Dependence of Heliospheric Lyα Absorption on the Interstellar Magnetic Field

Abstract

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.

Local Interstellar Parameters as They Are Inferred from Analysis of Observations Inside the Heliosphere

“…The analysis of Lyman-alpha absorption spectra toward nearby stars is presented in Wood et al (2007Wood et al ( , 2008 and will not be touched here.…”

“…One of the most direct diagnostics of the heliospheric interface is provided by the crossings of the heliospheric termination shock (TS) by Voyager 1 and 2 Costa et al (1999) T (H, TS) = 12000 K deflection of H atoms Lallement et al (2005) time-variation of H atom properties Quemerais et al (2008aQuemerais et al ( , 2008b Voyager, Pioneer, etc. and by Voyager 2 83.7 AU in August 2007 Richardson and Stone (2008) Absorption spectra Wood et al (2007Wood et al ( , 2008 toward nearby stars (HST) 2-3 kHz emission measured n(e, CHISM) = 0.04 cm −3 1.8 kHz emission cut-off by Voyagers ∼158 AU Gurnett et al (2006), and ref. therein at 94 AU in December 2004 and at 83.7 AU in August-September 2007, respectively.…”

Ionization rates in the heliosheath and in astrosheaths

“…Moreover, for nearby stars the relative speeds between the star and the interstellar medium can differ by factors between 0.2 to 2 compared with the interstellar medium that surrounds the solar system (Wood et al 2007). For these high relative speeds cross-sections of other processes and different species may become important.…”

Pressure and Ionization Balances in the Circum-Heliospheric Interstellar Medium and the Local Bubble

“…While one might conclude that the constraint on the EM determined by CHIPS could be weakened considerably by this depletion, it appears unlikely that this is happening: observations of X-ray absorption features from highly ionized Fe and Ne elsewhere within the Galaxy by Yao et al (2006) indicate a normal abundance ratio in the hot ISM.…”

“…For some time, the value and direction of the magnetic field in the CHISM was unknown, so one could hold out some hope that magnetic pressure could keep the CHISM from collapsing under the external pressure from the LB. Now, however, 3 independent studies using different observational methods indicate that the magnetic field is inclined relative to the upwind direction and creates an asymmetry in the shape of the heliosphere, but it has a strength that ranges somewhere between 1.8 and 2.5 µG (Lallement et al 2005;Opher et al 2007;Wood et al 2007). If the strength were as high as 3 µG, B 2 /(8kπ) = 1700 cm −3 K. This much pressure supplementing the thermal pressure still falls well short of the apparent pressure of the LB.…”