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Paper: Heliospheric Structure: The Bow Wave and the Hydrogen Wall
Volume: 484, Outstanding Problems in Heliophysics: From Coronal Heating to the Edge of the Heliosphere
Page: 255
Authors: Zank, G. P.; Heerikhuisen, J.; Wood, B. E.; Pogorelov, N. V.; Zirnstein, E.; McComas, D. J.
Abstract: IBEX observations indicate that the local interstellar flow speed is less than previously thought (23.2 km/s). Reasonable local interstellar medium (LISM) parameters show that the LISM flow may be either marginally super-fast magnetosonic or sub-fast magnetosonic. We address whether a LISM model that is barely super-fast or sub-fast magnetosonic can account for Lyman-alpha absorption observations, and if the LISM flow transition is mediated by neutral hydrogen (H). We use three 3D self-consistently coupled MHD - kinetic H models with different LISM magnetic field strengths (2, 3, and 4 μG) and plasma and neutral H number densities to investigate these questions. The 2 μG model admits a broad bow shock-like structure, the 3 μG model a very broad smooth super-fast-sub-fast transition resembling a bow wave, and the 4 μG admits no bow shock. For both the 2 μG and the 3 μG models, the super-fast magnetosonic LISM flow passes through a critical point. Hot and fast neutral H can mediate a weak transition and impose a charge exchange length scale on the structure. The 3 μG model of the solar wind - LISM interaction is effectively bow shock-free. We model the observed Lyman-alpha absorption profiles along four sightlines finding that both super-fast magnetosonic models can account for the Lyman-alpha observations, with possibly the bow shock-free 3 μG model being slightly favored.
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