Stokes U map overlaid with polarisation vectors. The vector direction shows the orientation of the magnetic field, and the length is proportional to the polarisation intensity. Right panel: polarisation vectors only for an ellipse with a thickness of 1°, which defines the bubble boundaries. The outer limit of the ellipse is delimited by the yellow dashed line in both panels. The ellipse is centred on l = 348.5° and b = 14.5°, it has a major axis of 23°, a minor axis of 15°, and a tilt angle of 15.5°. The yellow contours show the total gas column density N_H, where the contour levels are at 2.5, 4.5, and 8.0 × 10²¹ cm⁻².