Comparison between the RM observations and the predictions from the model. Panel a) shows the RM data as a function of the distance to the centre of the Rosette. The two vertical dashed lines show the inner and outer radii of the nebula,  pc and  pc, respectively (Sect. 4.2.2). The blue curve shows the radial profile of the modelled RM for the best fit to the six RM measurements within (filled circles), the reference fit. The red curve is the result of fitting the highest four RM data points. The open circles correspond to the RM observations outside the Rosette, used to estimate the background RM, and are not included in the fit. Panel b) presents the reduced χ² from both fits. The stars indicate the best fit parameters B₀ and θ₀, which correspond to the minimum for each fit. The reference fit (blue) gives B₀ = 7 μG and θ₀ = 25° for χ² = 271 and N_dof = 4. The second fit (red) gives B₀ = 24 μG and θ₀ = 70° for χ² = 76 and N_dof = 2. The contours are at 10 and 30% above the corresponding minimum values of .