2024ApJ...961..235P

Lyα 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 Lyα 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 Lyα 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 Lyα profiles toward nose targets (α Cen and 36 Oph). Differences in Lyα 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 Lyα 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 μG, α_BV ≈ 40°) than in the Moscow model, we find better agreement with observed Lyα 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 Lyα profiles.