Fast Shock, Slow Shock, or Bow Wave? Multi-fluid Simulations of the Solar Bow Shock

The interaction between the heliosphere and the local interstellar medium (LISM) has two alternate plausible scenarios, which can provide the same stand-off distance at the nose of the heliosphere. One is the superfast interaction with the presence of a fast bow shock, and the other is the subfast interaction with the presence of a slow bow shock [Zieger et al., 2013]. A third scenario could be a subfast interaction without a slow bow shock, when αBv, the angle between the interstellar magnetic field (BISM) and the bulk flow velocity of the interstellar wind (vISM), is greater than 45°, which would result in a fast bow wave extending far upstream in the LISM. However, the latter scenario would fail to produce an observable hydrogen wall in the Ly α absorption profiles of nearby stars, as demonstrated by Zank et al. [2013]. The Voyager observations of heliospheric asymmetries support a subfast interaction with strong BISM and small αBv [Opher et al., 2009a; Izmodenov et al. 2009]. IBEX observations also indicated that the interstellar wind is slower than previously thought and therefore most likely subfast [McComas et al., 2012]. The ribbon model of Chalov et al. [2010] uses a strong BISM with small αBv, too. However, other competing ribbon mechanisms [e.g. Heerikhuisen et al., 2010; Schwadron and McComas, 2013] prefer a much weaker magnetic field and larger angles. More recently, Scherer and Fichtner [2014] argued that including the He+ component of the LISM yields a higher fast magnetosonic Mach number that still supports the existence of a marginal fast bow shock. Up to date, there is no general consensus on the nature and existence of a bow shock ahead of the heliosphere.

In this paper, we study the nature of LISM-heliosphere interaction for the different possible scenarios, explore the possible relationship between the geometry of the slow bow shock and the IBEX ribbon, investigate the role of He+ in producing turbulence downstream of the fast bow shock, and predict plasma and magnetic field parameters along the Voyager trajectories beyond the heliopause. We employ 3-D global multi-fluid MHD simulations of the heliospheric interface varying the LISM parameters in different interaction scenarios. We fit the edge of the slow bow shock to the observed location of the IBEX ribbon. We also run 1-D local multi-fluid simulations of the fast bow shock with two ion species (H+ and He+) in the LISM and determine the level and nature of turbulence downstream of the quasi-parallel, quasi-perpendicular, and oblique regions of the bow shock. Finally, we validate our global simulations of the heliospheric interface with Voyager 1 and 2 magnetic field and plasma observations within and beyond the heliopause. This work is supported by NASA within the SHIELD DRIVE Science Center and by NASA's IBEX Explorer mission.