The Structure of the Galactic Magnetic Field toward the High-Latitude Clouds

We present the results of an optical polarization survey toward the galactic anticenter, in the area 5h ≥ α ≥ 2h and 6° ≤ δ ≤ 12°. This region is characterized by the presence of a stream of high-velocity H I as well as high galactic latitude molecular clouds. We used our polarization data together with 100 μm IRAS maps of the region to study the relation between the dust distribution and the geometry of the magnetic field. We find that there is a correlation between the percent polarization and the 100 μm flux such that P(%) ≤ (0.16 ± 0.05)F100. When the IRAS flux is converted into H I column densities this becomes P(%) ≤ (0.13 ± 0.03)N20 - 0.22, which is consistent with previous interstellar medium studies on the relation between reddening and polarization. This correlation indicates that our survey is as deep as IRAS and that the magnetic field geometry does not change strongly with the optical depth in the lines of sight that we have studied. The implied lower limit to the distance of our survey is 500-700 pc at galactic latitudes b = -20° and 100 pc at b = -50°. Our main finding is that the magnetic field is perpendicular to the H I high-velocity stream as well as the molecular cloud MBM 16 in the high-latitude region. The field is also perpendicular to the velocity gradient in the stream. Closer to the plane, the magnetic field is parallel to the dust filaments that extend like a plume toward the halo. Our observations indicate that the galactic magnetic field toward the high-latitude clouds is toroidal. We propose a model in which flux tubes that rise out of the galactic plane become force-free and predominantly toroidal at high latitudes. The high-latitude clouds may be gas streams that are falling back toward the galactic plane within such buoyant braided ropes of magnetic flux.