Magnetic field strength in AGB circumstellar envelopes as a function of radial distance. The black boxes indicate the literature values reported from maser measurements (e.g. Herpin et al. 2006; Leal-Ferreira et al. 2013; Vlemmings et al. 2005; Rudnitski et al. 2010). The black star indicates the surface magnetic field measurement for the star χ Cyg (Lèbre et al. 2014). The measurements for CW Leo are indicated in red. These are the CN Zeeman measurements from Duthu et al. (2017, red dash-dotted box) and the CO structure-function analysis result reported in Sect. 5.3 (red dashed-dotted line). For the CO result, we used the relation between n_H2 and R for the range of radii probed by the ALMA observations (Eq. (6)). This range is indicated by the vertical dotted lines. The stellar surface is indicated by the long-dashed vertical line. For the region probed by ALMA around CW Leo, we indicate in orange the lower (dashed) and upper (solid) magnetic field limits based on the analysis of the ¹²CO and H¹³CN radiative interaction rates. The relation between radius and magnetic field strength that fits the observations and modelled radiative rates described in Sect. 4.2 is presented as the blue dashed relation (we extrapolated out to the maximum scale probed with the ALMA polarisation observations). It is clear that this relation is not in agreement with the other magnetic field strength observations.