  Faraday rotation measure at radio wavelengths is commonly used to diagnose
large-scale magnetic fields. It is argued that the length-scales on which
magnetic fields vary in large-scale diffuse astrophysical media can be inferred
from correlations in the observed RM. RM is a variable which can be derived
from the polarised radiative transfer equations in restrictive conditions. This
paper assesses the usage of RMF (rotation measure fluctuation) analyses for
magnetic field diagnostics in the framework of polarised radiative transfer. We
use models of various magnetic field configurations and electron density
distributions to show how density fluctuations could affect the correlation
length of the magnetic fields inferred from the conventional RMF analyses. We
caution against interpretations of RMF analyses when a characteristic density
is ill-defined, e.g. in cases of log-normal distributed and fractal-like
density structures. As the spatial correlations are generally not the same in
the line-of-sight longitudinal direction and the sky plane direction, one also
needs to clarify the context of RMF when inferring from observational data. In
complex situations, a covariant polarised radiative transfer calculation is
essential to capture all aspects of radiative and transport processes, which
would otherwise ambiguate the interpretations of magnetism in galaxy clusters
and larger-scale cosmological structures.
