A New Description of the Outer Boundary of the Solar System

Professor Merav Opher and her group study the boundary between our solar system and the interstellar medium of the Milky Way Galaxy. Recent models had indicated that under the expected conditions of the interstellar magnetic field (ISMF) there should be no bow shock ahead of the heliosphere. In FY13 the group discovered (work led by Dr. B. Zieger) that even though a fast bow shock might be ruled out, the conditions in the local interstellar medium are such that a slow bow shock will be present, most likely along NASA’s Voyager 1 probe’s trajectory. This means that Voyager 1 will encounter ISM conditions mediated by the slow bow shock while Voyager 2 should not. The work was featured as an AGU Research Spotlight in EOS (“The solar system has a bow shock after all, it’s just slow”).

3-D view of the slow magnetosonic surface with the prospective crossing locations of Voyager 1 (V1) and Voyager 2 (V2). The angle between the interstellar magnetic field (B_ISMF) and the normal to this isosurface (θ_Bn) is plotted as contour lines on the surface. The slow bow shock is restricted to the quasi-parallel region of blueish colors north-west of the nose. The white line indicates the intersection of the isosurface with the B_ISMF–v_ISMF plane.

3-D view of the slow magnetosonic surface with the prospective crossing locations of Voyager 1 (V1) and Voyager 2 (V2). The angle between the interstellar magnetic field (B_ISMF) and the normal to this isosurface (θ_Bn) is plotted as contour lines on the surface. The slow bow shock is restricted to the quasi-parallel region of blueish colors north-west of the nose. The white line indicates the intersection of the isosurface with the B_ISMF–v_ISMF plane.

Professor Opher’s group has developed a 3D magneto-hydrodynamic (MHD) model including time-dependent solar cycle conditions to describe the heliosheath (project lead by PhD student Elena Provornikova). They compared the results with the data from Voyager 1 (V1) and 2 (V2) out to 30 time the Earth’s orbital distance. The main conclusions are that time dependent effects alone cannot explain the vastly different behavior of the flows in the heliosheath seen by V1 and V2. This has the implication that the standard 3D MHD models need to include additional physical effects to explain the shape and behavior of the flows in the heliosheath. The group also developed a model of the flows in the heliosheath as modified by the presence of a reconnected “sector region,” a region where the solar magnetic field reconnects. They have shown that the heliosheath flows are separated into two regions: the region within the reconnected sector fields behaves as a un-magnetized hydrodynamic flow collimated by the un-reconnected plasma outside of it. This scenario can explain the V1 measurements indicating that the spacecraft is immersed in a “quasi-stagnation region,” where the solar wind speeds are close to zero. The collimation of the flows within the sector region creates such a quasi-stagnation region ahead of the heliopause.