  Polarisation observations of masers in the circumstellar envelopes (CSEs)
around Asymptotic Giant Branch (AGB) stars have revealed strong magnetic
fields. However, masers probe only specific lines-of-sight through the CSE.
Non-masing molecular line polarisation observation can more directly reveal the
large scale magnetic field morphology and hence probe the effect of the
magnetic field on AGB mass-loss and the shaping of the AGB wind. Observations
and models of CSE molecular line polarisation can now be used to describe the
magnetic field morphology and estimate its strength throughout the entire CSE.
We use observations taken with ALMA of molecular line polarisation in the
envelope of two AGB stars (CW~Leo and R~Leo). We model the observations using
the multi-dimensional polarised radiative transfer tool PORTAL. We find
linearly polarised emission, with maximum fractional polarisation on the order
of a few percent, in several molecular lines towards both stars.
  We can explain the observed differences in polarisation structure between the
different molecular lines by alignment of the molecules by a combination of the
Goldreich-Kylafis effect and radiative alignment effects. We specifically show
that the polarisation of CO traces the morphology of the magnetic field.
Competition between the alignment mechanisms allows us to describe the
behaviour of the magnetic field strength with radius throughout the
circumstellar envelope of CW~Leo. The magnetic field strength derived using
this method is inconsistent with the magnetic field strength derived using a
structure function analysis of the CO polarisation and the strength previously
derived using CN Zeeman observations. In contrast with CW~Leo, the magnetic
field in the outer envelope of R~Leo appears to be advected outwards by the
stellar wind. {abridged abstract}
