Dependence of Heliospheric Lyα Absorption on the Interstellar Magnetic Field

We use newly developed 3D kinetic MHD models of the heliosphere to predict heliospheric H I Lyα absorption for various lines of sight. These predictions are compared with actual Lyα 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 μG and an angle of α = 15 − 45◦ with respect to the upwind direction of the ISM flow generally provide the best fits to the data, but the sensitivity of the Lyα 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 Lyα 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 Lyα 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. Subject headings: MHD — solar wind — interplanetary medium — ultraviolet: stars JILA, University of Colorado, 440 UCB, Boulder, CO 80309-0440; [email protected], [email protected]. Lomonosov Moscow State University, Dept. of Aeromechanics and Gas Dynamics, Moscow 119899, Russia; [email protected]. Institute for Problems in Mechanics RAS, Prospekt Vernadskogo 101-1, Moscow 117526, Russia; and Space Research Institute (IKI) RAS