Propagation of Stream Interfaces: An LFM-helio Study

During the last solar minimum, the heliosphere was dominated by steady-state Stream Interaction Regions (SIRs). Carrington Rotation (CR) 2060, which occurred during this period, contained multiple SIRs and was free of transient phenomena which made it a good interval for the study of SIRs. We have used the Lyon-Fedder-Mobarry (LFM) heliospheric 3-D magnetohydrodynamic (MHD) model, the LFM-helio, to examine the radial evolution of SIRs, particularly the speed of the stream interface, from 0.1 AU to 2.0 AU. The LFM-helio is an adaptation of the magnetospheric LFM MHD code to heliospheric plasmas and fields. The ideal MHD equations are solved on a uniform spherical grid, excluding 10 degree cones centered at the poles. The inner boundary condition is obtained using the Wang-Sheeley-Arge (WSA) coronal model driven by photospheric magnetic field measurements for CR 2060. The global nature of the LFM-helio solution facilitates the study of the steepening of SIRs as well as their speed through the ambient plasma. For the SIR considered, the location of the stream interface is determined using multiple definitions, namely: the location of maximum total pressure, the location of maximum flow vorticity, the location of null azimuthal flow velocity and the location of steepest gradient of entropy. The speed of the plasma at these locations is compared to the mean speed of the interface to determine whether the interfaces are convected with the solar wind or propagate through it. In order to elucidate the physics of the evolution of the SIR, we also ran the LFM-helio using an idealized inner boundary condition. The idealized inner boundary specified the solar wind speed so as to ensure the presence of an SIR. Specifically, a source of fast wind was located at the same latitude as, and longitudinally near, a source of slow wind. The combined effect of radially outward plasma flow and rotation of the inner boundary align the fast wind behind the slow wind, creating an SIR. The magnetic field, plasma density and temperature were specified using empirical relations consistent with those used for the WSA. The evolution of the SIR produced by the idealized and realistic inner boundaries were compared in order to elucidate the factors determining the speed of the interface.