IMF Bx influence on the magnetotail neutral sheet geometry and dynamics

1. Bz IMF < 0 Run Bx = +6 nT Y = 0 , t = 1h20m – growth phase t = 112 min (maximum of NS displacement) Bx = +6 nT Bx • CCMC, BAT-S-RUS. • No dipole tilt • Vx = -400 km/s • N = 5 cm-3 • T = 100.000 K • Vy, Vz = 0 • By = 0 • Bz – variable (to study the dynamics) • 2 runs, 3 hours: • Bx = 6 nT (top) • Bx = -6 nT (bottom) == ZNS± 3 Re Bz = -6 nT Bz 10 Loading of MF from SW 0 ~ 3 Re ! -10 ΔF agree with Cowley model S -15 VA2 C North LobeVAN = 1310 km/s Bx = -6 nT VA1 South LobeVAS = 990 km/s 1. FN > FS , consistent with Cowley model of penetrating Bx IMF 2. ΔBn (at MP) ≈ΔBLobe during flux loading (Bz IMF <0) Δt ~ 4 - 6 min S 2 different displacement modes are observed, with/without accompanying sharp ΔP VA1 < VA2 substorm expansion Bz = -6 nT A Figure 3: Top-to-bottom: IMF BZ, vertical displacement of the neutral sheet, and pressure imbalance between N and S lobes. Figure 2: Example of MHD output (left) and simulation input (right). All parameters are fixed and only Bz IMF is vary as shown on Fig.3. Geometry of asymmetric reconnection: Alfven discontinuity shifts to the region with lowest Alfvenic speed. Figure 4: 1st (SW driving) mode. Figure 5: 2nd (substorm) mode. Spacecraft data • Aug – Sept, 2001-2006 years. • -19.5 < X < -15 Re • |Y| < 8 Re • 1 min averaged C1 data • 5 min averaged SW data • dZ = Z(C1) – Z(TF.04) , where • Z(C1) – Cluster 1 Zgsw coordinate • Z(TF.04) – NS Zgsw coordinate predicted • by Tsyganenko-Fairfield global NS • shape model (JGR, 2004). • Z(TF.04) = f(Xsc,Ysc,Ψ,BzIMF,ByIMF,PdSW) Hist.1: Cluster C1 crossings ΔZ ~ (0.5 – 1) Re for X = -(15::20) Re E. Gordeev, M. Amosova, V. Sergeev Saint-Petersburg State University, St.Petersburg, Russia Background Cowley (1981) suggested that Bx IMF can lead to asymmetric loading of tail lobes and, consequently, can affect the global magnetotail geometry (namely, position of the tail neutral sheet) . For IMF BX>0 (fig.1): North-South asymmetry of flux tube tensions leads to different displacement of opened flux tubes along X direction.  North lobe accumulates more fluxthan South lobe Imbalance of N- and S- pressures develops  Neutral sheet shifts southward to keeppressure balance across the tail. Goals of our study : 1/ - Confirm/Investigate the IMF BX effects and their dynamical appearance using global MHD simulations 2/ - Confirm the effect using magnetotail spacecraft observations IMF Bx influence on the magnetotail neutral sheet geometry and dynamics Figure 1: Sketch of reconnection in the presence of an IMF Bx field and Bz<0 (from Cowley ,1981) Global MHD simulation Interplanetary magnetic field (IMF) plays a crucial role in the solar wind – magnetosphere coupling and consequently in magnetospheric dynamics. A plenty of works revealed Bz and By IMF influence on energy circulation in the solar wind – magnetosphere system. At the same time the role of Bx component was overlooked since it does not show any significant correlation with geo-effective indices. • Global MHD models reveal significant IMF Bx-dependent effect in the position and geometry of magnetotail neutral sheet. • Neutral sheet motion displays 2 different modes: (1) - related to IMF southward turning and (2) - substorm expansion related mode. • Mode (1) is controlled by flux difference in tail lobes due to asymmetric loading (as predicted by Cowley). • Additional transient motion of NS (mode 2) was also found, that corresponds to redistribution of currents due to N-S asymmetry of plasma sheet magnetic reconnection. • Neutral sheet crossing data from Cluster spacecraft confirm IMF Bx effect. Conclusions