IMF Bx influence on the magnetotail neutral sheet geometry and dynamics E. Gordeev, M. Amosova, V. Sergeev Saint-Petersburg State University, St.Petersburg, Russia 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. Bz Bx 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 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 flux than South lobe  Imbalance of N- and S- pressures develops  Neutral sheet shifts southward to keep pressure balance across the tail. Background Global MHD simulation Figure 4: 1 st (SW driving) mode. 1. F N > F S, consistent with Cowley model of penetrating Bx IMF 2. Δ B n (at MP) ≈ ΔB Lobe during flux loading (Bz IMF <0) Bz IMF < 0 Loading of MF from SW ΔF agree with Cowley model Bz = -6 nT Bx = +6 nT Bz = -6 nT Bx = -6 nT ~ 3 Re ! CCMC, BAT-S-RUS. No dipole tilt Vx = -400 km/s N = 5 cm -3 T = K Vy, Vz = 0 By = 0 Bz – variable (to study the dynamics) 2 runs, 3 hours: Bx = 6 nT (top) Bx = -6 nT (bottom) Y = 0, t = 1h20m – growth phase 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. substorm expansion Δt ~ min 2 different displacement modes are observed, with/without accompanying sharp ΔP Run Bx = +6 nT Figure 3: Top-to-bottom: IMF BZ, vertical displacement of the neutral sheet, and pressure imbalance between N and S lobes t = 112 min (maximum of NS displacement) V A 1 < V A 2 VA1VA1 VA2VA2 S A C S Geometry of asymmetric reconnection: == Z NS ± 3 Re North Lobe V A N = 1310 km/s South Lobe V A S = 990 km/s Alfven discontinuity shifts to the region with lowest Alfvenic speed. Figure 5: 2 nd (substorm) mode. Spacecraft data Aug – Sept, years < 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 Figure 1: Sketch of reconnection in the presence of an IMF Bx field and Bz<0 (from Cowley,1981) 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