Lyα Absorption in a “Croissant-like” Heliosphere

Ly α absorption profiles have been used to detect astrospheres and heliospheric absorption from the hydrogen wall and heliotail. Using magnetohydrodynamic models of the heliosphere, we can compare simulated to observed Ly α profiles to probe the neutral hydrogen within and near the heliosphere. There is an ongoing controversy whether the heliosphere has a long “comet-like” tail or a short “croissant-like” tail. Here we present the first Ly α absorption investigation using a croissant-like heliosphere. With identical boundary conditions we compare the BU model, which presents a croissant-like tail, and the Moscow model, which presents a comet-like tail. The BU and Moscow models present nearly identical Ly α profiles toward nose targets ( α Cen and 36 Oph). Differences in Ly α profiles are shown toward the tail target (HD 35296). Despite the shortened heliotail of the croissant model, significant downwind heliosheath absorption is seen, just 5% shallower and shifted by 4 km s ^−1 . This implies that an extended tail model is not required to reproduce the heliosheath Ly α absorption observations. Finer observation gratings may be able to resolve this shift. Additionally, when using higher interstellar medium (ISM) neutral and plasma densities and lower magnetic field (∣ B _LISM ∣ = 3.2 μ G, α _BV ≈ 40°) than in the Moscow model, we find better agreement with observed Ly α profiles. None of the models presented show agreement in all directions simultaneously. Furthermore, we show that for the ISM conditions with the least certainty ( n _p,LISM , n _H,LISM , T _LISM , B _LISM ), B _LISM has the most significant effect on the structure of the hydrogen wall and Ly α profiles.