Interstellar Dust at the Magnetic Wall of the Heliosphere

February 5, 2008 Two sets of data show that small interstellar grains captured in interstellar magnetic fields, → BIS, draped over the heliosphere appear to polarize the light of nearby stars. The polarizing grains couple to → BIS, while larger grains couple to the cloud velocity. The maximum polarization direction, Pmax, is offset in ecliptic longitude by ∆λ ∼ +35 from the upwind direction, and the polarization peak is enhanced for stars near the ecliptic plane. A band of weak polarization stretches through the southern ecliptic hemisphere between the Pmax region and the downwind direction. The Pmax direction indicates that → BIS at the heliosphere is directed towards l∼105o, forming an angle of ∼75o with the inflowing gas. Grain alignment appears stable as the grains approach the heliosphere. The disruption of grain alignment in the tenuous local interstellar material (ISM) by stochastic collisions is ∼600 times slower than in the denser clouds for which grain alignment theory has been developed, however grain alignment mechanisms are still fast, and are boosted by compression of → BIS at the heliosphere. The polarization vanishes where the outer heliosheath magnetic fields become tangled or reconnect. A similar offset seen in energetic neutral atoms may form inside of the polarization region, where the plasma is pinched by opposing magnetic fields. The composition of dust in the interstellar cloud surrounding the solar system appears to be similar to olivines, and significant quantities of carbonaceous grains can be ruled out.