  Analysis of Lagrangian Coherent Structures (LCSs) has been showed to be a
valid mathematical approach to explain the formation of transport barriers in
magnetized plasmas. Such LCSs, borrowed from fluid dynamics theory, can be
considered as the hidden skeleton of the system and can be used for studying a
wide spectrum of transport mechanisms even in plasmas. In this paper, we
demonstrate that such structures can be particularly useful for underlying the
hidden paths governing the motion of magnetic field lines in chaotic magnetic
fields. To perform such an analysis, we developed a numerical tool able to
detect LCSs for a general dynamical system. The tool is able to deal with
general coordinate systems and it is shown to match with other techniques
already used to analyse chaotic magnetic fields, e.g. connection length. After
the description of the computational tool, we focus on the heat transport
equation and the comparison between temperature profile and topology of the
LCSs. We provide evidence that numerical simulations are able to reproduce the
temperature profiles similar to those observed in RFP experiments and that our
tool successfully predicts the location of temperature gradients. The results
suggest that, inside the chaotic region, the field-lines motion is far from
stochastic and that the presence of hidden patterns allows the development of
high temperature gradients.
