Constraints on the Structure of the Heliospheric Interface Based on Lyα Absorption Spectra

Spectroscopic observations of the hydrogen Lyα lines from nearby stars taken by the Hubble Space Telescope (HST) sometimes show absorption signatures from the heliosphere. This absorption is a unique diagnostic of material in the outermost parts of our heliosphere. We summarize how the HST data have been used to test various models of the heliosphere, particularly new 3-dimensional MHD models that have recently become available. We also focus on new detections of heliospheric absorption in very downwind directions, which can only be modeled using heliospheric model codes with extended grids in the downwind direction. We illustrate a couple attempts to reproduce this absorption using a couple of these extended tail models.