Heliospheric structure: The bow wave and the hydrogen wall

G. P. Zank, J. Heerikhuisen, B. E. Wood, N. V. Pogorelov, E. Zirnstein, D. J. McComas

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Abstract

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 Lyα 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 μG) as well as plasma and neutral H number densities. The 2 and 3 μG models are fast magnetosonic far upwind of the heliopause whereas the 4 μG model is fully subsonic. The 2 μG model admits a broad (∼50-75 AU) bow-shock-like structure. The 3 μG model has a smooth super-fast-sub-fast magnetosonic transition that resembles a very broad, ∼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 μG and the 3 μG models, the super-fast magnetosonic LISM flow passes through a critical point located where the fast magnetosonic Mach number M = 1 and Qe = γ/(γ-1)UQm, where Qe and Qm are the plasma energy and momentum source terms due to charge exchange, U is the LISM flow speed, and γ is the plasma adiabatic index. Because the Mach number is only barely super-fast magnetosonic in the 3 μG 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 Lyα absorption profiles along the four sightlines considered (α Cen, 36 Oph, DK UMa, and χ1 Ori - upwind, sidewind, and downwind respectively); (2) both the super-fast magnetosonic models can account for the Lyα observations, with possibly the bow-shock-free 3 μG 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.

Original languageEnglish (US)
Article number20
JournalAstrophysical Journal
Volume763
Issue number1
DOIs
StatePublished - Jan 20 2013
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

Keywords

  • ISM: magnetic fields
  • Sun: heliosphere
  • interplanetary medium
  • local interstellar matter
  • plasmas
  • shock waves
  • solar wind

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    Zank, G. P., Heerikhuisen, J., Wood, B. E., Pogorelov, N. V., Zirnstein, E., & McComas, D. J. (2013). Heliospheric structure: The bow wave and the hydrogen wall. Astrophysical Journal, 763(1), [20]. https://doi.org/10.1088/0004-637X/763/1/20