Observed NPS in the radio band and its geometry model. Panel a: 408 MHz sky map (Haslam et al. 1982, T_B in mK). The gray contours indicate T_B = 25 K, 40 K, 50 K, and 60 K. The right window shows the zoom-in view of the NPS, and the polygon marks the region for radio intensity statistics (similar to the region for gamma-ray analysis in Jóhannesson & Porter 2021). Panel b: 3D view and the three-slice view of the shell (coordinate values are in units of kpc). Panel c: projected thickness (D_s) of the radiative shell, accounting for the NPS/Loop I in Galactic coordinates with (R_in, R_out) = (5.0 kpc, 7.5 kpc). The crosses and the dashed line mark the outlines of Loop I and the northern Fermi bubble, respectively (see Su et al. 2010 for the coordinates). Panel d: projected thickness maps for (R_in, R_out) = (4.0 kpc, 7.5 kpc) (left) and (6.0 kpc, 7.5 kpc) (right).