Charting the Interstellar Magnetic Field behind the Interstellar Boundary Explorer (IBEX) Ribbon
Abstract:The relation between the interstellar magnetic field (ISMF) controlling the configuration of the “ribbon” of energetic neutral atoms discovered by IBEX, and the ISMF in deep space, can be probed with polarized starlight. Starlight is polarized in a dichroic interstellar medium formed by interstellar dust grains that are aligned with respect to the ISMF. Our ongoing survey of polarized starlight traces the ISMF within 40 parsecs. The local ISMF direction was evaluated using the weighted means of the linear polarization vectors. The dominant nearby magnetic field direction is within 7.6 (+14.9,-7.6) degrees of the ISMF direction that is traced by the IBEX ribbon. A low level of random magnetic turbulence is obtained from the polarization data that best trace the IBEX ribbon field direction, 9 (+/-1) deg, which explains the continuity of the IBEX ISMF out into space where it can be traced by starlight polarization. The ISMF direction is perpendicular to the velocity of the cloud around the heliosphere, and it orders the kinematics of the other local interstellar clouds.
These results are obtained only after a well-defined subset of the polarization data is omitted from the sample. A separate analysis shows that these polarization vectors are oriented toward the upwind direction of the interstellar gas flowing into the heliosphere. This group of polarization data traces an elongated filamentary-type feature that is perpendicular to the hydrogen deflection plane. We suggest a heliosheath origin for the grains that create this polarized feature.
One characteristic shown by the polarization and IBEX field directions is that the ribbon ISMF extends to the boundaries of the BICEP2 region where the polarized CMB background has been studied. Inside of the BICEP2 region other nearby magnetic components are also present. The sightline toward the star Capella suggests that the polarization mechanism is very efficient in this nearby cloud.