Tentative Identification of Interstellar Dust in Nose of the Heliosphere

Observations of the weak polarization of light from nearby stars, reported by Tinbergen (1982), are consistent with polarization by small, radius<0.14 μm, interstellar dust grains entrained in the magnetic wall of the heliosphere. The region of maximum polarization is towards ecliptic coordinates (λ,β)∼(295,0), corresponding to (l,b) = (20,–21), while the dust cone direction shows a marginally significant minimum in polarization. The direction of maximum polarization is offset along the ecliptic longitude by ∼35 from the nose of the heliosphere, and extends to low ecliptic latitudes. An offset is also seen between the region with the best aligned dust grains, λ∼281→330, and the upwind direction of the undeflected large grains, λ ∼ 259, β ∼ +8 , which are observed by Ulysses and Galileo to be flowing into the heliosphere. In the aligned-grain region, the strength of polarization anti-correlates with ecliptic latitude, indicating that the magnetic wall is predominantly at negative ecliptic latitudes. An extension of the magnetic wall to β <0 is consistent with predictions by Linde (1998). A consistent interpretation follows if the maximum-polarization region traces the heliosphere magnetic wall in a direction inclined to the local interstellar magnetic field, BIS, while the region of best-aligned dust samples the region where BIS stretchs smoothly over the heliosphere with maximum compression. These data are consistent with a tilt of ∼60 of BIS with respect to the ecliptic plane, and parallel to the galactic plane. Interstellar dust grains captured in the heliosheath may also introduce a weak, but important, large scale contaminant for the cosmic microwave background signal with a symmetry consistent with the relative tilts of BIS and the ecliptic. Subject headings: heliosphere — interstellar : dust, interstellar — dust: polarization