A NEW DETECTION OF LYα ABSORPTION FROM THE HELIOTAIL

Abstract

We present new Hubble Space Telescope observations of H i Lyα absorption toward the F8 V star HD 35296. This line of sight is only a few degrees from the downwind direction of the local interstellar medium flow vector. As a consequence, Lyα absorption from the heliotail is detected in the spectrum, consistent with three previous downwind detections of heliotail absorption. The clustering of the heliotail absorption detections around the downwind direction demonstrates that the heliotail is pointed close to that direction, limiting the extent to which the interstellar magnetic field might be distorting and deflecting the heliotail. We explore this issue further using three-dimensional MHD models of the global heliosphere. The three computed models represent the first three-dimensional MHD models with both a kinetic treatment of neutrals and an extended grid in the tail direction, both of which are necessary to model Lyα absorption downwind. The models indicate only modest heliotail asymmetries and deflections, which are not large enough to be inconsistent with the clustering of heliotail absorption detections around the downwind direction. The models are reasonably successful at reproducing the observed absorption, but they do overpredict the Lyα opacity by a factor of 2-3. We discuss implications of these results in light of observations of the heliotail region from the Interstellar Boundary Explorer mission.

Evidence for a Weak Wind From the Young Sun

“…There appears to be a small inconsistency in the wavelength calibration of the two Mg ii lines, leading to a mediocre fit to the h line in Figure 1(a). We have seen similar inconsistencies in the past with these two lines (e.g., Wood et al 2014). Note that Table 1 lists the rest wavelengths of both closely separated fine structure components of the D i and H i Lyα lines, but in the figures we show only the combined absorption.…”

Toward More Realistic Analytic Models of the Heliotail: Incorporating Magnetic Flattening via Distortion Flows

“…it increases with both B ism and distance −z, the fact that the derived displacement does not depend monotonously on z is somewhat at odds with the notion of the heliotail axis being deflected by a fixed angle against the inflow axis (which would imply m fit ∼ z for fixed B ism ). At this point, we may only note that while the simulations at hand do not support a corresponding scaling relation, the absolute magnitude of deflection is indeed rather small, in agreement with Wood et al (2014), who present observational constraints to argue for a deflection angle of the order of 10…”

Response times of Cassini/INCA > 5.2 keV ENAs and Voyager ions in the heliosheath over the solar cycle

“…The consistency of the ENA intensities, taken together with the convergence of the ENA energy spectra between the nose and anti-nose directions, provide indisputable evidence that the global heliosphere must be roughly symmetric. However, we must keep in mind that, as explained in [25], a perfect symmetry not only would require a negligible ram pressure of the interstellar flow [8], but also, models including Hubble Space Telescope observations of H-Lyα absorption have been interpreted as showing a nose-tail asymmetry [37]. Thus, although this nose-tail asymmetry can be in fact attributed to the non-negligible anisotropic ram pressure of the interstellar medium, we stress that such distortion would differ substantially from any heliosheath structure that includes a very prolonged tail [16].…”