Evidence for an interstellar dust filament in the outer heliosheath

A recently discovered filament of polarized starlight that traces a coherent magnetic field is shown to have severalproperties that are consistent with an origin in the outer heliosheath of the heliosphere: (1) the magnetic field that provides the best fit to the polarization position angles is directed toward ℓ = 357°. 3, b = 17°. 0 (±11?. 2); this direction is within 6°. 7 ± 11°.2 of the observed upwind direction of the flow of interstellar neutral helium gas through the heliosphere. (2) The magnetic field is ordered; the component of the variation of the polarization position angles that can be attributed to magnetic turbulence is ±9°. 6. (3) The axis of the elongated filament can be approximated by a line that defines an angle of 80° ± 14° with the plane that is formed by the interstellar magnetic field (ISMF) vector and the vector of the inflowing neutral gas (the BV plane). We propose that this polarization feature arises from aligned interstellar dust grains in the outer heliosheath where the interstellar plasma and magnetic field are deflected around the heliosphere. This interpretation suggests that the polarization is seen where stream lines of the flow and the draped ISMF lines are approximately parallel to each other and perpendicular to the sightline. An ordered magnetic field is required so that grain alignment is not disrupted during the interaction. The filament location is consistent with dust plumes previously predicted to form around the heliosphere. The proposed outer heliosheath location of the polarizing grains can be tested with three-dimensional models that track torques on asymmetric dust grains as they propagate through the heliosheath plasma, and using these models to evaluate grain alignment and the asymmetric extinction of the grains.