Evolution of the field strength with distance (adapted from Vlemmings 2012; Duthu et al. 2017). The four boxes show the SiO masers (the red box represents the results of this work for a saturated maser, the hatched box for an unsaturated maser using θ₂ = 45°, the dotted box the results from Herpin et al. 2006), the H₂O and OH masers, and CN. The r⁻² solar-type and r⁻¹ toroidal magnetic fields are discussed in Sect. 6. The star close to the SiO maser box is for the magnetic field measured by Lèbre et al. (2014) in χ Cyg. The vertical dashed line indicates the stellar surface for the Miras.