The Galactic global magnetic field is thought to play a vital role in shaping
Galactic structures such as spiral arms and giant molecular clouds. However,
our knowledge of magnetic field structures in the Galactic plane at different
distances is limited, as measurements used to map the magnetic field are the
integrated effect along the line of sight. In this study, we present the
first-ever tomographic imaging of magnetic field structures in a Galactic
spiral arm. Using optical stellar polarimetry over a 17′×10′ field of
view, we probe the Sagittarius spiral arm. Combining these data with stellar
distances from the Gaia mission, we can isolate the contributions of five
individual clouds along the line of sight by analyzing the polarimetry data as
a function of distance. The observed clouds include a foreground cloud (d<200 pc) and four clouds in the Sagittarius arm at 1.23 kpc, 1.47 kpc, 1.63
kpc, and 2.23 kpc. The column densities of these clouds range from 0.5 to 2.8×1021cm−2. The magnetic fields associated with each
cloud show smooth spatial distributions within their observed regions on scales
smaller than 10 pc and display distinct orientations. The position angles
projected on the plane-of-sky, measured from the Galactic north to east, for
the clouds in increasing order of distance are 135∘, 46∘,
58∘, 150∘, and 40∘, with uncertainties of a few degrees.
Notably, these position angles deviate significantly from the direction
parallel to the Galactic plane.Comment: Accepted for publication in Ap