HELIOSPHERIC STRUCTURE: THE BOW WAVE AND THE HYDROGEN WALL

Recent IBEX observations indicate that the local interstellar medium (LISM) flow speed is less than previously thought (23.2 km s(-1) rather than 26 km s(-1)). Reasonable LISM plasma parameters indicate that the LISM flow may be either marginally super-fast magnetosonic or sub-fast magnetosonic. This raises two challenging questions: (1) Can a LISM model that is barely super-fast or sub-fast magnetosonic account for Ly alpha observations that rely critically on the additional absorption provided by the hydrogen wall (H-wall)? and (2) If the LISM flow is weakly super-fast magnetosonic, does the transition assume the form of a traditional shock or does neutral hydrogen (H) mediate shock dissipation and hence structure through charge exchange? Both questions are addressed using three three-dimensional self-consistently coupled magnetohydrodynamic plasma-kinetic H models with different LISM magnetic field strengths (2, 3, and 4 mu G) as well as plasma and neutral H number densities. The 2 and 3 mu G models are fast magnetosonic far upwind of the heliopause whereas the 4 mu G model is fully subsonic. The 2 mu G model admits a broad (similar to 50-75 AU) bow-shock-like structure. The 3 mu G model has a smooth super-fast-sub-fast magnetosonic transition that resembles a very broad, similar to 200 AU thick, bow wave. A theoretical analysis shows that the transition from a super-fast to a sub-fast magnetosonic downstream state is due to the charge exchange of fast neutral H and hot neutral H created in the supersonic solar wind and hot inner heliosheath, respectively. For both the 2 mu G and the 3 mu G models, the super-fast magnetosonic LISM flow passes through a critical point located where the fast magnetosonic Mach number M = 1 and Q(e) = gamma/(gamma - 1)U Q(m), where Q(e) and Q(m) are the plasma energy and momentum source terms due to charge exchange, U is the LISM flow speed, and. is the plasma adiabatic index. Because the Mach number is only barely super-fast magnetosonic in the 3 mu G case, the hot and fast neutral H can completely mediate the transition and impose a charge exchange length scale on the structure, making the solar-wind-LISM interaction effectively bow-shock-free. The charge exchange of fast and hot heliospheric neutral H therefore provides a primary dissipation mechanism at the weak heliospheric bow shock, in some cases effectively creating a one-shock heliosphere (i.e., a heliospheric termination shock only). Both super-fast magnetosonic models produce a sizeable H-wall. We find that (1) a sub-fast magnetosonic LISM flow cannot model the observed Ly alpha absorption profiles along the four sightlines considered (alpha Cen, 36 Oph, DK UMa, and chi(1) Ori-upwind, sidewind, and downwind respectively); (2) both the super-fast magnetosonic models can account for the Ly alpha observations, with possibly the bow-shock-free 3 mu G model being slightly favored. Subject to further modeling and comparison against further lines of sight, we conclude with the tantalizing possibility that IBEX may have discovered a class of interstellar shocks mediated by neutral H.