Annual seminar, Apatity, 26 February – 01 March, 2013 / [ed. board: A. G. Yahnin, A. A. Mochalov]. - Апатиты : Издательство Кольского научного центра РАН, 2013. - 215 с. : ил., табл.

THE MAGNETOSPHERIC MAGNETIC FIELD DEFORMATION: EFFECTS OF DOUBLE-LOOP SUBSTORM CURRENT WEDGE A.V. Nikolaev1, V.A. Sergeev1, N.A. Tsyganenko1, V. Angelopoulos2, H. Singer3 ^Saint-Petersburg State University, St.-Petersburg, Russia 2University o f California, Los Angeles, USA 3Space Weather Prediction Center (NOAA), Boulder, USA Abstract. Recent studies of magnetic field dipolarization amplitudes simultaneously observed by one GOES and four THEMIS spacecraft radially-distributed in the magnetotail have confirmed a double-loop (Rl-like plus R2-like) geometry of the Substorm Current Wedge (SCW) and introduce and test the new magnetospheric quantitative model SCW2L. To demonstrate and quantify the double-loop current system field line twisting effect we map fixed neutral sheet points to the ionosphere using T89 and SCW2L magnetic field models and show the possible shapes of active ionospheric structures (such as auroral bulge, westward traveling surge) which can appear when the substorm current system arise. Using quantitative SCW2L model and varying its parameters (current intensities, their ratio, field line stretching amplitude and currents intensity) we show that: (1) the poleward footprints expansion maximal amplitude may reach ALat ~ 9° CGLat during rare strong substorms (Ii = 2 MA), ALat ~ 5-6° CGLat in moderate conditions (Ii = 1 MA) and ALat< 3° during low level disturbances (Ii < 0.5 MA); (2) the SCW-related magnetic disturbances can naturally explain the auroral bulge poleward expansion, westward traveling surge formation, auroral streamers morphology and their clockwise and westward/eastward movement. Our study is a continuation of earlier observational and theoretical studies devoted to the SCW morphology and its impact on the aurora displays and magnetospheric magnetic field configuration. Introduction The classical view on SCW (also referred to as the R1 loop) presented in Fig. 1 shows wedge-shaped structure (northern hemisphere) and its currents directions. The history of R1 loop models is long enough and we will not dwell on it noting that the recent realistic version of magnetospheric model (FW-SCW) detailed in Sergeev et al., [2011] proved to be suitable for accurate magnetic field computations both in space and Earth's surface. However, geostationary magnetic observations compared with FW- SCW predictions systematically mismatched and indicated the need for additional “region 2” sense current loop also referred to as the R2 loop (see Fig. 1, green curve) to be added to traditional single-loop model. Previously using twin-loop SCW model version (SCW2L, presented in Fig. 1) and ground-based/spacecraft observations we estimated radial distance to R2 loop “Physics o f Auroral Phenomena", Proc. XXXVI Annual Seminar, Apatity, pp. 51 - 54, 2013 P o la r © Kola Science Centre, Russian Academy of Science, 2013 W w inst^tuteT'^ Fig. 1 The classic Substorm Current Wedge (“region 1” sense, red curve) and additional opposite polarity R2 current loop (green curve, “region 2” sense). location (RT2), current intensities and their ratio during real substorms expansion phase. Statistical studies of relationship between dipolarization amplitudes at 6.6 Re and 11 Re shown that magnetotail stretching amplitude (RCF parameter which is based on BZ0 value at geostationary just prior to onset) corresponds to events o f different intensities and associated R2 equatorial current locations. The strongly stretched configuration (RCF > 6) allows earthward moving plasma injections, carrying dipolarization front (DF) and generating R2 wedge currents, penetrate closer to Earth (RT2 < 6.6 Re), while dipole-like configuration (RCF < 3) causes BBFs stop tailward the geostationary (RT2 > 6.6 Re). In current study we apply SCW2L model to quantitatively investigate its magnetospheric effects and describe common auroral features appearing in azimuthal current wedge sector and at its edges. SCW2L model setup The SCW2L model is parameterized by total R1 and R2 currents (termed as I] and I2), east and west SCW longitudes (Pe and Pw), and radial distances to equatorial location o f R1 and R2-sense currents (RT, and RT2, correspondingly). The shape of magnetic field lines, carrying SCW currents, is described by T89 model [Tsyganenko, Fig. 2 View from the North: a) R2 (Rl) loop location and configuration with concave (I) and convex (II) equatorial current segments; b) corresponding Bz magnetic radial profiles calculated along X axis in XY plane (Y=0). The calculations performed for Pw = 155° SM Lon, Pe = 205 SM Lon. 51