Magnetic-field morphology of the C.1 shell. Comparison between Planck polarization data (top row) and the Stokes parameters produced with a toy model (bottom row) describing the magnetic-field geometry compressed on the surface of a shell seen in projection (in yellow). Units and dynamic range in both rows are the same. Top right panel: we overlay on the polarization-fraction map (right panel) black contours of HI emission at 30, 35, 40, 45, and 50 K tracing the 7.4 km s⁻¹ component of C.1 (dashed-line circle in all panels). The position of the CrA molecular cloud is shown with white (left and right panels) and gray (central panels) contours of at 4 × 10²¹ cm⁻². The location of the Coronet cluster (CrA-A) is marked in yellow in the left panel. Top row: field of view is 20 × 20 deg² centered in Galactic coordinates of (l, b) = (6°, −20°).