Dependence of Heliospheric Lyman-alpha Absorption on the Interstellar Magnetic Field

Abstract

We use newly developed 3D kinetic MHD models of the heliosphere to predict heliospheric H I Lyman-alpha absorption for various lines of sight. These predictions are compared with actual Lyman-alpha spectra from the Hubble Space Telescope, many of which have yielded previous detections of heliospheric absorption. We find that the absorption predicted by the models is weakly affected by both the magnitude and orientation of the assumed ISM magnetic field. Models with B=1.25-2.5 micro-Gauss and an angle of alpha=15-45 deg with respect to the upwind direction of the ISM flow generally provide the best fits to the data, but the sensitivity of the Lyman-alpha absorption to many model input parameters makes it difficult to fully characterize the region of parameter space allowed by the data. We also use the models to assess the degree to which heliospheric asymmetries induced by the ISM field should be apparent in Lyman-alpha absorption. An ISM field that is skewed with respect to the ISM flow vector results in substantial azimuthal asymmetries in both the hydrogen wall and heliosheath, but only the heliosheath asymmetries yield potentially detectable asymmetries in Lyman-alpha absorption; and then only in downwind directions, where comparison with the data is complicated by few actual absorption detections and an insufficient model grid extent.