One‐ versus Two‐Shock Heliosphere: Constraining Models with Goddard High Resolution Spectrograph Lyα Spectra toward α Centauri

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Abstract
Redshifted Lyα absorption toward α Cen has been interpreted by Linsky & Wood and Frisch et al. as evidence for decelerated interstellar hydrogen piled up on the upstream side of the heliosphere. We utilize newly developed two-dimensional multifluid models of the solar wind interaction with the interstellar material to corroborate this interpretation by synthesizing the Lyα absorption profile predicted for this "hydrogen wall." Both subsonic and supersonic inflow into the heliosphere are considered, corresponding to one-shock and two-shock global morphologies, respectively. We find that these two extremes give observably different redward absorption characteristics in the Lyα profiles, and our preliminary conclusion is that the Lyα profiles seen toward α Cen favor a barely subsonic model (Mach number 0.9). For such a model to hold, additional interstellar pressure terms, such as cosmic-ray or magnetic pressures, must contribute. To make this conclusion more certain, an extended-model parameter survey is required, coupled with Lyα data along other lines of sight.