The interstellar magnetic field close to the sun. II.

P. C. Frisch, B. G. Andersson, A. Berdyugin, V. Piirola, R. Demajistre, H. O. Funsten, A. M. Magalhaes, D. B. Seriacopi, D. J. McComas, N. A. Schwadron, J. D. Slavin, S. J. Wiktorowicz

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The magnetic field in the local interstellar medium (ISM) provides a key indicator of the galactic environment of the Sun and influences the shape of the heliosphere. We have studied the interstellar magnetic field (ISMF) in the solar vicinity using polarized starlight for stars within 40pc of the Sun and 90° of the heliosphere nose. In Frisch et al. (Paper I), we developed a method for determining the local ISMF direction by finding the best match to a group of interstellar polarization position angles obtained toward nearby stars, based on the assumption that the polarization is parallel to the ISMF. In this paper, we extend the analysis by utilizing weighted fits to the position angles and by including new observations acquired for this study. We find that the local ISMF is pointed toward the galactic coordinates ℓ, b=47° ± 20°, 25° ± 20°. This direction is close to the direction of the ISMF that shapes the heliosphere, ℓ, b=33° ± 4°, 55° ± 4°, as traced by the center of the "Ribbon" of energetic neutral atoms discovered by the Interstellar Boundary Explorer (IBEX) mission. Both the magnetic field direction and the kinematics of the local ISM are consistent with a scenario where the local ISM is a fragment of the Loop I superbubble. A nearby ordered component of the local ISMF has been identified in the region ℓ ≈0° → 80° and b ≈0° → 30°, where PlanetPol data show a distance-dependent increase of polarization strength. The ordered component extends to within 8pc of the Sun and implies a weak curvature in the nearby ISMF of ∼025pc-1. This conclusion is conditioned on the small sample of stars available for defining this rotation. Variations from the ordered component suggest a turbulent component of ∼23°. The ordered component and standard relations between polarization, color excess, and Ho column density predict a reasonable increase of N(H) with distance in the local ISM. The similarity of the ISMF directions traced by the polarizations, the IBEX Ribbon, and pulsars inside the Local Bubble in the third galactic quadrant suggest that the ISMF is relatively uniform over spatial scales of 8-200pc and is more similar to interarm than spiral-arm magnetic fields. The ISMF direction from the polarization data is also consistent with small-scale spatial asymmetries detected in GeV-TeV cosmic rays with a galactic origin. The peculiar geometrical relation found earlier between the cosmic microwave background dipole moment, the heliosphere nose, and the ISMF direction is supported by this study. The interstellar radiation field at ∼975 Å does not appear to play a role in grain alignment for the low-density ISM studied here.

Original languageEnglish (US)
Article number106
JournalAstrophysical Journal
Issue number2
StatePublished - Dec 1 2012
Externally publishedYes

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science


  • ISM: bubbles
  • ISM: magnetic fields
  • Sun: heliosphere
  • local interstellar matter
  • methods: data analysis
  • polarization


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