Midplane cuts, as in Fig. 1, but for the first derivative of the Faraday rotation depth along the LOS, dϕ/dℓ=(ϕ(ℓ + dℓ) − ϕ(ℓ))/dℓ, computed from the synthetic observation produced using POLARIS. The red (blue) colors correspond to increases (decreases) in RM along each LOS toward an observer placed at the center of the selected bubble. The position of the observer is marked by a yellow cross. We note that the finite number of shells used to sample the rays causes the regular concentric ring pattern visible in the plots.