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tokens: 3 shows the measured fractional linear polarization as a function of wavelength for the six stars measured in the B , V , R , and I bands. Fixing K = 1.15 ( Serkowski et al. 1975 ) and leaving λ max and p max as free parameters, we fit the six polarization profiles to Eq. ( 1 ) using weighted least-squares. Fig. 3. Wavelength dependence of polarization for six of the stars in the R -band sample (USNO-B1 identifiers shown in the legend). Debiased fractional linear polarization ( p̂ ) measured in the Johnsons-Cousins B , V , R , and I bands (horizontal axis shows effective wavelength). For each star, a line shows the best-fit Serkowski law (Eq. ( 1 ), with K set to 1.15). The fit λ max are in the range 580 ± 46 nm–643 ± 34 nm, while the fit p max are in the range 0.9%−1.7%. Although limited by the uncertainties of the measurements, we find no evidence for substantial differences between the best-fit value of λ max in this sample compared to canonical values ( Serkowski et al. 1975 ). If grain alignment were unusually efficient in this region with smaller grains able to be aligned, we might have expected a shift of λ max toward shorter wavelengths than the typical 550 nm ( Martin et al. 1999 ). This is not observed. 3.3. Magnetic field morphology If the magnetic field morphology is responsible for the high submillimeter polarization fractions, then any region with high p 353 should have a magnetic field that lies mostly in the plane of the sky with no substantial change in orientation either along the line of sight or within the beam. The orientation of the magnetic field as projected on the plane of the sky is probed by the optical polarization angle θ R as well as the submillimeter polarization angle rotated by 90°. Within an 80′ beam centered on this region we find χ 353 + 90° =112.8 ± 0.6°. The polarization angles inferred from stellar polarimery do not vary significantly across the region (Fig. 4 , left). The standard deviation of the distribution of θ R is consistent with it arising from measurement uncertainties, suggesting little beam depolarization. Fig. 4. Left : distribution of stellar polarization angles, θ R for stars farther than 250 pc (solid line). The dashed vertical line shows the χ 353 +90° from Planck at 80′ resolution. Right : probability density functions of the orientation of linear HI structures ( θ RHT ) for different velocity ranges (shown on the right). We only consider velocity ranges within the peak of the HI intensity spectrum. The filled circles in the bottom mark the mean value, ⟨ θ RHT ⟩ for each velocity range. All angles are measured in the Galactic reference frame, according to the IAU convention. Linear structures seen in HI have also been shown to trace the interstellar magnetic field ( Clark et al. 2014 ). Recently, Clark (2018) proposed that changes in the orientation of these structures in velocity space can be used to probe variations of the magnetic field orientation along the line of sight. By