Numerical simulations are used to explore the interaction between solar coronal mass ejections (CMEs) and the structured, ambient global solar wind flow into which they are launched. In particular, the simulations are used to estimate how interplanetary dynamical processes affect the propagation of the injected CME material and how the ambient interplanetary magnetic field (IMF) is distorted in the interaction. The solar wind is composed of alternating patterns of slow flow (with high density and low temperature) and fast flow (with low density and high temperature). The slow flow is thought to be the interplanetary extension of the coronal streamer belt, while the fast flow is known to emerge from coronal holes. The spiral interplanetary magnetic field (IMF) reverses polarity across a very thin layer (the heliospheric current sheet) which is embedded in the slow, dense streamer belt flow circling the Sun. CMEs have been identified as a prime causal link between solar activity and large, non-recurrent geomagnetic storms [Gosling et al., 1991]. Interplanetary events with a southward IMF component Bz < -10 nT and with a duration larger than 3 hours have a one-to-one causal relationship with intense (Dst < -100 nT) geomagnetic storms [Gonzales and Tsurutani, 1987]. Great magnetic storms are caused by a combination of Bz from injected magnetic cloud structures with Bz generated by interplanetary dynamic interactions [Tsurutani et al., 1999]. The solar wind is composed of alternating patterns of slow flow (with high density and low temperature) and fast flow (with low density and high temperature). The slow flow is thought to be the interplanetary extension of the coronal streamer belt, while the fast flow is known to emerge from coronal holes. The spiral interplanetary magnetic field (IMF) reverses polarity across a very thin layer (the heliospheric current sheet) which is embedded in the slow, dense streamer belt flow circling the Sun. CMEs have been identified as a prime causal link between solar activity and large, non-recurrent geomagnetic storms [Gosling et al., 1991]. Interplanetary events with a southward IMF component Bz < -10 nT and with a duration larger than 3 hours have a one-to-one causal relationship with intense (Dst < -100 nT) geomagnetic storms [Gonzales and Tsurutani, 1987]. Great magnetic storms are caused by a combination of Bz from injected magnetic cloud structures with Bz generated by interplanetary dynamic interactions [Tsurutani et al., 1999]. Meridional cross sections of interplanetary disturbances 2 days after CME launch. Distributions of velocity (gray scale), injected mass (color scale), bulk density (contours), and IMF (projected unit vectors) are shown in slices across pulse center. The dynamic distortion of the CME depends strongly upon where it is launched with respect to the streamer belt. Schematic view of the outflow conditions at the inner boundary. The distribution of the mass density is shown with enhanced color intensity. The solid line in the middle of the streamer belt indicates location of the heliospheric current sheet. Numerical Simulation of Interplanetary Disturbances V J Pizzo and Dusan Odstrcil NOAA Space Environment Center and CIRES Abstract Background Simulated CME Interplanetary Structure The evolution and appearance of CMEs can be strongly affected by their interaction with background solar wind structures. This interaction also distorts the IMF and enhances its southward component, Bz. Substantial Bz excursions can be generated by:  shock compression of the spiral IMF  draping around a CME by mass flow convection  distortion of the IMF by rarefaction waves trailing the CME The amplitude of Bz generated by interplanetary dynamics depends mainly upon the details of the configuration of the corotating streamer belt and the initial position and speed of the CME with respect to the streamer belt flow. The evolution and appearance of CMEs can be strongly affected by their interaction with background solar wind structures. This interaction also distorts the IMF and enhances its southward component, Bz. Substantial Bz excursions can be generated by:  shock compression of the spiral IMF  draping around a CME by mass flow convection  distortion of the IMF by rarefaction waves trailing the CME The amplitude of Bz generated by interplanetary dynamics depends mainly upon the details of the configuration of the corotating streamer belt and the initial position and speed of the CME with respect to the streamer belt flow. Numerical Model Conclusions A three-dimensional magnetohydrodynamic (MHD) model is used in a Sun- centered spherical coordinate system between 0.14 and 1.04 AU [Odstrcil and Pizzo 1999]. An explicit finite-difference TVDLF (Total-Variation-Diminishing Lax- Friedrichs) algorithm is applied with dimensional splitting on a non-staggered numerical grid. A tilted-dipole outflow configuration is specified at the inner boundary near the Sun, and a structured, corotating solar wind flow with a spiral IMF is established by dynamic relaxation. The CME is modeled as a simple plasma blob injected at high speed into the ambient spiral IMF. No magnetic ejecta are currently included in the calculation, although such embedded structures will be included in subsequent calculations. Four different cases of a CME launch are considered, as illustrated in the figure at top center. A three-dimensional magnetohydrodynamic (MHD) model is used in a Sun- centered spherical coordinate system between 0.14 and 1.04 AU [Odstrcil and Pizzo 1999]. An explicit finite-difference TVDLF (Total-Variation-Diminishing Lax- Friedrichs) algorithm is applied with dimensional splitting on a non-staggered numerical grid. A tilted-dipole outflow configuration is specified at the inner boundary near the Sun, and a structured, corotating solar wind flow with a spiral IMF is established by dynamic relaxation. The CME is modeled as a simple plasma blob injected at high speed into the ambient spiral IMF. No magnetic ejecta are currently included in the calculation, although such embedded structures will be included in subsequent calculations. Four different cases of a CME launch are considered, as illustrated in the figure at top center. Simulated Spacecraft Profiles Temporal profiles of interplanetary disturbances generated by CMEs launched near the slow streamer belt flow. Schematic views of the initial outflow conditions for four different cases are included at the top-right of each panel, and the location of the observing points (at 1 AU) is projected into that region. Observing points at 1 AU are at the pulse centerline (top row), at 15 deg north (middle row), and 15 deg south (bottom row). Radial velocity, number density, total magnetic field, and north-south magnetic field are shown from top to bottom. The solid and dashed lines in the number density plots show the total and injected number density, respectively. Shaded areas indicate intervals where the injected densities define the main body of the CME plasma. IMF Distortions Input Flow Configurations North-south IMF as function of time and heliographic latitude at 1 AU. Profiles are obtained from the temporal evolution of variables stored at 5 deg intervals along a meridional cut through the pulse center. The heliographic locations of these 17 observing points are projected onto a schematic view of outflow conditions at 0.14 AU (left column). Light filling indicates a region where the solar wind velocity is less than 500 km/s; this approximately indicates the location of the slow streamer belt flow. Dark filling indicates a region where the injected mass density is larger than 5 cm^3, which approximately indicates the extent of the CME. The pulse centerline is indicated by the thick profile at the center of each panel.