Following the interstellar magnetic field from the heliosphere into space with polarized starlight

Abstract

Starlight that becomes linearly polarized by magnetically aligned dust grains provides a viable diagnostic of the interstellar magnetic field (ISMF). A survey is underway to map the local ISMF using data collected at eight observatories in -3both hemispheres. Two approaches are used to obtain the magnetic structure: statistically evaluating magnetic field directions traced by multiple polarization position angles, and least-squares fits that provide the dipole component of the magnetic field. We find that the magnetic field in the circumheliospheric interstellar medium (CHM), which drives winds of interstellar gas and dust through the heliosphere, drapes over the heliopause, and influences polarization measurements. We discover a polarization band that can be described with a great circle that traverses the heliosphere nose and ecliptic poles. A gap in the band appears in a region coinciding both with the highest heliosheath pressure, found by IBEX, and the center of the Loop I superbubble. The least-squares analysis finds a magnetic dipole component of the polarization band with the axis oriented toward the ecliptic poles. The filament of dust around the heliosphere and the warm helium breeze flowing through the heliosphere trace the same magnetic field directions. Regions along the polarization band near the heliosphere nose have magnetic field orientations within 15 • of sightlines. Regions in the IBEX ribbon have field directions within 40 • of the plane of the sky. Several spatially coherent magnetic filaments are within 15 pc. Most of the low frequency radio emissions detected by the two Voyager spacecraft follow the polarization band. The geometry of the polarization band is compared to the Local Interstellar Cloud, the Cetus Ripple, the BICEP2 low opacity region, Ice Cube IC59 galactic cosmic ray data, and Cassini results.Subject headings: ISM: clouds, dust, magnetic fields -Physical processes: polarization -Sun: heliosphere 1 An average local linear polarization of 0.0004% per parsec yields n(H o )∼ 0.1 cm −3 , based on the relation P % =9E(B-V) for percentage polarization P % , color excess E(B-V) (Serkowski et al. 1975;Fosalba et al. 2002) and N(H o )+N(H 2 )=5.8 × 10 21 E(B-V) cm −2 mag (Bohlin et al. 1978). Ionization effects must be included to obtain accurate ratios (Frisch et al. 2015b). Note that the polarization pseudo-vector is aligned parallel to the ISMF direction (Lazarian 2007) so that these conversion formulae provide a lower limit to the amount of interstellar gas corresponding to the observed polarization strengths.

Polarization due to rotational distortion in the bright star Regulus

“…Yet even at this level accurate subtraction is crucial to measuring intrinsic polarization induced by rapid rotation. Only recently has sufficient precision been obtained in measurements of stars within the LHB 17,26,28,30 to enable accurate interstellar subtraction in this region.…”

The wavelength dependence of interstellar polarization in the Local Hot Bubble

“…Arzoumanian et al 2011), meaning different trends may manifest on adjacent sight lines. made small surveys of parts of the LHB (Bailey et al 2010;Cotton et al 2016b,c;Frisch et al 2016;Cotton et al 2017b), and the regions just beyond it (Berdyugin et al 2014), and begun to reveal its structure. However, with still few measurements, the data within the LHB is sparse and each study monochromatic, meaning we do not know the wavelength dependence of interstellar polarization.…”